LTC5100 3.3V, 3.2Gbps VCSEL Driver U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 155Mbps to 3.2Gbps Laser Diode Driver for VCSELs* 60ps Rise and Fall Times, 10ps Deterministic Jitter Eye Diagram is Stable and Consistent Across Modulation Range and Temperature 1mA to 12mA Modulation Current Easy Board Layout, Laser can be Remotely Located if Desired No Input Matching or AC Coupling Components Needed On-Chip ADC for Monitoring Critical Parameters Digital Setup and Control with I2CTM Serial Interface Emulation and Set-Up Software Available** Operates Standalone or with a Microprocessor On-Chip DACs Eliminate External Potentiometers Constant Current or Automatic Power Control First and Second Order Temperature Compensation On-Chip Temperature Sensor Extensive Eye Safety Features Single 3.3V Supply 4mm × 4mm QFN Package U APPLICATIO S ■ ■ The LTC5100 supports fully automated production with its extensive monitoring and control features. Integrated 10-bit DACs eliminate the need for external potentiometers. An onboard 10-bit ADC provides the laser current and voltage, as well as monitor diode current and temperature. Status information is available from the I2C serial interface for feedback and statistical process control. An internal digital controller compensates laser temperature drift and provides extensive laser safety features. Gigabit Ethernet and Fibre Channel Transceivers SFF and SFP Transceiver Modules Proprietary Fiber Optic Links , LTC and LT are registered trademarks of Linear Technology Corporation. I2C is a trademark of Philips Electronics N.V. *Vertical Cavity Surface Emitting Laser **Downloadable from www.linear.com U ■ The LTC®5100 is a 3.2Gbps VCSEL driver offering an unprecedented level of integration and high speed performance. The part incorporates a full range of features to ensure consistently outstanding eye diagrams. The data inputs are AC coupled, eliminating the need for external capacitors. The LTC5100 has a precisely controlled 50Ω output that is DC coupled to the laser, allowing arbitrary placement of the IC. No coupling capacitors, ferrite beads or external transistors are needed, simplifying layout, reducing board area and the risk of signal corruption. The unique output stage of the LTC5100 confines the modulation current to the ground system, isolating the high speed signal from the power supply to minimize RFI. TYPICAL APPLICATIO 3.3V 24LC00 EEPROM IN SOT-23 PACKAGE VDD SDA ADC MD 3.2Gbps Electrical Eye Diagram SCL DIGITAL CONTROLLER DAC 10nF SRC EN FAULT DAC 50Ω 1mA/DIV MODA IN+ SERIALIZER + MODB 100Ω IN– VSS ARBITRARY DISTANCE – 3.2Gbps MODULATOR 50ps/DIV 5100 TA01 5100 F01 WARNING: POTENTIAL EYE HAZARD. SEE “EYE SAFETY INFORMATION” Figure 1. VCSEL Transmitter with Automatic Power Control sn5100 5100fs 1 LTC5100 ORDER PART NUMBER MD SRC EN TOP VIEW 16 15 14 13 VSS 1 LTC5100EUF 12 VSS IN + 2 11 MODA 17 IN – 3 10 MODB VSS 4 6 7 8 VDD(HS) 9 5 SCL VDD, VDD(HS) ............................................................. 4V IN+, IN– (Cml_en = 1) (Note 6) Peak Voltage ........... VDD(HS) – 1.2V to VDD(HS) + 0.3V Average Voltage...... VDD(HS) – 0.6V to VDD(HS) + 0.3V + IN , IN– (Cml_en = 0) (Note 4) .. –0.3V to VDD(HS) + 0.3V Cml_en = 0 (Note 4) Peak Difference Between IN+ and IN– .............. ±2.5V Average Difference Between IN+ and IN– ....... ±1.25V MODA, MODB (Transmitter Disabled) .... –0.3V to 2.75V MODA, MODB (Transmitter Enabled) ............ VDD(HS) – 2.75V to 2.75V EN, SDA, SCL, FAULT ..................... –0.3V to VDD + 0.3V MD, SRC ................................................... –0.3V to VDD Ambient Operating Temperature Range .. – 40°C to 85°C Storage Temperature Range ................ – 65°C to 125°C VDD W (Note 1) U W U PACKAGE/ORDER I FOR ATIO SDA W W AXI U RATI GS FAULT U ABSOLUTE VSS UF PART MARKING 5100 UF PACKAGE 16-LEAD (4mm × 4mm) PLASTIC QFN TJMAX = 125°C, θJA = 37°C/W EXPOSED PAD IS VSS (PIN 17) MUST BE SOLDERED TO PCB GROUND PLANE Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1% resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5. PARAMETER CONDITIONS MIN TYP MAX UNITS 3.135 3.3 3.465 V Power Supply VDD, VDD(HS) Operating Voltage VDD + VDD(HS) Quiescent Current, Excluding the SRC Pin Current (Note 2) ● VDD = 3.465V Transmitter Disabled, Power_down_en = 1 4.5 mA Transmitter Enabled, Is_rng = Im_rng = 3 Impp = 24mA 54 mA High Speed Data Inputs (IN+ and IN– Pins) (Test Circuit, Figure 5) Input Signal Amplitude Peak-to-Peak Differential Voltage (The SingleEnded Peak-to-Peak Voltage is One Half the Differential Voltage) Common Mode Input Signal Range (Note 3) Cml_en = 0 (Note 4) 500 to 2400 0 mVP-P VDD(HS) V 80 to 120 Ω Common Mode Input Resistance Cml_en = 0 (Note 5) 50 kΩ Open-Circuit Voltage Cml_en = 0 (Note 5) 1.65 Differential Input Resistance V SRC Pin Current, IS Full-Scale IS Current Minimum Operating Current (Note 7) Is_rng = 0 Is_rng = 1 Is_rng = 2 Is_rng = 3 6 12 18 24 mA mA mA mA 1/16 of Full-Scale IS Current Resolution SRC Pin Voltage Range 9 18 27 36 10 1.2 Bits VDD – 200mV V sn5100 5100fs 2 LTC5100 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1% resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5. PARAMETER CONDITIONS MIN TYP MAX UNITS 6 – IM 12 – IM 18 – IM 24 – IM 9 – IM 18 – IM 27 – IM 36 – IM mA mA mA mA ±25 % Laser Bias Current, IB Full-Scale Current (Note 8) Is_rng = 0 Is_rng = 1 Is_rng = 2 Is_rng = 3 Absolute Accuracy SRC Pin and MODA, MODB Pin Currents Within Specified Voltage Ranges Resolution 10 Bits Linear Tempco Resolution 122 ppm/°C ±15625 ppm/°C Second Order Tempco Resolution 3.81 ppm/°C2 Second Order Tempco Range ±488 ppm/°C2 Linear Tempco Range Temperature Stability Ib_tc1 = 0, Ib_tc2 = 0 Off-State Leakage Transmitter Disabled, VSRC = 1.2V ±500 ppm/°C µA 50 MODA, MODB Pin Current, IM Full Scale, Peak-to-Peak Modulation Current (Note 9) Im_rng = 0 Im_rng = 1 Im_rng = 2 Im_rng = 3 6 12 18 24 9 18 27 36 mA mA mA mA 1/8 of Full-Scale Peak-to-Peak Modulation Current Minimum Operating Current (Note 10) Resolution (Note 11) 9 Current Stability Im_tc1 = 0, Im_tc2 = 0 Voltage Range Peak Transient Voltage on MODA and MODB Absolute Accuracy of the Modulation Current Linear Tempco Resolution Bits ±500 1.2 ppm/°C 2.7 V ±25 % 122 ppm/°C ±15625 ppm/°C Second Order Tempco Resolution 3.81 ppm/°C2 Second Order Tempco Range ±484 ppm/°C2 Linear Tempco Range Maximum Bit Rate 3.2 Gbps Modulation Current Rise and Fall Times 20% to 80% Measured with K28.5 Pattern at 2.5Gbps 60 ps Deterministic Jitter, Peak-to-Peak (Note 12) Measured with K28.5 Pattern at 3.2Gbps 10 ps Random Jitter, RMS (Note 13) 1 psRMS Pulse Width Distortion 10 ps Automatic Power Control (Note 14) Minimum Operating Current for the Monitor Diode (Note 15) 20% of Full Scale Monitor Diode Current Temperature Stability Imd_tc1 = 0, Imd_tc2 = 0 ±500 ppm/°C Monitor Diode Bias Voltage (Note 16) IMD ≤ 1600µA 1.45 V sn5100 5100fs 3 LTC5100 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1% resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5. PARAMETER CONDITIONS MIN TYP MAX UNITS Automatic Power Control (Note 14) Temperature Compensation (Note 17) Linear Tempco Resolution Linear Tempco Range 254 • Imd_nom/1024 ppm/°C ±32300 • Imd_nom/1024 ppm/°C ADC Resolution 10 Bits 9 18 27 36 mA mA mA mA Source Current Measurement, IS (SRC Pin Current) Full Scale Is_rng = 0 Is_rng = 1 Is_rng = 2 Is_rng = 3 ±3% of Full Scale ±25% of Reading Accuracy Average Modulation Current Measurement, IM (Note 18) Full Scale Im_rng = 0 Im_rng = 1 Im_rng = 2 Im_rng = 3 9 18 27 36 mA mA mA mA ±3% of Full Scale ±25% of Reading Accuracy Laser Diode Voltage Measurement Full Scale 3.5 Accuracy ±150mV ±10% of Reading V Monitor Diode Current Measurement (Note 19) Full Scale Imd_rng = 0 Imd_rng = 1 Imd_rng = 2 Imd_rng = 3 34 136 544 2176 Zero Scale ADC Code = 0 1/8 of Full Scale Resolution Relative to Reading 0.2 µA µA µA µA % ±25% of Reading Accuracy Temperature Measurement Full Scale Celsius Sensitivity 239 °C 0.500 °C/LSB 400 800 1200 1600 mV mV mV mV Termination Resistor Voltage Measurement Full Scale Is_rng = 0 Is_rng = 1 Is_rng = 2 Is_rng = 3 ±30mV ±10% of Reading Accuracy Safety Shutdown, Undervoltage Lockout (UVLO) Undervoltage Detection Undervoltage Detection Hysteresis VDD Decreasing 2.8 V 150 mV sn5100 5100fs 4 LTC5100 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C; VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1% resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5. PARAMETER CONDITIONS MIN TYP MAX UNITS Bias Current Limit, IB(LIMIT) Set Point Resolution 7 Bits 9 18 27 36 mA mA mA mA Expressed in % Over the Imd Set Point 50 % Expressed in % Under the Imd Set Point –50 % Time from the Fault Occurance to Reduction of the Laser Bias Current to 10% of Nominal 100 µs Set Point Range Is_rng = 0 Is_rng = 1 Is_rng = 2 Is_rng = 3 Optical Power Limit Automatic Power Control Mode Only, Apc_en = 1 Overpower Limit Underpower Limit Safety Shutdown Response Time FAULT Output, Open-Drain Mode, Flt_drv_mode = 0 Output Low Voltage IOL = 3.3mA 0.4 V Output High Leakage Current VFAULT = 2.4V 10 µA FAULT Output, Open-Drain Mode with 330µA Internal Pull Up, Flt_drv_mode = 1 Output Low Voltage IOL = 3.3mA Output High Current VFAULT = 2.4V 0.4 V µA –280 FAULT Output, Open-Drain Mode with 500µA Internal Pull Up, Flt_drv_mode = 2 Output Low Voltage IOL = 3.3mA Output High Current VFAULT = 2.4V 0.4 V µA –425 FAULT Output, Complementary Drive Mode, Flt_drv_mode = 3 Output High Voltage IOH = –3.3mA Output Low Voltage IOL = 3.3mA 2.4 V 0.4 V 0.8 V EN Input, Ib_gain or (Apc_gain in APC Mode) = 16, Im_gain = 4, Is_rng = 0, Im_rng = 0 Input Low Voltage Input High Voltage 2 V Input Low Current En_polarity = 0 (EN Active Low), VEN = 0V –10 µA Input High Current En_polarity = 0 (EN Active Low), VEN = VDD –10 to 10 µA Input Low Current En_polarity = 1 (EN Active High), VEN = 0V –10 to 10 µA Input High Current En_polarity = 1 (EN Active High), VEN = VDD 10 µA Transmit Enable Time Time from Active Transition on EN to 95% of Nominal Laser Power and 95% of Full Modulation. First Time Transmission is Enabled After Power On or with Rapid_restart_en = 0 100 ms Transmit Re-Enable Time Time from Active Transition on EN to 95% of Nominal Laser Power and 95% of Full Modulation. When Transmission is Re-Enabled After the First Time and with Rapid_restart_en = 1 1 ms Transmit Disable Time Time from Inactive Transition on EN to 5% of Nominal Laser Power 10 µs Minimum Pulse Width Required to Clear a Latched Fault 10 µs sn5100 5100fs 5 LTC5100 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDD = VDD(HS) = 3.3V, IS = 24mA; IM = 12mA (IMPP = 24mA); 49.9Ω, 1% resistor from SRC (Pin 14) to MODA (Pin 11); 50Ω, 1% load AC coupled to MODB (Pin 10); 10nF, 10% capacitor from SRC (Pin 14) to VSS; Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit in Figure 5. PARAMETER CONDITIONS MIN TYP MAX UNITS SCL, SDA SCL, SDA Input Low Voltage, VIL –0.5 0.3 • VDD V SCL, SDA Input High Voltage, VIH 0.7 • VDD VDD + 0.5 V SCL, SDA Input Low Current (Note 21) VSDA, VSCL = 0.1 • VDD –100 µA SCL, SDA Input High Current (Note 21) VSDA, VSCL = 0.9 • VDD –100 µA SCL, SDA Output Low Voltage IOL = 3mA 0 Hysteresis 0.4 280 V mV Serial Interface Timing (Note 20) SCL Clock Frequency 100 kHz 4 µs Low Period of the SCL Clock 4.7 µs High Period of the SCL Clock 4 µs Hold Time (Repeated) START Condition. After This Period the First Clock Pulse is Generated Set-Up Time for a Repeated START Condition Data Hold Time Data Set-Up Time µs 4.7 0 3.45 250 µs ns Input Rise Time of Both SDA and SCL Signals 1000 ns Output Fall Time of SCL and SDA from VIH(MIN) to VIL(MAX) with a Bus Capacitance from 10pF to 400pF 300 ns Set-Up Time for STOP Condition Bus Free Time Between a STOP and START Condition 4 µs 4.7 µs Capacitive Load for Each Bus Line 400 pF Noise Margin at the LOW Level for Each Connected Device (Including Hysteresis) 0.1 • VDD V Noise Margin at the HIGH Level for Each Connected Device (Including Hysteresis) 0.2 • VDD V Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: The quiescent VDD and VDD(HS) currents refer to the current with zero SRC pin current (i.e., the laser is operating with zero bias current and zero modulation current). The total power supply current is the quiescent current plus the SRC pin current, IS, plus any current sinked from IN+ and IN–. Note 3: The peak transient voltage at the IN + and IN – pins must not exceed the range of –300mV to VDD(HS) + 300mV. Note 4: When Cml_en = 0 (not in CML mode), the termination is 100Ω differential with 50k common mode to VDD(HS)/2. Note 5: The common mode input resistance is measured relative to VDD(HS)/2 with the inputs tied together. Note 6: When Cml_en = 1 (CML mode), the termination is nominally 50Ω to VDD(HS) on each of the IN+ and IN– pins. Note 7: The SRC pin current can be programmed to near zero in each range, but the recommended minimum operating level is 1/16 of full scale. Note 8: The laser bias current is the average current delivered to the laser. It is equal to the SRC pin current minus the average modulation current at the MODA and MODB pins, or IB = IS – IM. Full scale for the bias current therefore depends on Is_rng and the actual modulation current. Note 9: The MODA and MODB pins are connected on-chip. The modulation current refers to the sum of the currents on these pins. IM refers to the total average current at the MODA and MODB pins. IMPP refers to the total peak-to-peak modulation current at the MODA and MODB pins. IMPP differs from the laser modulation current, IMOD. IMPP splits between the laser and the termination resistor according to IMOD = IMPP • RT/(RT + RLD), where RT is the value of the termination resistor and RLD is the dynamic resistance of the laser diode. sn5100 5100fs 6 LTC5100 ELECTRICAL CHARACTERISTICS Note 10: The modulation current can be programmed to near zero in each range, but the high speed performance is not guaranteed for currents less than the specified minimum. Note 11: The effective resolution of the modulation current is 9 bits because the modulation servo system uses only one-half of the 10-bit ADC range. Note 12: As defined in ANSI x3.230, Annex A, deterministic jitter is the peak-to-peak deviation of the 50% crossings of the modulation signal when compared to the ideal time crossings. The specification for the LTC5100 pertains to the electrical modulation signal. The K28.5 pattern is the repeating sequence 00111110101100000101. Note 13: Random jitter is the standard deviation of the 50% crossings of the electrical modulation signal as measured by an oscilloscope. It is measured with a 1GHz square wave after quadrature subtraction of the random jitter of the pulse generator and oscilloscope. Peak-to-peak random jitter is defined as 14 times the RMS random jitter. Note 14: The LTC5100 digitizes and servo controls the logarithm of the monitor diode current. Many of the characteristics of the APC system, such as range and resolution, are determined by the ADC. Note 15: The minimum practical operating current for the monitor diode is determined by servo settling time considerations. Note 16: IMD must be less than 25µA, 100µA, 400µA and 1600µA corresponding to Imd_rng = 0, 1, 2, 3. Note 17: The temperature coefficients of the monitor diode current depend on the IMD setting because of the logarithmic relationship between the set point and the monitor diode current. Imd_nom is the digital code setting for the nominal monitor diode current. Imd_nom lies between 0 and 1023. Note 18: The ADC digitizes the average modulation current, which is 50% of the peak-to-peak current for a 50% duty cycle signal. Note 19: The LTC5100 ADC digitizes the logarithm of the monitor diode current. This implies that the ADC resolution is a constant percentage of reading and that the monitor diode current is non-zero when the ADC reads zero. See the Design Notes for further information. Note 20: Serial interface timing is guaranteed by design from –40°C to 85°C. Note 21: The LTC5100 has 100µA nominal pull-up current sources on the SCL and SDA pins to eliminate the need for external pull-up resistors when connected to a single EEPROM device. The LTC5100 meets the maximum rise time specification of 1000ns with external I2C bus capacitances up to 25pF. Example: 10pF EEPROM + 150mm trace ~ 25pF. Note 22: VDD and VDD(HS) must be tied together on the PC board. sn5100 5100fs 7 LTC5100 U W TYPICAL PERFOR A CE CHARACTERISTICS VDD = VDD(HS) = 3.3V, TA = 25°C, Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit shown in Figure 5. Optical Eye Diagram at 3.2Gbps with 850nm VCSEL 100µW/DIV Optical Eye Diagram at 2.5Gbps with 850nm VCSEL Effect of Peaking Control on the Electrical Eye Diagram 3mA/DIV 125µW/DIV 5100 G01 50ps/DIV EMCORE MODE LC-TOSA VCSEL 215 PRBS, 7dB EXTINCTION RATIO, 300µW AVG PWR, 2.4GHz 4TH ORDER BESSEL-THOMPSON LOWPASS FILTER 5100 G02 60ps/DIV EMCORE MODE LC-TOSA VCSEL 215 PRBS, 10dB EXTINCTION RATIO, 300µW AVG PWR, 1.87GHz 4TH ORDER BESSEL-THOMPSON LOWPASS FILTER 50ps/DIV 5100 G03 3.2Gbps, 223 PRBS, Im_rng = 2, IMPP = 12mA, PEAKING = 4, 8, 16, 30 Electrical Eye Diagram at 25°C 3.2Gbps, 223 PRBS, IMPP = 3mA Electrical Eye Diagram at 25°C 3.2Gbps, 223 PRBS, IMPP = 12mA Electrical Eye Diagram at 25°C 3.2Gbps, 223 PRBS, IMPP = 24mA 0.5mA/DIV 2mA/DIV 49ps RISING 54ps FALLING Im_rng = 0 PEAKING = 16 50ps/DIV 5100 G04 4mA/DIV 51ps RISING 59ps FALLING Im_rng = 2 PEAKING = 16 50ps/DIV Electrical Eye Diagram at –40°C, 3.2Gbps, 223 PRBS, IMPP = 12mA 47ps RISING 56ps FALLING Im_rng = 2 PEAKING = 16 50ps/DIV 5100 G07 50ps/DIV 5100 G06 Electrical Eye Diagram at 85°C, 3.2Gbps, 223 PRBS, IMPP = 12mA 2mA/DIV 2mA/DIV 50ps RISING 62ps FALLING Im_rng = 3 PEAKING = 16 5100 G05 2mA/DIV 57ps RISING 67ps FALLING Im_rng = 2 PEAKING = 16 50ps/DIV 5100 G09 sn5100 5100fs 8 LTC5100 U W TYPICAL PERFOR A CE CHARACTERISTICS VDD = VDD(HS) = 3.3V, TA = 25°C, Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Test circuit shown in Figure 5. Rise and Fall Times vs IM at the Midpoint of Each Im_rng Rise and Fall Times vs IMPP for Im_rng = 3 65 3 58 tFALL 2 56 54 Im_rng 0 52 Im_rng 0 3 1 tRISE 2 1 7 60 55 DETERMINISTIC JITTER (ps) 60 8 tFALL 20% TO 80% RISE AND FALL TIMES (ps) tRISE 50 45 3 6 9 IMPP (mA) 15 12 3 0 6 9 15 18 IMPP (mA) 12 21 3 2 Im_rng 2 Im_rng 1 40 Im_rng 0 30 10 TRANSMIT DISABLED AND Power_down_en = 1 0 4 8 12 16 Impp (mA) 47.9 47.8 47.7 Im_rng = 3 Impp = 12mA Ib = 0.0mA (INCLUDES SRC PIN CURRENT REQUIRED TO SUPPLY THE AVERAGE MODULATION CURRENT 47.6 47.4 –40 24 –20 40 20 0 TEMPERATURE (°C) 60 5100 G16 1.01 27 80 5100 G20 10 Impp (mA) 1 100 5100 G18 Laser Modulation Current vs Temperature, Im_rng = 1 1.01 NORMALIZED TO UNITY AT 25°C Imd_tc1 = Imd_tc2 = 0 0.99 0.97 –40 NORMALIZED TO UNITY AT 25°C Im_tc1 = Im_tc2 = 0 1.00 0.99 0.98 0.98 0.98 60 24 100 IMPP (NORMALIZED) IMD (NORMALIZED) 0.99 0 20 40 TEMPERATURE (°C) 21 1000 80 1.00 1.00 –20 18 5100 G15 Monitor Diode Current vs Temperature in APC Mode NORMALIZED TO UNITY AT 25°C Ib_tc1 = Ib_tc2 = 0 0.97 –40 15 5100 G17 Laser Bias Current vs Temperature in CCC Mode 1.01 12 10000 47.5 20 9 48.0 TRANSMIT ENABLED TRANSMIT DISABLED AND Power_down_en = 0 6 Modulator Output Resistance vs Modulation Level 48.1 Im_rng 3 20 3 0 Supply Current vs Temperature IDD + IDD(HS) (mA) 50 0 27 5100 G14 Supply Current vs Modulation Level (Excluding Laser Current) 60 24 IMPP (mA) 5100 G13 IDD + IDD(HS) (mA) 4 40 0 IB (NORMALIZED) 5 1 50 0 6 OUTPUT RESISTANCE (Ω) 20% TO 80% RISE AND FALL TIMES (ps) 62 Deterministic Jitter vs IMPP for Im_rng = 3 –20 0 20 40 TEMPERATURE (°C) 60 80 5100 G21 0.97 –40 –20 0 20 40 TEMPERATURE (°C) 60 80 5100 G22 sn5100 5100fs 9 LTC5100 U W TYPICAL PERFOR A CE CHARACTERISTICS VDD = VDD(HS) = 3.3V, TA = 25°C, Cml_en = 0, Lpc_en = 1, transmitter enabled, unless otherwise noted. Hot Plug with EN Active in CCC Mode Hot Plug with EN Active in APC Mode Start-Up with Slow Ramping Supply in APC Mode VDD VDD VDD SCL SCL SCL FAULT FAULT LASER OUTPUT LASER OUTPUT 10ms/DIV EEPROM READ LASER OUTPUT 10ms/DIV 5100 G23 10ms/DIV 5100 G24 5100 G25 Transmitter Enable, Rapid Restart Transmitter Disable Transmitter Enable FAULT VDD VDD VDD FAULT FAULT FAULT EN EN EN LASER OUTPUT LASER OUTPUT LASER OUTPUT 5µs/DIV 10µs/DIV 5µs/DIV 5100 G26 5100 G28 5100 G27 Response to Fault Fault Recovery Time FAULT FAULT VMD FAULT 5µs WIDE PULSE ON EN EN LASER OUTPUT 10µs/DIV LASER OUTPUT 10ms/DIV 5100 G29 5100 G30 sn5100 5100fs 10 LTC5100 U U U PI FU CTIO S VSS (Pins 1, 4, 9, 12, 17): Ground for Digital, Analog and High Speed Circuitry. These pins are internally connected. Connect Pins 1, 4, 9 and 12 to the ground plane with minimal trace lengths. Place a minimum of four vias (preferably nine vias) to the ground plane in the Exposed Pad area. Most of the high speed modulation current is returned through the Exposed Pad (Pin 17). IN+, IN– (Pins 2, 3): High Speed Laser Modulation Inputs. The inputs are differential with internal termination resistors. The input amplifier is internally AC coupled. With current mode logic (CML) enabled, the inputs are independently terminated to VDD(HS) with 50Ω resistors. With CML disabled, the inputs provide 100Ω differential termination and permit rail-to-rail common mode range. The input pins can be AC coupled with external capacitors. When externally AC coupled, the input pins self-bias to VDD(HS)/2. The Cml_en bit selects the termination mode. FAULT (Pin 5): Signals One of Five Safety Fault Conditions: laser overcurrent, overpower, underpower, power supply undervoltage and memory load error. The pin can be programmed active high or active low with the Flt_pin_polarity bit. The FAULT pin can be programmed to four different drive modes with the Flt_drv_mode bits. SDA, SCL (Pins 6, 7): Serial Interface Data and Clock Signals. The pins are open drain with a 100µA internal pullup current. An external pull-up resistor can be added to drive larger capacitive loads. MODA, MODB (Pins 11, 10): High Speed Laser Modulation Outputs. MODA and MODB are connected on-chip and driven by an open-drain output transistor. One of these pins should be connected to the laser. The other should be connected to a termination resistor. See the Applications Information section for details. MD (Pin 13): Monitor Diode Input for Automatic Power Control of the Laser Bias Current. The MD pin allows connection to the cathode or anode of the monitor diode. The Md_polarity bit selects the polarity of the monitor diode. SRC (Pin 14): Current Source for Biasing the Laser. See the Applications Information section for details. EN (Pin 15): Transmitter Enable and Disable Input. This input is TTL compatible and can be programmed for active high or active low operation with the En_polarity bit. An internal 10µA current source disables the transmitter if the EN pin becomes disconnected. This safety feature operates whether the EN pin is active high or active low. VDD (Pin 16): Power Input for Digital and Low Speed Analog Circuitry. Connect this pin to VDD(HS) (Pin 8) with a short trace. No bypassing is needed at the VDD pin if the trace length to the VDD(HS) bypass capacitor is less than 10mm long. VDD(HS) (Pin 8): Power Input for the High Speed Laser Modulation Circuitry. Filter this pin with a ferrite bead and bypass the pin directly to the ground plane with a 10nF ceramic capacitor. sn5100 5100fs 11 LTC5100 W BLOCK DIAGRA UNDERVOLTAGE DETECTION VDD 16 Over_current Over_pwr Under_pwr Under_voltage En_polarity EN 15 FAULT PIN DRIVER TRANSMIT AND Fault FAULT CONTROLLER Transmit_en 5 FAULT Mem_load_errorr Flt_drv_mode Flt_pin_polarity 100µA 100µA LOGARITHMIC AMPLIFIER DATA BUS + CURRENT ATTENUATOR IMD(MON) SDA 6 SERIAL DIGITAL INTERFACE SCL 7 Over_pwr Under_pwr LASER POWER CONTROLLER Md_polarity Imd_rng IS(MON) IM(MON) Ib LIMIT DAC 7 BITS + – – Over_current Im_rng Is_rng IS(MON) IM(MON) VLD TEMP SENSOR – POWER LIMIT COMPARATORS REGISTER SET 13 MD VDD 10-BIT ADC IMD(MON) Is_rng VTERM(MON) IS(MON) sel IS DAC 10 BITS Transmit_en IS 14 SRC IM DAC 10 BITS VDD(HS) 8 Is_rng VDD(HS) + VTERM(MON) CML_en – 20pF PEAKING DAC 5 BITS 12 VSS VSS 1 50Ω 50Ω IN+ 2 + IN– 3 – IM Transmit_en VLD 11 MODA 10 MODB IM(MON) VSS 4 9 VSS Im_rng 17 5100 F02 VSS (EXPOSED PAD) Figure 2. Block Diagram sn5100 5100fs 12 LTC5100 W FU CTIO AL DIAGRA S U U VDD 16 Imd rng Md polarity 5 FAULT EN 15 SDA 6 IMD Imd adc SCL 7 ADC CURRENT ATTENUATOR LOG AMP 13 MD VDD + USER_ADC. Data Apc gain Imd set Imd nom TEMP SENSOR Imd tc1 – – + Imd error ∑ Is dac DAC MINIMUM GAIN CTRL TEMP COMP Imd tc2 Is rng ADC IS IS T int adc 14 SRC + ADC T ext – USER_ADC. Data T nom Ext temp en Im tc1 Im tc2 USER_ADC. Data TEMP COMP + – – VTERM VLD ADC Im gain Im set Im nom + ADC + Im error ∑ Im dac DAC – Peaking VDD(HS) 8 DAC 12 VSS VSS 1 IN+ IM 100Ω IN– 3 11 MODA + 2 10 MODB + – Im adc VSS 4 ADC – 17 9 VSS Im rng 5100 F03 VSS (EXPOSED PAD) Figure 3. Functional Diagram—Automatic Power Control Mode sn5100 5100fs 13 LTC5100 W FU CTIO AL DIAGRA S U U VDD 16 5 FAULT EN 15 13 MD SDA 6 VDD SCL 7 Is rng (BIAS CURRENT) + + Is rng Is adc ADC – – Im rng Ib_set Ib nom TEMP SENSOR Ib tc1 + – Ib_error Is dac DAC Ib gain TEMP COMP Ib tc2 ∑ IS IS T int adc 14 SRC + ADC T ext – USER_ADC. Data USER_ADC. Data Im tc1 + – – VTERM T nom Ext temp en + ADC VLD ADC TEMP COMP Im tc2 Im gain + Im nom Im_error ∑ Im dac DAC – Peaking VDD(HS) 8 DAC 12 VSS VSS 1 + IN IM + 2 100Ω IN– 3 11 MODA 10 MODB + – Im adc VSS 4 ADC – 17 9 VSS Im rng 5100 F04 VSS (EXPOSED PAD) Figure 4. Functional Diagram—Constant Current Control Mode sn5100 5100fs 14 LTC5100 TEST CIRCUIT 1.8V POWER SOURCE VDD 16 VDD 1 FROM BERT EN 14 SRC 10nF 13 MD VSS VSS MODA ZO = 50Ω 2 IN + ZO = 50Ω 3 IN – 4 15 10nF LTC5100 MODB VSS VSS FAULT SDA 5 6 12 50Ω 11 10 ZO = 50Ω 9 TO SCOPE MICROWAVE BLOCKING CAPACITOR SCL VDD(HS) 7 8 10nF 5100 F05 RESISTORS: 0402 SURFACE MOUNT CAPACITORS: 0402 SURFACE MOUNT, X7R DIELECTRIC Figure 5. Test Circuit U U U EQUIVALE T I PUT A D OUTPUT CIRCUITS LTC5100 LTC5100 VDD VDD 10µA VDD 0 EN ACTIVE LOW 15 VDD 100µA SDA, SCL 6, 7 EN En_polarity 1 EN ACTIVE HIGH 10µA VSS 5100 F06 5100 F07 Figure 6. Equivalent Circuit for the EN Pin Figure 7. Equivalent Circuit for the SDA and SCL Pins VDD LTC5100 3.3mA DRIVER 250µA VDD Md_polarity M2 Imd 0 400µA VDD COMPLEMENTARY DRIVE 250µA PULL-UP LTC5100 400µA PULL-UP VDD MD FAULT 6 3.3mA DRIVER 1 M1 5100 F08 Figure 8. Equivalent Circuit for the FAULT Pin. All Switches are Open in Open-Drain Mode Imd 13 5100 F09 Figure 9. Equivalent Circuit for the MD Pin sn5100 5100fs 15 LTC5100 U U U EQUIVALE T I PUT A D OUTPUT CIRCUITS LTC5100 VDD(HS) RON ≅ 3Ω VDD(HS) 2 LTC5100 50k 20pF VDD VDD(HS) M2 IN + SRC 50Ω 50Ω 3 Cml_en 25k 25k TO INPUT AMPLIFIER VDD(HS) IN – MODA 14 11 VDD(HS) M1 1M MODB 10 50k 5100 F10 Figure 10. Equivalent Circuit for the IN+ and IN– Pins 5100 F11 Figure 11. Equivalent Circuit for the SRC, MODA and MODB Pins U OPERATIO OVERVIEW (Refer to Figure 1 and the Block Diagram in Figure 2) provides both constant current and automatic power control of the laser bias current. In automatic power control mode, special circuitry maintains constant settling time in spite of variations in the laser slope efficiency and monitor diode response characteristics. The LTC5100 is optimized to drive common cathode VCSELs in high speed fiber optic transceivers. The chip incorporates several features that make it very compact and easy-to-use while delivering exceptional high speed performance. Only a capacitor, a resistor and a small EEPROM (excluding laser diode and power supply filtering) are needed to build a complete fiber optic transmitter. Digital control over the I2C serial interface allows fully automated laser setup to improve manufacturing efficiency. The LTC5100’s extensive set of eye safety features meet GBIC and SFF requirements but go beyond the standards with open-pin protection, redundant transmitter enable controls and other interlocks. The high speed inputs of the LTC5100 are internally terminated in 50Ω and internally AC coupled, eliminating all external components at the inputs. The modulation output is DC coupled to the laser and presents a high quality resistive drive impedance to deliver very fast and clean eye diagrams in spite of laser impedance variations. The modulation output is capable of driving significant lengths of transmission line, allowing the LTC5100 to be placed at an arbitrary distance from the laser. This feature allows for packaging flexibility within the module. 10-bit integrated DACs set laser bias and modulation levels, eliminating the cost and space of digital potentiometers. A multiplexed ADC allows monitoring of temperature and laser operating conditions in production or field operation. Laser bias and modulation currents are digitally temperature compensated to second order for tight control of average power and extinction ratio. The LTC5100 The LTC5100 minimizes electromagnetic interference (EMI) with several architectural features. The unique design of the driver output forces the high speed modulation current to circulate only in the laser and ground system. The high speed amplifier chain and the digital circuitry are internally filtered and decoupled to further reduce power supply noise generation. sn5100 5100fs 16 LTC5100 U OPERATIO LASER BIAS AND MODULATION Terminology and Basic Calculations Modulator Architecture Figure 12 through Figure 16 define terminology that is used throughout this data sheet. The current delivered by the SRC pin is called IS. The average modulation current delivered by the chip at the MODA, MODB pins is called IM. The laser bias current, IB, is defined as the average current in the laser. IB is the difference between the source current and average modulation current. The LTC5100 drives common cathode lasers using a method called “shunt switching”. As shown in Figure 12, shunt switching involves sourcing DC current into the laser diode and shunting part of that current with a high speed current switch to produce the required modulation. The SRC pin provides the DC current and the MODA, MODB pins (which are connected on chip) provide the high speed modulation current. This technique results in a very fast, single-ended driver that confines the high speed modulation current to the laser and ground system. The LTC5100 actually uses a modified shunt switching scheme in which the source current is delivered through a “termination” resistor, RT, that is bypassed to ground with a large capacitor. The resistor brings three advantages to the modulation stage. First, it gives the modulator a precise resistive output impedance to damp ringing and absorb reflections from the laser. Second, the resistor isolates the capacitance of the SRC pin from the high speed signal path, further improving modulation speed. Third, the resistor and capacitor heavily filter the high speed output signal so that it does not modulate the power supply and cause radiation or interference. On-chip decoupling of the high speed amplifiers further reduces power supply noise generation. LTC5100 VDD SRC 3.2Gbps MODULATOR MODA, MODB 11, 10 IM CT 10nF IS 14 RT 50Ω TYP The peak-to-peak modulation current delivered by the chip is called IMPP. IMPP is twice the value of IM because the high speed data is assumed to have a 50% duty cycle. The peak-to-peak modulation current is divided between the termination resistor and the laser. The peak-to-peak modulation amplitude in the laser is called IMOD. The relationship between IMPP and IMOD depends on the relative values of the termination resistor and the laser dynamic resistance. IS IB IMOD IMPP IM 0 5100 F13 Figure 13. Components of the LTC5100 Source and Modulation Currents (The Laser Bias Current is Also Shown) The relationships between the source, bias, and modulation currents are as follows. IB = IS – IM (1) IMPP = 2 • IM (2) IMOD = IS = IB + IM IB IM RT •I (RT + RLD ) MPP (3) where RT is the termination resistor value. RLD is the dynamic resistance of the laser, defined in Figure 15. IMPP = 2 • IM VSS Figure 12. Simplified Laser Bias and Modulation Circuit The expression for IB in Equation 1 shows that the maximum achievable laser bias current is a function of the maximum source current, IS, and the average modulation sn5100 5100fs 17 LTC5100 U OPERATIO current, IM. The maximum value of IS is given in the Electrical Characteristics and the value of IM depends on the laser characteristics and the termination resistor value. V RLD VLD The logic “1” and “0” current levels in the laser are given by: I1 = IB + IMOD 2 (4) IMOD 2 (5) I0 = IB – ILD I0 IB I1 5100 F15 Figure 15. Approximate VI Curve for a Laser Diode IMOD IB ITH 0 L 5100 F14 P1 Figure 14. Components of the Laser Bias and Modulation Currents η The power levels corresponding to I1 and I0 are P1 and P0, as shown in Figure 16. P1 = η(I1 – ITH) (6) P0 = η(I0 – ITH) (7) where η is the slope efficiency and Ith is the laser threshold current, defined in Figure 16. The average optical power and extinction ratio are given by: PAVG = ER = P1 + P0 2 P1 P0 (8) (9) The average voltage on the laser diode relative to ground is VLD (see Figure 12 and Figure 15). The voltage on the SRC pin is: VS = VLD + IS • RT (10) = VLD + (IB + IM) • RT The value VS is important because VS must not exceed the limits given in the Electrical Characteristics. PAVG P0 ILD ITH I0 IB I1 5100 F15 IMOD Figure 16. Approximate LI Curve for a Laser Diode The voltage across the termination resistor is: VTERM = VSRC – VMODA (11) = Is • RT The LTC5100 can digitize the voltage across the termination resistor using the on-chip ADC, which can give a more accurate measurement of Is than that given by digitizing the current internally. See the Electrical Characteristics for details. Temperature Compensation The LTC5100 digitally compensates the temperature drift of the laser bias current, laser modulation current and sn5100 5100fs 18 LTC5100 U OPERATIO monitor photodiode current. In each case the fundamental calculation is the same. The LTC5100’s digital controller multiplies the nominal value of the quantity (IB, IM or IMD) by a quadratic function of temperature. Temperature measurements are supplied either by an on-chip temperature sensor or by an external microprocessor, according to the setting of Ext_temp_en. The general temperature compensation formula is: I = I_nom • (TC2 • 2 –18 • ∆T2 + TC1 • 2–13 • ∆T + 1) (12) where I is the digital representation of the laser bias current, modulation current or monitor diode current (IB, IM or IMD). When using the internal temperature sensor (Ext_temp_en = 0), the temperature measurements are taken by the onchip ADC, and ∆T is the change in the LTC5100 die temperature relative to a user defined nominal temperature: ∆T = T_int_adc – T_nom (13) When using an external temperature source (Ext_temp_en = 1), the temperature measurements are provided in digital form by a microprocessor or host computer and ∆T is the change in temperature relative to a user defined nominal temperature: ∆T = T_ext –T_nom (14) T_int_adc, T_ext, and T_nom are 10-bit, unsigned numbers scaled at 0.5K/LSB. The maximum temperature that can be represented is therefore 210 • 0.5°K = 512°K or 239°C. TC1 and TC2 are the first and second order temperature coefficients. They correspond to the registers Im_tc1 and Im_tc2 for modulation current, Ib_tc1 and Ib_tc2 for bias current and Imd_tc1 and Imd_tc2 for monitor diode current. In each case TC1 and TC2 are 8-bit signed numbers in two’s complement format. The range of the temperature coefficients is therefore –128 to +127. When TC1 is multiplied by its weighting coefficient of 2–13 in Equation 12, the effective value of the first order temperature coefficient is 122ppm/°C per LSB. The full-scale range is approximately ±15500 ppm/°C. When TC2 is multiplied by its weighting coefficient of 2–18 in Equation 12, the effective value of the second order temperature coefficient is 3.81ppm/°C2 per LSB. The full-scale range is approximately ±484 ppm/°C2. Note that Equation 12 is applied to the digital representation of the currents, not the physical current themselves. This is a particularly important point where monitor diode current is concerned, because the digital representation of the monitor diode current is the logarithm of the current. Thus the temperature compensation is of the logarithm of the monitor diode current and not the current itself. Notation Used for Registers and Bit Fields The LTC5100 has a large set of registers, many of which are subdivided into fields of bits. Register names are given in all capitals (SYS_CONFIG) and bit fields are given in mixed case (Apc_en). For example, the bit that enables Automatic Power Control mode is contained in the System Configuration register. This bit is denoted by: SYS_CONFIG.Apc_en In many cases this bit field will simply be referred to as “Apc_en.” The functional diagrams of Figure 3 and Figure 4 show registers and bit fields within registers between horizontal bars. For example, the “Data” field in the ADC register is shown as: USER_ADC.Data A write operation to this register is shown as: USER_ADC.Data A register read operation is shown as: Peaking Range Selection for the Source and Modulation Currents The source and modulation currents each have four ranges of operation to optimize ADC and DAC resolution as well as high frequency performance. The source current range is controlled by two bits called Is_rng. Similarly, the modulation current range is controlled by two bits called Im_rng. The maximum current that can be delivered is proportional to the range, so the current output is 1, 2, 3 or 4 times the typical base value of 9mA for the source current and 4.5mA for the average modulation current or 9mA peakto-peak. sn5100 5100fs 19 LTC5100 U OPERATIO Figure 17 depicts the current ranges for the source current. The guaranteed full scale is 6mA per range. The minimum operating level should be limited to 1/16 of full scale to avoid the coarse relative quantization seen in any ADC or DAC when operated at low levels. The source range, Is_rng, should be selected as low as possible such that the source current, IS, stays within the guaranteed current limits over temperature, considering the laser temperature characteristics. From Equation 1 we can see that the source current is the sum of the laser bias and the average modulation currents: IS = IB+ IM (15) Is_rng should be chosen to support the total current required for laser bias and modulation, taking temperature changes in IB and IM into account. 36 IS (mA) High Speed Aspects of the Modulation Output Is_rng = 3 27 Is_rng = 2 18 Is_rng = 1 RECOMMENDED MINIMUM IS 1/16 OF FULL SCALE 9 4.5 Figure 18 depicts the current ranges for the average modulation current. This is the average modulation current at the MODA and MODB pins of the chip (recall that the MODA and MODB pins are connected on-chip). The peakto-peak modulation at the pins of the chip is twice the average. Guaranteed full scale is 3mA average or 6mA pp per range. The minimum operating level should be limited to 1/8 of full scale to preserve the quality of the eye diagram. Operating below 1/8 full scale causes increased overshoot and undershoot. The modulation range, Im_rng, should be selected as low as possible such that the modulation current, IM, stays within the guaranteed current limits over temperature. The modulation current varies over temperature to compensate the loss in slope efficiency typical of most VCSELs. Therefore, the choice of Im_rng should take temperature changes into account. Is_rng = 0 0 5100 F17 The LTC5100 modulation output presents a resistive drive impedance with very low reflection coefficient. This output design suppresses ringing and reflections to maintain the quality of the eye diagrams in spite of laser impedance variations. The reflection coefficient is sufficiently low that the LTC5100 can drive the laser over an arbitrary length of transmission line, as shown in Figure 19. A well designed transmission line stretching the entire length of a typical transceiver module goes virtually unnoticed in this system. The only practical limitation on interconnect length to the laser is high frequency line loss. Figure 17. Ranges for the Source Current LTC5100 18 VDD IM (mA) SRC Im_rng = 3 IS 14 13.5 C1 Im_rng = 2 9 3.2Gbps MODULATOR Im_rng = 1 RECOMMENDED MINIMUM IS 1/8 OF FULL SCALE 4.5 Im_rng = 0 0 LBWA MODA LBWB MODB IM 11 10 CT 10nF RT 50Ω TYP IS = IB + IM ZO = RT TRANSMISSION LINE IB M1 VSS 5100 F18 Figure 18. Ranges for the Modulation Current Figure 19. High Speed Details of the Modulation Output sn5100 5100fs 20 LTC5100 U OPERATIO Figure 19 shows how the LTC5100 achieves a low reflection coefficient. The unavoidable capacitance of the high speed driver transistor, bond pads and ESD protection circuitry (C1) is compensated by the inductance of the bond wires (LBWA and LBWB). The high speed behavior of the circuit in Figure 19 can be understood in greater detail by examining the simplified circuit in Figure 20. In Figure 20 the switched current source (M1 in Figure 19) launches a current step (1) toward the termination resistor (2A) and toward the transmission line (2B) connected to the laser. The laser is typically mismatched to the line impedance and reflects a portion of the incident wave (3) back toward the MODB pin. There it encounters an L-C-L structure composed of the bond wires and driver capacitance. This structure is carefully designed as a lumped element approximation to the transmission line impedance. It therefore transmits wave (3) through the IC package without reflecting energy back toward the laser. The traveling wave passes through the chip largely unimpeded (4) and is absorbed by the matched termination resistor, RT. The matched termination is provided by the termination resistor, RT, decoupled by capacitor CT. CT forms an AC short across the entire frequency range contained in the modulation data. The termination resistor, RT, need not be 50Ω. 50Ω is best for electrical testing because it matches the impedance of most high frequency instruments. RT can be made smaller, 35Ω, for example, to more closely match a laser with low dynamic impedance or to allow more voltage headroom at the SRC pin. This may be necessary for lasers that run at high voltages or high bias currents. RT can be made larger, 70Ω for example, to more closely match a laser with high dynamic impedance or if a narrow, high impedance PC board trace is needed to connect to the laser. Figure 21 shows that the high speed modulation current is confined to the ground system, laser and back termination network. No high speed current circulates in the power supply where it could cause radiation and interference problems. HIGH SPEED DATA INPUTS The high speed data inputs, IN+ and IN–, are internally terminated in 50Ω and internally AC coupled, eliminating the need for external termination resistors and AC coupling capacitors. Figure 10 shows the equivalent circuit for the high speed data pins. By default, the high speed data inputs are terminated differentially with 100Ω for compatibility with LVDS, PECL and similar differential signaling standards (Cml_en = 0). Alternately, the inputs can be programmed for 50Ω single-ended termination to the power supply for biasing a current mode logic (CML) driver. To select CML compatibility, program Cml_en to 1. Although internally AC coupled, the inputs are biased with high valued resistors (50k equivalent) to VDD(HS)/2, so the LTC5100 remains compatible with external AC coupling capacitors. When externally AC coupled, the inputs selfbias to approximately VDD(HS)/2. Internal AC coupling gives the LTC5100 rail-to-rail input common mode capability. The inputs can be driven as much as 300mV beyond the rail during peak excursions. The AC coupling circuit is a distributed highpass filter with VDD LTC5100 SRC 4 2A 3 LBWB LBWA 11 MODA RT 50Ω TYP MODA 2B 10 C1 ZO = RT 3.2Gbps MODULATOR MODB TRANSMISSION LINE MODB NO HIGH SPEED CURRENT 14 50Ω 10nF 11 12 10 9 M1 1 5100 F20 Figure 20. Wave Propagation in the Laser Interconnect VSS EXPOSED PAD 5100 F21 Figure 21. High Speed Current Flow in the Modulation Output sn5100 5100fs 21 LTC5100 U OPERATIO approximately second order characteristics. The design maximizes the flatness of the step response over extended periods, giving optimal performance during long strings of ones or zeros in the data. MODULATION CURRENT CONTROL IN APC AND CCC MODES The LTC5100 controls the modulation current with a digital servo control loop using feedback from the on-chip ADC. Figure 3 and Figure 4 are Functional Diagrams of the LTC5100 operating in Automatic Power Control (APC) mode and Constant Current Control (CCC) modes, respectively. These diagrams show the organization and operation of the servo control loops for laser bias and laser modulation. Either diagram can be used to understand the modulation current control loop. Servo Control The average modulation current is controlled by a digital servo loop (shown in the lower half of Figure 3). The nominal modulation current, Im_nom, is multiplied by a temperature compensation factor, producing a 10-bit digital set point value, Im_set. Im_set is the target value for average modulation current. The ADC digitizes the average modulation current, producing a 10-bit value Im_adc. The difference between the target value and the actual value produces the servo loop error signal, Im_error. Im_error is multiplied by a constant, Im_gain, to set the loop gain. The result is integrated in a digital accumulator and applied to a 10-bit DAC, increasing or decreasing the modulation amplitude as required to drive the loop error to zero. The servo loop adjusts the modulation amplitude every four milliseconds, producing 250 servo iterations per second. The modulation servo loop operates on the average modulation current, which is one-half of the peak-to-peak value for a 50% duty cycle signal. The analog electronics in the high speed modulator ensure that controlling the average modulation current is equivalent to controlling the peakto-peak current. The ADC input for average modulation current is scaled such that code 512 is the nominal full-scale value, corresponding to 4.5mA per range. Thus, if Im_rng = 0 and Im = 4.5mA, the ADC digitizes code 512. The control system for the modulation current effectively has 9-bit resolution, because at most one-half of the 10-bit ADC range is utilized. This provision maximizes the compliance voltage range of the modulation output. The difference equation for the modulation servo loop is: Im_ gain •Im_ error (16) 8 Im_ gain = Im_ adcn− 1 + • Im_ set – Im_ adcn− 1 8 Im_ adcn = Im_ adcn–1 + ( ) Im_gain is a 3-bit digital value, so the scaling factor, Im_gain/8, takes on the discrete values 0, 1/8, 2/8, …, 7/8. If Im_gain = 4, then Im_gain/8 = 0.5 and the error in the control loop is cut in half with each servo iteration. In this case the step response of the loop is given by: n Im_ gain Im_ adcn = Im_ set • 1 – 1 – 8 (17) The step response has the familiar exponential settling characteristic of a first order system. The step response is shown in Figure 22 for Im_gain = 4. The remaining error is reduced by one-half with each servo iteration. In seven iterations, or about 28ms, the modulation current settles to under 1% in this example. The measured step response, including the modulation envelope, is shown in the Typical Performance Characteristics. Im_set Im_adc 1 0 4 2 3 4 5 6 7 8 SERVO ITERATIONS 8 12 16 20 24 28 32 TIME (ms) 5100 F22 Figure 22. Step Response of the Average Modulation Current for Im_gain = 4 sn5100 5100fs 22 LTC5100 U OPERATIO Reducing Im_gain slows the settling time and increasing Im_gain speeds the settling time. For example, with Im_gain = 1, the residual loop error is cut by 1/8 with each servo iteration. In this case it would take 35 servo iterations (about 140ms) to settle to 1%. With Im_gain = 7, the residual servo loop error is cut by 7/8 with each servo iteration. In this case it would take only three servo iterations (about 12ms) to settle to 1%, but the servo loop will tend to “hunt” or oscillate at a low level with such a high loop gain. Temperature Compensation The set point value for the modulation current, Im_set in Figure 3 and Figure 4, changes with temperature to compensate the temperature dependence of the laser diode’s slope efficiency. Temperature measurements are supplied either by an on-chip temperature sensor or by an external microprocessor, according to the setting of Ext_temp_en. The temperature compensated expression for Im_set is given by: Im_ tc2 • 2–18 • ∆T2 Im_ set = Im_ nom • (18) + Im_ tc1 • 2–13 • ∆T + 1 Im_tc1 and Im_tc2 are the first and second order temperature coefficients for the modulation current. LASER BIAS CURRENT CONTROL IN APC MODE Figure 3 is a functional diagram of the LTC5100 operating in automatic power control (APC) mode. In APC mode, the LTC5100 servo controls the average optical power with Imd_rng LTC5100 feedback from a monitor photodiode. Setting Apc_en = 1 selects this mode. In APC mode the monitor diode current can be temperature compensated with first and second order temperature coefficients. Figure 9 shows an equivalent circuit for the MD pin and Figure 23 shows details of the monitor diode circuit. The Md_polarity bit selects whether the monitor diode sources or sinks current from the MD pin. A programmable attenuator and logarithmic amplifier permit a very wide range of monitor diode currents spanning 4.25µA to 2176µA (typical) with constant 0.2% set point resolution. The attenuator divides the monitor diode current by 1, 4, 16 or 64 depending on the value of Imd_rng. Two bits called Imd_rng control the attenuator setting, selecting a full scale current range of 34, 136, 544 or 2176µA typical. A 5kHz lowpass filter provides antialiasing and limits noise. The logarithmic amplifier compresses the dynamic range of the monitor diode current and plays a role in maintaining constant and predictable settling times regardless of the photodiode characteristics. Range Selection Figure 24 depicts the current ranges for the monitor diode current. The full-scale range of the monitor diode current is 34µA • 4Imd_rng typical where Imd_rng = 0, 1, 2 or 3. The minimum operating level should be limited to 20% of full scale to ensure adequate settling time of the optical power output of the laser. The range should be selected so that the monitor diode current stays within the guaranteed current limits over temperature. VDD Md_polarity Md_polarity = 1 5kHz LOWPASS FILTER ATTENUATOR ÷1, ÷4, ÷16, ÷64 MD POLARITY CONTROL 13 Md_polarity = 0 LOG AMP 10-BIT ADC Imd_adc 5100 F32 Figure 23. Detail of the Monitor Photodiode Circuit sn5100 5100fs 23 LTC5100 U OPERATIO 2176 IMD (µA) (LOG SCALE) Imd_rng = 3 544 RECOMMENDED MINIMUM IS 20% OF FULL SCALE Imd_rng = 2 136 Imd_rng = 1 34 Imd_rng = 0 7 0 5100 F24 Figure 24. Operating Ranges for the Monitor Diode Current The SRC pin current range, Is_rng, should be chosen so that the SRC pin can supply the required bias current over temperature. See the section titled Range Selection for the Source and Modulation Currents. response, γ (Amps/Watt). These parameters vary widely from laser to laser. If nothing is done to compensate the variations in η and γ, the settling time of the optical power output will vary over an unacceptably wide range. For example, a 4:1 variation in slope efficiency and a 5:1 variation in monitor diode response could create a 20:1 variation in settling time. The LTC5100 uses two techniques to fully compensate for variations in the laser and monitor diode characteristics, achieving constant settling times under all conditions. First, taking the logarithm of the monitor diode current precisely compensates variations in the monitor diode response. Second, multiplying the error signal by the modulation current precisely compensates for variations in laser slope efficiency. The difference equation for the APC loop is: Im_ adcn = Im d _ adcn−1 + A • Im d _ error = Im d _ adcn−1 + A • (Im d _ set – Im d _ adcn−1) Servo Control The average optical power is controlled by a digital servo loop shown in the upper half of Figure 3. The loop sets and controls the logarithm of the monitor diode current. The logarithm of the nominal monitor diode current, Imd_nom, is multiplied by a temperature compensation factor, producing a 10-bit digital set point value, Imd_set. Imd_set is therefore the temperature compensated logarithm of the target value for monitor diode current. The ADC digitizes the logarithm of the monitor diode current, producing a 10-bit value called Imd_adc. The difference between the target value and the actual value produces the servo loop error signal, Imd_error. Imd_error is multiplied by a constant, Apc_gain, to set the loop gain. Imd_error is also multiplied by the set point value of the modulation current to further stabilize the servo dynamics, as explained below. The result is integrated in a digital accumulator and applied to a 10-bit DAC, increasing or decreasing the SRC pin current (and consequently the laser bias current) as required to drive the loop error to zero. The servo loop adjusts the laser bias current every four milliseconds, producing 250 servo iterations per second. The open-loop gain of the APC loop is proportional to the laser slope efficiency, η (Watts/Amp), and monitor diode (19) where A is the small-signal loop gain, given by: A= Apc_gain 1 + Is _ rng 1 • • 32 1 + Im_ rng ln(8) ER – 1 RT + RLD • • ER + 1 RT (20) where: ln(8) = 2.079 is the natural logarithm of 8 ER is the extinction ratio RT is the termination resistance RLD is the dynamic resistance of the laser diode Apc_gain is a 5-bit digital value, so the scaling factor, Apc_gain/32, takes on the discrete values 0, 1/32, 2/32, …, 31/32. In practice, the extinction ratio is usually high (ER >> 1), and RT ~ RLD, so Equation 20 simplifies to: A≈ Apc _ gain 1 + Is _ rng • 32 1 + Im_ rng (21) sn5100 5100fs 24 LTC5100 U OPERATIO Equation 20 shows that the loop gain is completely independent of the slope efficiency and monitor diode response. Consequently the servo dynamics and settling time are independent of these highly varying quantities. The Apc_gain quantity can be set to compensate for the selected values of Is_rng and Im_rng as well as the extinction ratio, termination resistance and laser dynamic resistance. microprocessor, according to the setting of Ext_temp_en. The temperature compensated expression for Imd_set is given by: The step response of the APC loop is: Imd_tc1 and Imd_tc2 are the first and second order temperature coefficients for the monitor diode current. Equation 23 applies to the digital representation of the monitor diode current. Recall that Imd_set is the digital set point for the logarithm of the monitor diode current. This fact has two important implications. First, the first order temperature coefficient in Equation 23 (Imd_tc1) results in an exponential change in the physical monitor diode current with temperature. However, the monitor diode temperature drift is usually very small, and the exponential is well approximated as linear. Second, if Imd_tc2 = 0, the relative temperature sensitivity of the physical current is given by: Imd_adcn = Imd_set • [1 – (1 – A)n] (22) The step response given in Equation 22 has the familiar exponential settling characteristic of a first order system. The step response is shown in Figure 25 for A = 0.5. The remaining error is reduced by one-half with each servo iteration. In seven iterations, or about 28ms, the modulation current settles to under 1% in this example. The measured step response, including the modulation envelope, is shown in the Typical Performance Characteristics. Choosing A = 0.5 is nearly optimal because it results in smooth, exponential settling. A = 1 will settle in about two servo iterations or 8ms, but “hunting” or low level oscillation will be seen in the laser bias current. A > 1 results in overshoot and A > 2 results in sustained high level oscillation. Imd _ set = Imd _ tc2 • 2–18 • ∆T 2 Imd _ nom • + Imd _ tc1• 2–13 • ∆T + 1 (23) dIMD 1 Imd _ nom • = ln(8)• 2–13 •Imd _ tc1• dT IMD 1024 (24) where IMD is the physical monitor diode current in Amps. Equation 24 shows that the temperature coefficient of the physical current depends on the nominal monitor diode current. For example, if Imd_nom = 512 and Imd_tc1 = 4, the physical temperature compensation would be: Imd_set Im_adc 1 0 4 2 3 4 5 6 7 8 SERVO ITERATIONS 8 12 16 20 24 28 32 TIME (ms) 5100 F25 Figure 25. Step Response of the Monitor Diode Current for a Total Loop Gain of 0.5 dIMD 1 512 • = ln(8)• 2–13 • 4 • = 508ppm/°C (25) dT IMD 1024 The effect of Imd_tc2 on the physical monitor diode current has no simple physical interpretation. In most cases it will be sufficient to set Imd_tc2 to zero and use the first order temperature coefficient, Imd_tc1 to correct monitor diode drift. Temperature Compensation The set point value for the monitor diode current, Imd_set in Figure 3, can be changed with temperature to compensate the temperature dependence of the monitor diode response. Temperature measurements are supplied either by an on-chip temperature sensor or by an external LASER BIAS CURRENT CONTROL IN CCC MODE Figure 4 is a functional diagram of the LTC5100 operating in constant current control (CCC) mode. In CCC mode, the LTC5100 sets the laser bias current directly. Setting Apc_en = 0 selects this mode. In CCC mode the laser bias sn5100 5100fs 25 LTC5100 U OPERATIO current can be temperature compensated with first and second order temperature coefficients. with each servo iteration. In this case the step response of the loop is given by, assuming Im_nom = 0 : Servo Control Ib _ adcn = The laser bias current is controlled by a digital servo loop (shown in the upper half of Figure 4) and can be understood as follows. The nominal bias current, Ib_nom, is multiplied by a temperature compensation factor, producing a 10-bit digital set point value, Ib_set. Ib_set is the target value for the laser bias current. The ADC digitizes the SRC pin current and the average modulation current, producing 10-bit values Is_adc and Im_adc. The laser bias current is the difference between the SRC pin current and the average modulation current (Equation 1). The system generates a digital representation of the laser bias current by calculating: Ib _ gain • (Is _ rng + 1) n Ib _ set • 1 – 1 – 32 Ib_adc = Is_rng • Is_adc – Im_rng • Im_adc The difference between the target value and the actual value is the servo loop error signal, Ib_error. Ib_error is multiplied by a constant, Ib_gain, to set the loop gain. The result is integrated in a digital accumulator and applied to a 10-bit DAC, increasing or decreasing the SRC pin current as required to drive the loop error to zero. The servo loop adjusts the SRC pin current every four milliseconds, producing 250 servo iterations per second. The simplified difference equation for the bias current servo loop is, assuming Im_nom = 0: Ib _ adcn = The step response has the familiar exponential settling characteristic of a first order system. The step response is shown in Figure 26 for Ib_gain • (Is_rng + 1) = 16. The remaining error is reduced by one-half with each servo iteration. In seven iterations, or about 28ms, the laser bias current settles to under 1% in this example. The measured step response is shown in the Typical Performance Characteristics. (26) where Ib_adc is the result of a calculation. (The ADC never digitizes the laser bias current directly.) (27) Ib _ gain • (Is _ rng + 1)• Ib _ error 32 Ib _ gain • (Is _ rng + 1) = Ib _ adcn−1 + 32 • (Ib _ set – Ib _ adcn−1) Ib _ adcn−1 + Ib_gain is a 5-bit digital value, so the scaling factor, Ib_gain/32, takes on the discrete values 0, 1/32, 2/32, …, 31/32. If Ib_gain • (Is_rng + 1) = 16, then Ib_gain • (Is_rng + 1)/32 = 0.5 and the error in the control loop is cut in half (28) Ib_set Ib_adc 1 0 4 2 3 4 5 6 7 8 SERVO ITERATIONS 8 12 16 20 24 28 32 TIME (ms) 5100 F26 Figure 26. Step Response of the Laser Bias Current for (Ib_gain) • (Is_rngtl ) = 16 Reducing Ib_gain slows the settling time and increasing Ib_gain speeds the settling time. For example, with Ib_gain • (Is_rng + 1) = 1, the residual loop error is cut by 1/32 with each servo iteration. In this case it would take 145 servo iterations (about 580ms) to settle to 1%. With Ib_gain • (Is_rng + 1) = 31, the residual servo loop error is cut by 31/ 32 with each servo iteration. In this case it would take only two servo iterations (about 8ms) to settle to 1%. Setting Im_nom ≠ 0 slows the settling time of the laser bias current somewhat. This effect can easily be compensated by increasing Ib_gain. Temperature Compensation The set point value for the laser bias current, Ib_set in Figure 4, can change with temperature to compensate the temperature dependence of the laser diode’s threshold current. Temperature measurements are supplied either sn5100 5100fs 26 LTC5100 U OPERATIO by an on-chip temperature sensor or by an external microprocessor, according to the setting of Ext_temp_en. The temperature compensated expression for Ib_set is given by: TRANSMIT ENABLE, FAULT DETECTION AND EYE SAFETY (29) The LTC5100 is compatible with the Gigabit Interface Converter (GBIC) specification, but includes additional features and safety interlocks. Figure 27 shows the state diagram for enabling the transmitter and detecting faults. Ib_tc1 and Ib_tc2 are the first and second order temperature coefficients for the laser bias current. The EN pin and Soft_en control bit enable and disable the transmitter. The EN pin may be programmed for active high or active low operation with the En_polarity bit. Ib _ tc2 • 2–18 • ∆T 2 Ib _ set = Ib _ nom • + Ib _ tc1• 2–13 • ∆T + 1 POWER ON RESET OR Operating_mode = 0 TIMEOUT 1 READ EEPROM SUCCEEDED OR Operating_mode = 1 3 WAIT 64ms FAILED Mem_load_error = 1 Operating_mode = 1 2 READY (DISABLED) ENABLE Transmitter_enabled = 0 ENABLE 4 SETTLING FAULT DETECTION DISABLED (ENABLED) Transmitter_enabled = 1 ENABLE AND Rapid_restart_en = 0 100ms TIMEOUT 8 5 FAULT SETTLED (ENABLED) ENABLE AND Rapid_restart_en = 1 FAULT DETECTION ENABLED Transmit_ready = 1 6 READY (SETTLED Transmitter_enabled = 0 Transmit_ready = 0 AND DISABLED) FAULT PIN ASSERTED FAULTED Transmitter_enabled = 0 (DISABLED) Transmit_ready = 0 Faulted = 1 ENABLE AND Rep_flt_inhibit = 1 9 Faulted = 0 READY (DISABLED) Faulted_once = 1 ENABLE SETTLING (ENABLED) 7 SETTLING (ENABLED) ENABLE 10 ENABLE ENABLE 40ms TIMEOUT ENABLE AND Rep_flt_inhibit = 0 FAULT DETECTION DISABLED Transmit_ready = 1 FAULT DETECTION DISABLED Transmitter_enabled = 1 100ms TIMEOUT 25ms TIMEOUT 11 SETTLING (ENABLED) FAULT DETECTION ENABLED Transmit_ready = 1 Faulted_once = 0 FAULT ENABLE = EN AND Soft_en FAULT = Over_current OR Over_power OR Under_power 12 FAULTED POWER ON RESET TWICE (DISABLED) Faulted = 1 Faulted_once = 1 Faulted_twice = 1 5100 F27 Figure 27. State Diagram for Transmitter Enable and Fault Detection sn5100 5100fs 27 LTC5100 U OPERATIO The EN pin and the Soft_en bit must both be active to enable the transmitter, providing an extra degree of safety and allowing full software control of the transmitter enable function. As shown in Figure 6, the EN pin has a weak 10µA current source that pulls it to the inactive state in case of an accidental open on the pin. The EN and Soft_en bits are inhibited until the LTC5100 has successfully loaded its registers from an EEPROM or the Operating_mode bit has been set, signaling that a microprocessor has assumed control of the chip. The first time the transmitter is enabled after initial power up, the servo loops find the correct DAC settings for bias and modulation current through a feedback process. Initial settling is typically within 300ms. If the transmitter is disabled and subsequently re-enabled, the previously determined DAC settlings are restored. In this case settling occurs typically within 1ms. This feature is called “Rapid Restart” and can be overridden by setting the Rapid– restart_en bit to zero. Table 1. Fault Detection and Handling The LTC5100 has sophisticated eye safety and fault handling features. Five types of faults are detected: low supply voltage, excessive laser bias current, overpower, underpower and EEPROM memory load failure. Table 1 summarizes these five faults and how they are handled in the LTC5100. Faults are latched in compliance with GBIC requirements. Faults can be independently enabled (except for low supply voltage and memory load failure) and are recorded in an internal register for readout over the serial bus. If two faults occur simultaneously, the fault with the highest priority (see Table 1) is recorded in the FLT_STATUS register. This register indicates the cause of the fault and is cleared only when read (not when the fault itself is cleared.) Low supply voltage and memory load failure are considered hard faults and cannot be masked or overridden. They prevent the transmitter from begin enabled until they are cleared. Normally, a fault automatically disables the transmitter and shuts down the laser. In some systems it may be desirable to allow data transmission to continue after a FAULT TYPE LASER OVERCURRENT LASER OVERPOWER LASER UNDERPOWER EEPROM MEMORY LOAD FAULT POWER SUPPLY UNDERVOLTAGE SOFTWARE FORCED FAULT Fault Occurs When Laser Bias Current Exceeds the Value in the IB_LIMIT Register Monitor Diode Current is 50% Greater Than the Set Point Monitor Diode Current is 50% Less than the Set Point EEPROM Load Starts But Fails to Complete VDD Drops Below 2.8V The Flt_pin_override and Force_flt Bits are Set Priority 5 4 3 2 1 NA Cleared by Power-On Reset Yes Yes Yes Yes No Yes Latched in the FLT_STATUS Register Yes Yes Yes Yes Yes No (Not Part of the FLT_STATUS Register) Cleared from the FLT_STATUS Register on Read Yes Yes Yes Yes Yes No (Not Part of the FLT_STATUS Register) Latched at the FAULT Pin Yes Yes Yes Yes No (The FAULT Pin Signals a Fault as Long as the Supply Voltage Remains Too Low) Yes (Actually Latched in the FLT_CONFIG Register) Enabled by Over_current_en Over_pwr_en and Apc_en Under_pwr_en and Apc_en Always Enabled Always Enabled Flt_pin_override Glitch Rejection 4µs 4µs 4µs NA 200mV Typical Hysteresis NA sn5100 5100fs 28 LTC5100 U OPERATIO fault has occurred. For example, the software in the host system may need to evaluate the cause of the fault before shutting down the laser. If Auto_shutdn_en = 1, the LTC5100 automatically disables the transmitter after a fault. If Auto_shutdn_en = 0, data transmission continues after a fault. The transmitter is not disabled until the host system drives the EN pin inactive or clears the Soft_en bit. Low power supply voltage and memory load errors are considered hard faults and always disable the transmitter, regardless of the setting of Auto_shutdn_en. appropriate laser safety features of the LTC5100, and take any additional precautions needed to ensure compliance of the end product with the requirements of the relevant regulatory agencies. In particular, the LTC5100 produces laser currents in response to digitally programmed commands. The user must ensure software written to control the LTC5100 does not cause excessive levels of radiation to be emitted by the laser. POWER CONSUMPTION AND POWER MANAGEMENT The LTC5100 implements the GBIC protocol for preventing software from repeatedly re-enabling a faulted transmitter. When a first fault is detected, it can be cleared by disabling the transmitter. If the transmitter is re-enabled and a second fault occurs within 25ms after fault detection is enabled, the transmitter is permanently disabled. Only cycling power to the LTC5100 can clear this condition. This feature is called “Repeated Fault Inhibit” and can be overridden by setting the Repeated_flt_inhibit bit to zero. The power consumption of the LTC5100 is dependent on several variables, including the modulation current range (set by Im_rng), the laser bias and modulation levels, and the state of the transmitter (whether enabled or disabled.) If Power_down_en = 1, the LTC5100 turns off its high speed amplifiers when the transmitter is disabled, reducing supply current to less than 5mA (typical). See the Typical Performance Chacteristics for further information. The FAULT pin can be configured active high or active low with the Flt_pin_polarity bit. The FAULT pin can be programmed for open drain, 330µA internal pull-up, 500µA internal pull-up or complementary (push-pull) drive with the two Flt_drv_mode bits. Refer to Figure 8 for an equivalent circuit of the FAULT pin. HIGH SPEED PEAKING CONTROL The FAULT pin can be overridden in software for testing purposes or to allow a microprocessor in the transceiver module to fully control the module’s fault output. If the Flt_pin_override bit is set, then the Force_flt bit fully controls the state of the FAULT pin. The state of the LTC5100 can be monitored by reading the FLT_STATUS register. See Table 21 for a description of the status bits. EYE SAFETY INFORMATION Communications lasers can emit levels of optical power that pose an eye safety risk. While the LTC5100 provides certain fault detection features, these features alone do not ensure that a laser transmitter using the LTC5100 is compliant with IEC 825 or the regulations of any particular agency. The user must analyze the safety requirements of their transceiver module or system, activate the The LTC5100 has the ability to selectively peak the falling edge of the modulation waveform to accelerate the turn-off of the laser diode. The 5-bit PEAKING register controls this function. See the Typical Performance Chacteristics for further information. Lower values in the PEAKING register increase the falling edge peaking. ANALOG-TO-DIGITAL CONVERSION Overview The ADC in the LTC5100 is a 10-bit, dual slope integrating converter with excellent linearity and noise rejection. A multiplexer allows digitizing six quantities: • • • • • • SRC pin current, IS Average modulation current, IM Laser diode voltage, VLD Monitor diode current, IMD Termination resistor voltage, VTERM Die temperature, T All of these measurements are available to the user via the I2C serial bus. sn5100 5100fs 29 LTC5100 U OPERATIO Conversion Sequence The ADC has a 1ms conversion time and operates in a fourcycle sequence. Three of these cycles are dedicated to the needs of the servo controllers for laser bias and modulation current. One cycle is available to the user to convert any desired quantity. Table 2 shows how the four conversion time slots are allocated. The temperature compensation and servo loop calculations are done during the User cycle. The source and modulation DACs are also updated during this cycle. Table 2. ADC Conversion Sequence APC MODE CCC MODE RESULT STORED IN REGISTER 1 T T T_INT_ADC 2 IM IM IM_ ADC 3 IMD IS IMD_ADC/IS_ADC 4 User User USER_ADC CYCLE User Access to the ADC The results of each conversion cycle in Table 2 are stored in user accessible registers. The last die temperature measurement can be read over the I2C bus at any time by reading the T_INT_ADC register. Note that the quantity converted during the third cycle depends on whether the chip is in APC or CCC mode. The result of the third conversion cycle is stored in a register that is called IMD_ADC in APC mode and IS_ADC in CCC mode. There is only one register, but it is given two names to indicate the quantity it actually holds. The fourth cycle, called the user cycle, is available to digitize any of the six multiplexed signals. The result can be read out over the I2C serial bus. The signal to be digitized during the user cycle is selected by setting the three-bit field USER_ADC.Adc_src_sel (see Table 23). For example, setting Adc_ src_sel = 2 programs the multiplexer to select the laser diode voltage, VLD. During the next user conversion cycle, VLD is converted and stored to the USER_ADC. Data field. When the conversion is complete, USER_ADC.Valid is set and USER_ADC.Adc_src indicates the signal source whose converted value is stored in USER_ADC.Data. Reading or writing the USER_ADC register clears the Valid bit. The Valid bit remains cleared until the next user conversion is complete. USER_ADC.Adc_src always corresponds to the signal source whose data is stored in USER_ADC.Data, not the source that was most recently selected by writing USER_ADC.Adc_src_sel. The Valid bit and ADC_src field are useful for monitoring when the ADC has updated the USER_ADC.Data field. Table 3 gives an extended example of accessing the USER_ADC register. Note that the content of the USER_ADC register is different for writing and for reading, even though the I2C command used to access this register is the same in both cases. See Table 23 and Table 24 for a detailed definition of the bit fields in the USER_ADC register. Table 23 also shows how to convert ADC digital codes to real-world quantities. DIRECT MICROPROCESSOR CONTROL OF THE LASER BIAS AND MODULATION CURRENT Setting Lpc_en to zero turns off the LTC5100’s digital Laser Power Controller (see Figure 2). The source and modulation DACs (Is_dac and Im_dac) can then be written from the I2C serial bus, allowing an external microprocessor or test computer to directly control the source and modulation currents. DIGITAL CONTROL AND THE I2C SERIAL INTERFACE The LTC5100 has extensive digital control and monitoring features. These features can be used during final assembly of a transceiver module to set up the laser and verify performance. In normal operation, the LTC5100 can operate standalone or under microprocessor supervision. Operating standalone, the LTC5100 automatically loads its configuration and laser operating parameters (bias current, modulation current, monitor diode current) from a small external EEPROM at power up. Operating under microprocessor supervision, the microprocessor is in total control of setting up the LTC5100. I2C Serial Interface Protocol The digital interface for the LTC5100 is I2C, a 2-wire serial bus standard that is fully documented in “I2C-Bus and How sn5100 5100fs 30 LTC5100 U OPERATIO Table 3. Example of User ADC Cycle Access ADC CYCLE SIGNAL SOURCE 1 2 WRITE TO Adc_src_sel READ FROM ADC_USER REGISTER Adc_src Valid Data COMMENT T VTERM 0 VTERM(1) Selected Signal Source is VTERM IM VTERM 0 VTERM(1) 3 IS VTERM 0 VTERM(1) 4 User (VTERM) VTERM 0 VTERM(1) 1 T VTERM 1 VTERM(2) ADC Updates Data with New Data, Setting Valid 2 IM VTERM 0 VTERM(2) User Selects New Signal Source, VLD, Clearing Valid 3 IS VTERM 0 VTERM(2) 4 User (VLD) VTERM 0 VTERM(2) 1 T VLD 1 VLD(1) 2 IM VLD 1 VLD(1) 3 IS VLD 0 VLD(1) 4 User (VLD) VLD 0 VLD(1) 1 T VLD 1 VLD(2) 2 IM VLD 1 VLD(2) 3 IS VLD 1 VLD(2) 4 User (VLD) VLD 1 VLD(2) VLD VLD WRITE S READ S ADC Updates Data with New Data, Setting Vaild and Changing Adc_src to Reflect the Source of the New Data User Reads the ADC_USER Register, Clearing Valid ADC Updates Data with New Data, Setting Valid LTC5100 COMMAND A LOW BYTE A HIGH BYTE A P ADDRESS W A BYTE (7 BITS) 0x0A LTC5100 LTC5100 N COMMAND A S ADDRESS R A LOW BYTE A HIGH BYTE P ADDRESS W A A BYTE (7 BITS) 0x0A 0x0A 5100 F28 Figure 28. I2C Serial Read/Write Sequences (LTC5100 Responses are Shown in Bold Italics) to Use It, V1.0” by Philips Semiconductor. The 7-bit I2C bus address for the LTC5100 is 0x0A (hex). When the Read/Write bit that follows is a “1”, the resulting 8-bit word becomes 0x15. When the Read/Write bit is a “0”, the 8-bit word becomes 0x14. To communicate with the LTC5100, the bus master transmits the LTC5100 address followed by a command byte and data as defined by the I2C bus specification and shown in Figure 28 and Table 4. Note that 16 bits of data are always transmitted, low byte first, high byte last. Within each transmitted byte, the bit order is MSB .. LSB. The register set and I2C command set for the LTC5100 are documented in Table 7 through Table 30. Table 4. Legend for the I2C Protocol SYMBOL S W R A NA P MEANING Start Write Read Acknowledge No Acknowledge Stop sn5100 5100fs 31 LTC5100 U OPERATIO Standalone Operation Operating_mode = 1. Table 5 shows the memory map for the EEPROM. On power-up the LTC5100 becomes an I2C bus master and attempts to load its configuration data from an external EEPROM. If an EEPROM responds, the LTC5100 reads 16-bytes of data and transfers this data to the internal register set. When a 16-byte transfer is completed without error, the LTC5100 becomes ready to enable the transmitter and begin driving the laser. If a bus error occurs during this transfer, the load sequence is aborted and a Mem_load_error is generated, preventing the transmitter from being enabled until a successful memory load attempt is completed or until an external agent sets the Operating_mode bit. Every 64ms another attempt is made to load the EEPROM until the memory is read or until The LTC5100 generates I2C address 0xAE (1010_1110 binary) when accessing the EEPROM, making it compatible with a wide range of EEPROM sizes. Table 6 details how the LTC5100 interacts with EEPROMs from 128 bits to 16k bits and from where it gets its data. The LTC5100 supports hot plugging in standalone mode. If the Soft_en bit is set in the EEPROM and the EN pin is active, the LTC5100 loads its configuration data from the EEPROM and immediately enables the transmitter. The transmitter is typically enabled and settled within the 300ms t_init period required by the GBIC specification. Table 5. EEPROM Memory Map BIT BYTE 7 15 Reserved 14 Ib_gain(4:0)/Apc_gain(4:0) 13 Reserved 12 T_nom(7:0) 11 Im_tc2(7:0) 6 5 4 3 2 1 0 Peaking (4:0) Im_gain(2:0) 10 Im_tc1(7:0) 9 Reserved 8 Im_nom(7:0) 7 Ib_tc2(7:0)/Imd_tc2(7:0) 6 Ib_tc1(7:0)/Imd_tc2(7:0) 5 Reserved 4 Ib_nom(7:0)/Imd_nom(7:0) 3 Reserved 2 Lpc_en 1 Reserved Ib_limit 0 Cml_en Md_polarity Imd_rng(1:0) T_nom(9:8) Im_rng(1:0) Im_nom(9:8) Is_rng(1:0) Ib_nom(9:8)/Imd_nom (9:8) Rep_flt_inhibit Rapid_restart_en Flt_drv_mode Auto_shutdn_en Flt_pin_polarity Flt_pin_override Force_flt Ext_temp_en Power_down_en Apc_en Over_pwr_en Under_pwr_en Over_current_en En_polarity Soft_en Operating_mode sn5100 5100fs 32 LTC5100 U OPERATIO Table 6. Effective Base Addresses for Various Sized EEPROMs GENERIC PART NUMBER Bits Bytes 24LC00 24LC01B 24LC02B 24LC04B 24LC16B 128 1k 2k 4k 16k 16 128 256 512 2048 1010xxx. 1010xxx. 1010xxx. 1010.xxa 1010cba. Word Address Space (Binary) xxxx_nnnn xnnn_nnnn nnnn_nnnn nnnn_nnnn nnnn_nnnn LTC5100 Generates Device Address 1010_111. = 0xAE 1010_111. = 0xAE 1010_111. = 0xAE 1010_111. = 0xAE 1010_111. = 0xAE LTC5100 Generates Word Address 0110_0000 = 0x60 0110_0000 = 0x60 0110_0000 = 0x60 0110_0000 = 0x60 0110_0000 = 0x60 Effective Base Address 0000_0000 = 0x00 0110_0000 = 0x60 0110_0000 = 0x60 0001_0110_0000 = 0x160 0111_0110_0000 = 0x760 Minimum Size EEPROM. Loads Every Byte in the EEPROM. EEPROM Not Big Enough for GBIC ID. LTC5100 Loads from 0x60 to 0x6F Standard GBIC EEPROM. Smallest EEPROM That is Big Enough to Hold the LTC5100 Data and the GBIC ID. LTC5100 Loads from 0x60 to 0x6F, the First 16 Bytes of the Vendor Area LTC5100 Loads from an Area Outside the GBIC ID Data Area LTC5100 Loads from an Area Outside the GBIC ID Data Area Device Address (Binary) Comments Microprocessor Controlled Operation An external microprocessor or a test computer can take full control of the LTC5100 by setting the Operating_mode bit. When this bit is set, the LTC5100 stops searching for an external EEPROM and takes commands from the microprocessor. It is even possible to combine standalone and microprocessor controlled modes. If an EEPROM is present, the LTC5100 will load its configuration registers from the EEPROM at power-up. A microprocessor or test computer can then read and write the LTC5100 registers at will. The primary purpose of the Operating_mode bit is to stop the LTC5100’s EEPROM load attempts. Once the LTC5100 has loaded itself from an EEPROM (if present), it is not technically necessary to set the Operating_mode bit to communicate with the LTC5100. The LTC5100 attempts to read the EEPROM every 64ms until it successfully loads its registers or until the Operating_mode bit is set. There is a finite chance that the microprocessor and the LTC5100 will generate an I2C bus collision if an EEPROM load attempt coincides with the microprocessor’s attempt to access the LTC5100. In this case, the microprocessor will receive a NACK (not-acknowledged) response to its transmissions. The microprocessor needs only to cease transmission in accordance with the I2C protocol and try again. If the microprocessor makes this second attempt within 64ms (typical), it is guaranteed not to collide with the LTC5100. sn5100 5100fs 33 LTC5100 U U REGISTER DEFI ITIO S Table 7. Register Set Overview REGISTER NAME REGISTER GROUP System Operating Configuration Laser Setup Coefficients Temperature Fault Monitoring and Eye Safety ADC DAC CONSTANT CURRENT CONTROL MODE AUTOMATIC POWER CONTROL MODE I2C COMMAND CODE (HEX) READ/WRITE ACCESS REFERENCE INFORMATION SYS_CONFIG “ 0x10 R/W Table 8 LOOP_GAIN “ 0x1E R/W Table 9 PEAKING “ 0x1F R/W Table 10 Reserved “ 0x08 R/W Table 11 IB IMD 0x15 R/W Table 12 IB_TC1 IMD_TC1 0x16 R/W Table 13 IB_TC2 IMD_TC2 0x17 R/W Table 14 IM “ 0x19 R/W Table 15 IM_TC1 “ 0x1A R/W Table 16 IM_TC2 “ 0x1B R/W Table 17 T_EXT “ 0x0D R/W Table 18 T_NOM “ 0x1D R/W Table 19 FLT_CONFIG “ 0x13 R/W Table 20 FLT_STATUS “ 0x12 R Table 21 IB_LIMIT “ 0x11 R/W Table 22 USER_ADC “ 0x18 R/W Tables 23, 24 T_INT_ADC “ 0x05 R/W Table 25 IM_ADC “ 0x06 R/W Table 26 IS_ADC IMD_ADC 0x07 R/W Table 27 IS_DAC “ 0x01 R/W Table 28 IM_DAC “ 0x02 R/W Table 29 PWR_ LIMIT_DAC “ 0x03 R Table 30 sn5100 5100fs 34 LTC5100 U U REGISTER DEFI ITIO S Table 8. Register: SYS_CONFIG—System Configuration (I2C Command Code 0x10) REGISTER .BITFIELD BIT .Reserved 15:8 RESET VALUE (BIN) FUNCTION AND VALUES .Cml_en 7 0 Current Mode Logic Enable 0: Floating Differential Input Termination: 100Ω Across IN+ and IN– 1: CML Compatible Input Termination: 50Ω from IN+ to VDD(HS) and from IN– to VDD(HS) .Md_polarity 6 0 Monitor Diode Polarity 0: Cathode Connected to the MD Pin, Sinking Current from the Pin 1: Anode Connected to the MD Pin, Sourcing Current Into the Pin .Ext_temp_en 5 0 External Temperature Enable Selects the Source of Temperature Measurements for Temperature Compensation. 0: Internal Temperature Sensor 1: Externally Supplied Through the Serial Interface .Power_down_en 4 1 Power Down Enable Allow Power Reduction When the Transmitter is Disabled. 0: No Power Reduction When the Transmitter is Disabled. 1: Reduce Power Consumption When Transmitter is Disabled by Turning Off the High Speed Amplifiers. .Apc_en 3 0 Automatic Power Control Enable Select the Means of Controlling the Laser Bias Current. 0: Constant Current Control 1: Automatic Power Control Using Feedback from the Monitor Diode .En_polarity 2 0 EN Pin Polarity Set the Input Polarity of the EN Pin. 0: Active Low: A Logic Low Input Level Enables the Transmitter. 1: Active High: A Logic High Input Level Enables the Transmitter. Note: In order to Enable the Transmitter, Both the EN Pin and Soft_en Bit Must be Asserted. .Soft_en 1 0 Soft Transmitter Enable Enables Transmitter Through the Serial Interface. 0: Disable the Transmitter 1: Enable the Transmitter (if the EN Pin is Active) Note: In order to Enable the Transmitter, Both the EN Pin and Soft_en Bit Must be Asserted. .Operating_mode 0 0 Digital Operating Control Mode Select Whether the LTC5100 Operates Autonomously or Under External Control. 0: Standalone Operation: Configuration Parameters are Loaded from an External EEPROM at Power Up. 1: Externally Controlled Operation: Configuration Parameters are Set by an External Microprocessor or Test Computer. sn5100 5100fs 35 LTC5100 U U REGISTER DEFI ITIO S Table 9. Register: LOOP_GAIN—Control Loop Gain (I2C Command Code 0x1E) REGISTER .BITFIELD BIT .Reserved 15:8 RESET VALUE (BIN) FUNCTION AND VALUES .Ib_gain 7 0 Bias Current or APC Loop Gain (.Apc_gain in APC Mode) 6 0 5 1 4 0 This Bit Field Modifies the Open-Loop Gain of the Bias Current Servo Control Loop. The Effect of This Bit Field Differs in Constant Current Control (CCC) Mode and in Automatic Power Control (APC) Mode. In CCC Mode, This Bit Field is Called lb_gain. In APC Mode, This Bit Field is Called Apc_gain. 3 0 Constant Current Control (CCC) Mode (Apc_en = 0): The Loop Gain and Settling Time are Independent of Is_rng. The Default Value of Ib_gain Yields Stable but Slow Settling of the Laser Bias Current for Any Value of Is_rng. Automatic Power Control (APC) Mode (Apc_en = 1): The Open-Loop Gain of the Bias Current Servo Loop Depends on the Value of Is_rng. The Default Value of Apc_gain Yields Stable but Potentially Slow Settling of the Laser Bias Current for any Value of Is_rng. Im_gain 2 0 Modulation Current Loop Gain 1 0 0 1 This Bit Field Modifies the Open-Loop Gain of the Modulation Current Servo Loop. The Open-Loop. Gain is Approximately Im_gain/32. The Loop Gain and Settling Time are Independent of Im_rng. The Default Value of Im_gain Yields Stable but Slow Settling of the Laser Modulation Current. Table 10. Register: PEAKING—High Speed Modulation Peaking (I2C Command Code 0x1F) REGISTER .BITFIELD BIT RESET VALUE (BIN) .Reserved 15:5 .Peaking 4 1 Peaking Control for the Modulation Output 3 0 2 0 1 0 This Bit Field Controls the High Speed Peaking of the Modulation Output. Decreasing the Value of Peaking Increases the Undershoot on the Falling Edge of the Modulation Signal. The Peaking Control can be Used to Compensate for Slow Laser Turn-Off Characteristics. 0 0 FUNCTION AND VALUES Table 11. Register: Reserved—Reserved for Internal Use. This Register is for Test Puposes Only. Do Not Write to this Register (I2C Command Code 0x08) REGISTER .BITFIELD BIT .Reserved 15:7 .Reserved 6 1 5 0 4 0 3 0 .Reserved RESET VALUE (BIN) 2 1 1 0 0 0 FUNCTION AND VALUES Reserved for Internal Use, Do Not Write. Reserved for Internal Use, Do Not Write. sn5100 5100fs 36 LTC5100 U U REGISTER DEFI ITIO S Table 12. Register: IB (IMD)—Laser Bias Current Register (Monitor Diode Current in APC Mode) (I2C Command Code 0x15) REGISTER .BITFIELD BIT .Reserved 15:12 .Is_rng RESET VALUE (BIN) FUNCTION AND VALUES 11 0 Source Current Range 10 0 Is_rng Sets the Full-Scale Range of the SRC Pin Current. The Table Below Shows the Available Ranges. Values for Is_rng Nominal FullScale SRC Pin Current (mA) Binary Value Decimal Value 00 0 9 01 1 18 10 2 27 11 3 36 See the Electrical Specifications for Guaranteed Limits in Each Range. .Ib_nom (.Imd_nom in APC Mode) 9 0 Bias Current or Monitor Diode Current Setting at the Nominal Temperature 8 0 This Bit Field has Different Functions Depending on Apc_en. 7 0 This Bit Field is an Unsigned 10-Bit Integer. 6 0 5 0 Constant Current Control (CCC) Mode (Apc_en = 0): Ib_nom Sets the Laser Bias Current at Temperature T = T_nom. The physical Bias Current at T_nom is Given by: 4 0 3 0 2 0 1 0 0 0 IB = Ib_nom • (Is _ rng + 1) • 9mA (typical) 1024 Automatic Power Control (APC) Mode (Apc_en = 1): Imd_nom Sets the Monitor Diode Current at the Temperature T = T_nom. The Physical Monitor Diode Current at T_nom is Given by: Imd _ nom IMD = 4.25µA • 4Imd _ rng • exp ln(8)• 1024 Table 13. Register: IB_TC1 (IMD_TC1)—Laser Bias/Monitor Diode Current First Order Temperature Coefficient (I2C Command Code 0x16) REGISTER .BITFIELD BIT .Reserved 15:8 .Ib_tc1 (.Imd_tc1 in APC Mode) RESET VALUE (BIN) FUNCTION AND VALUES 7 0 First Order Temperature Coefficient for Bias Current or Monitor Diode Current 6 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127. 5 0 This Bit Field has Different Functions Depending on Apc_en. 4 0 3 0 Constant Current Control (CCC) Mode (Apc_en = 0): Ib_tc1 Sets the First Order Temperature Coefficient for the Laser Bias Current. The Nominal Scaling is 2–13/°C or 122ppm/°C per LSB. 2 0 1 0 0 0 Automatic Power Control (APC) Mode (Apc_en = 1): Imd_tc1 Sets the First Order Temperature Coefficient for the Monitor Diode Current. See Laser Bias Current Control in APC Mode in the Operation Section for Details. sn5100 5100fs 37 LTC5100 U U REGISTER DEFI ITIO S Table 14. Register: IB_TC2 (IMD_TC2)—Laser Bias/Monitor Diode Current Second Order Temperature Coefficient (I2C Command Code 0x17) REGISTER .BITFIELD BIT .Reserved 15:8 .Ib_tc2 (.Imd_tc2 in APC Mode) RESET VALUE (BIN) FUNCTION AND VALUES 7 0 Second Order Temperature Coefficient for Bias Current or Monitor Diode Current 6 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127. 5 0 This Bit Field has Different Functions Depending on Apc_en. 4 0 3 0 Constant Current Control (CCC) Mode (Apc_en = 0): Ib_tc2 Sets the Second Order Temperature Coefficient for the Laser Bias Current. The Nominal Scaling is 2–18/°C2 or 3.81ppm/°C2 per LSB. 2 0 1 0 0 0 Automatic Power Control (APC) Mode (Apc_en = 1): Imd_tc2 Sets the Second Order Temperature Coefficient for the Monitor Diode Current. See Laser Bias Current Control in APC Mode in the Operation Section for Details. Table 15. Register: IM—Laser Modulation Current (I2C Command Code 0x19) REGISTER .BITFIELD BIT .Reserved 15:12 .Im_rng RESET VALUE (BIN) FUNCTION AND VALUES 11 0 Modulation Current Range 10 0 Im_rng Sets the Full-Scale Range of the Modulation Current Binary Value Decimal Value Nominal Full-Scale MODA and MODB Pin Current Peak-to-Peak (mA) Average (mA) 00 0 9 4.5 01 1 18 9 10 2 27 13.5 11 3 36 18 See the Electrical Specifications for Guaranteed Limits in Each Range. These Currents Represent the Peak-to-Peak Current at the MODA and MODB Pins. (The MODA and MODB Pins are Tied Together On Chip). .Im_nom 9 0 Modulation Current Setting at the Nominal Temperature 8 0 7 0 6 0 5 0 This Bit Field is an Unsigned 10-Bit Integer. Im_nom Sets the Average Modulation Current Delivered at the MODA and MODB Pins. (The MODA and MODB Pins are Tied Together On Chip). The Peak-to-Peak Current is Twice the Average Current for a Data Stream with 50% Duty Cycle. The Modulation Current Reaching the Laser Depends on its Dynamic Resistance Relative to the Termination Resistor. 4 0 3 0 2 0 1 0 0 0 sn5100 5100fs 38 LTC5100 U U REGISTER DEFI ITIO S Table 16. Register: IM_TC1—Laser Modulation Current First Order Temperature Coefficient (I2C Command Code 0x1A) REGISTER .BITFIELD BIT .Reserved 15:8 .Im_tc1 RESET VALUE (BIN) FUNCTION AND VALUES 7 0 First Order Temperature Coefficient for Modulation Current 6 0 5 0 4 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127. Im_tc1 Sets the First Order Temperature Coefficient for the Modulation Current. The Nominal Scaling is 2–13/°C or 122ppm/°C per LSB. 3 0 2 0 1 0 0 0 Table 17. Register: IM_TC2—Laser Modulation Current Second Order Temperature Coefficient (I2C Command Code 0x1B) REGISTER .BITFIELD BIT .Reserved 15:8 .Im_tc2 7 RESET VALUE (BIN) FUNCTION AND VALUES 0 Second Order Temperature Coefficient for Modulation Current 6 0 5 0 4 0 This Bit Field is a Signed 8-Bit, Two’s Complement Integer. Thus its Value Ranges from –128 to 127. Im_tc2 Sets the Second Order Temperature Coefficient for the Laser Bias Current. The Nominal Scaling is 2–18/°C2 or 3.81ppm/°C2 per LSB. 3 0 2 0 1 0 0 0 Table 18. Register: T_EXT—External Temperature (I2C Command Code 0x0D) REGISTER .BITFIELD BIT .Reserved 15:10 .T_ext RESET VALUE (BIN) 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 FUNCTION AND VALUES Externally Supplied Temperature for Temperature Compensation Calculations (Unsigned 10-Bit Integer) By Convention the Scaling of T_ext is 512K or 239°C Full Scale, Corresponding to 0.5°C/LSB. However, Any Scaling is Permissible as Long as the Temperature Compensation Coefficients are Also Appropriately Scaled. T_ext = (T + 273°C)/0.5°C, Where T is the External Temperature in Degrees Celsius. sn5100 5100fs 39 LTC5100 U U REGISTER DEFI ITIO S Table 19. Register: T_NOM—Nominal Temperature (Includes Imd_rng) (I2C Command Code 0x1D) REGISTER .BITFIELD BIT .Reserved 15:12 .Imd_rng .T_nom RESET VALUE (BIN) FUNCTION AND VALUES 11 0 Monitor Diode Current Range 10 0 Imd_rng Sets the Full-Scale Range of the Monitor Diode Current. MD Pin Current Range (µA) Binary Value Decimal Value Nom Min 00 0 4.25 34 01 1 17 136 10 2 68 544 11 3 272 2176 Nom Max 9 0 Nominal Temperature 8 0 7 0 6 0 T_nom is the Temperature with Respect to Which All Temperature Compensation Calculations are Made. T_nom is Usually the Temperature at Which the LTC5100 and Laser Diode were Set Up In Production. 5 0 The Scaling is 512K or 239°C Full Scale, Corresponding to 0.5°C/LSB 4 0 T_nom = (T + 273°C)/0.5°C, Where T is the Nominal Temperature in Degrees Celsius. 3 0 2 0 1 0 0 0 sn5100 5100fs 40 LTC5100 U U REGISTER DEFI ITIO S Table 20. Register: FLT_CONFIG—Fault Configuration (Refer also to Table 1) (I2C Command Code 0x13) REGISTER .BITFIELD .Reserved Rep_flt_inhibit BIT 15:12 11 RESET VALUE (BIN) 0 Rapid_restart_en 10 1 Flt_drv_mode 9 8 0 0 Lpc_en 7 1 Auto_shutdn_en 6 1 Flt_pin_polarity 5 1 Flt_pin_override 4 0 Force_flt 3 0 Over_pwr_en 2 1 Under_pwr_en 1 1 Over_current_en 0 1 FUNCTION AND VALUES Repeated Fault Inhibit 0: Allow Repeated Attempts to Clear a Fault and Re-enable the Transmitter. 1: Inhibit Repeated Attempts to Clear a Fault. Only One Attempt to Clear a Fault is Allowed. If the Fault Recurs Within 25ms of Re-enabling the Transmitter, the Transmitter is Disabled Until Power is Cycled. Rapid_restart_en 0: Rapid Restart Disabled: The Servo Controller Settings for the Laser Bias and Modulation Currents are Reset to Zero when the Transmitter is Disabled. When Re-enabled, the Laser Currents Start from Zero and Settle Typically Within the 300ms Standard Initialization Time, t_int, from the GBIC Specification. 1: Rapid Restart Enabled: The Servo Controller Settings for the Laser Bias and Modulation Currents are Retained when the Transmitter is Disabled. When Re-enabled, the Retained Servo Values are Loaded into the SRC_DAC and MOD_DAC, Allowing Settling Typically Within the 1ms Standard Turn-On Time, t_on, from the GBIC Specification. FAULT Pin Drive Mode 00: Open Drain (3.3mA Sink Capability) 01: Open Drain, 280µA Internal Pull Up 10: Open Drain, 425µA Internal Pull Up 11: Push-Pull (3.3mA Source and Sink Capability) Laser Power Controller (LPC) Enable 0: LPC Disabled: Allows External Control of the SRC_DAC and MOD_DAC Registers from the Serial Interface. This Setting Gives an External Microprocessor or Test Computer Full Control of the SRC_DAC and MOD_DAC Registers. 1: LPC Enabled: The LPC Continuously Updates the SRC_DAC and MOD_DAC Registers to Servo Control the Laser. (Any Values Written to These Registers Over the Serial Interface Will be Overwritten by the LPC.) Automatic Transmitter Shutdown Enable 0: Disabled: When a Fault Occurs the LTC5100 Continues to Drive the Laser. This Mode Allows a Microprocessor or Test Computer to Mediate the Decision to Shut Down the Transmitter. The Microprocessor can Turn Off the Transmitter by Driving the EN Pin Inactive or by Clearing the Soft_en Bit in the SYS_CONFIG Register. 1: Enabled: When a Fault Occurs, the Transmitter is Automatically Disabled. FAULT Pin Polarity 0: Active Low: The FAULT Pin is Driven Low to Signal a Fault. 1: Active High: The FAULT Pin is Driven High to Signal a Fault. FAULT Pin Override 0: The FAULT Pin is Driven Active when a Fault Occurs. 1: Internal Control of the FAULT Pin is Overridden. When a Fault Occurs, the Fault is Detected and Latched Internally, but the FAULT Pin Remains Inactive. This Mode Allows a Microprocessor or Test Computer to Mediate Fault Handling. The Microprocessor can Drive the FAULT Pin Active by Setting the Force_flt Bit. Force the FAULT Pin Output. Force_flt Gives a Microprocessor or Test Computer Full Control of the FAULT Pin, Allowing External Mediation of Fault Handling. 0: Force the FAULT Pin Inactive. 1: Force the FAULT Pin Active. This Bit Has No Effect Unless Flt_pin_override = 1. Enables Detection of a Laser Overpower Fault. 0: Disabled, 1: Enabled Enables Detection of a Laser Underpower Fault. 0: Disabled, 1: Enabled Enables Detection of a Laser Overcurrent Fault. 0: Disabled, 1 Enabled sn5100 5100fs 41 LTC5100 U U REGISTER DEFI ITIO S Table 21. Register: FLT_STATUS—Fault Status (I2C Command Code 0x12) REGISTER .BITFIELD BIT .Reserved 15:11 RESET VALUE (BIN) FUNCTION AND VALUES .Transmit_ready 10 0 Transmit Ready Indicates that the Laser Bias and Modulation Currents Have Settled to Within Specification and the LTC5100 is Ready to Transmit Data. A Fault Clears This Bit. 0: Not Ready, 1: Ready .Transmitter_ enabled 9 0 Transmitter Enabled Indicates That the Transmitter is Enabled and the Laser Bias and Modulation Currents Are on (Though Not Necessarily Settled.) The Transmitter is Enabled When the EN pin and Soft_en Bits are Active and No Faults Have Occurred. A Fault Clears This Bit. 0: Transmitter is Disabled, 1: Transmitter is Enabled. .En_pin_state 8 Varies .Faulted_twice 7 0 Faulted Twice (Only Active When Rep_flt_inhibit is Set) 0: Either No Faults or Only One Fault Has Been Detected. 1: A Second Fault Has Been Detected Within 25ms of Attempting to Clear a First Fault. The Transmitter is Disabled and Can Only be Re-enabled by Cycling the Power. .Faulted_once 6 1 Faulted Once (Only Active When Rep_flt_inhibit is Set) Indicates That a First Fault Has Been Detected. After a Fault Occurs, Faulted_once Will be Set at the Moment the Transmitter is Disabled (by Setting the EN pin of Soft_en Bit Inactive). If the Transmitter is Subsequently Re-enabled and a Second Fault Occurs Within 25ms, the Faulted_twice Bit is Set. If No Fault Occurs Within 25ms, the Faulted_once Bit is Cleared. 0: A First Fault Has Not Been Detected or Has Been Cleared. 1: A First Fault Has Been Detected. .Faulted 5 1 Faulted 0: The LTC5100 is Not in the Faulted State. 1: A Fault Has Occurred and the LTC5100 Has Entered the Faulted State (the Transmitter is Not Disabled Unless Auto_shutdn_en is Set). .Under_votlage Cleared-on-read 4 1 Undervoltage Fault Indicator (Always Enabled) Indicates That a Power Supply Undervoltage Event Occurred. 0: No Fault, 1: Undervoltage Fault Detected. The Undervoltage Bit is Always Set at Power Up. Read the FLT_STATUS Register Immediately After Power-Up to Clear This Bit. .Mem_load_error Cleared-on-read 3 0 Memory (EEPROM) Load Error Indicator (Always Enabled) Indicates That an Attempt to Load the Registers from EEPROM Was Started But Did Not Complete Successfully. 0: No Fault, 1: EEPROM Load Failed. .Over_power Cleared-on-read 2 0 Laser Overpower Fault Indicator (Enabled by Over_pwr_en) Indicates That a Laser Overpower Fault Occurred. Overpower Occurs When the Monitor Diode Current Exceeds its Set Point. An Overpower Fault Can Occur Only in APC Mode. 0: No Fault, 1: Overpower Fault Detected. .Under_power Cleared-on-read 1 0 Laser Underpower Fault Indicator (Enabled by Under_pwr_en) Indicates That a Laser Underpower Fault Occurred. Underpower Occurs When the Monitor Diode Current Falls Below its Set Point. An Underpower Fault Can Occur Only in APC Mode. 0: No Fault, 1: Underpower Fault Detected. .Over_current Cleared-on-read 0 0 Laser Overcurrent Fault Indicator (Enabled by Over_current_en) Indicates That the Laser Bias Current Exceeded the Value Set in the IB_LIMIT Register. 0: No Fault, 1: Overcurrent Fault Detected. EN Pin State Indicates the Logic Level on the EN Pin. The En_polarity Bit Has No Effect on En_pin_state. The Power-On Reset Value Reflects the State of the EN Pin. 0: EN Pin is Low. 1: EN Pin is High. sn5100 5100fs 42 LTC5100 U U REGISTER DEFI ITIO S Table 22. Register: IB_LIMIT—Laser Bias Current Limit (I2C Command Code 0x11) REGISTER .BITFIELD BIT .Reserved 15:7 .Ib_limit RESET VALUE (BIN) FUNCTION AND VALUES 6 0 Laser Bias Current Limit 5 0 This Bit Field is an Unsigned 7-Bit Integer 4 0 3 0 Sets the Detection Level for an Over_current Fault. When the Laser Bias Current Exceeds This Level an Over_current Fault is Generated (Provided Over_current_en is Set). 2 0 The Physical Bias Current Level is Given By: 1 0 0 0 IB(LIMIT) = Ib_limit • (Is _ rng + 1)• 9mA (typical) 128 Table 23. Register: USER_ADC—Writing (I2C Command Code 0x18) REGISTER .BITFIELD BIT .Reserved 15:3 .Adc_src_sel RESET VALUE (BIN) FUNCTION AND VALUES 2 0 ADC Source Select 1 0 Selects the Signal to be Converted by the ADC During the User ADC Cycle 0 0 User ADC Signal Sources Signal Select (Binary) Signal Name 000 IS Source Current (SRC Pin IS = ADC_code/1024 • (Is_rng + 1) • 9mA Current) 001 IM Average Modulation Current (MODA +MODB Pin Current) IM = ADC_code/1024 • (Im_rng + 1) • 9mA 010 VLD Laser Diode Voltage VLD = ADC_code/1024 • 3.5V 011 IMD Monitor Diode Current IMD = 4.25µA • 4lmd_rng • exp[In(8) • ADC_code/ 1024] 100 T Temperature T(°C) = ADC_code • 0.5°C – 273°C 101 VTERM Termination Resistor Voltage VTERM = ADC_code/1024 • (Is_rng + 1) • 400mV Description 110 Reserved Reserved 111 Reserved Reserved Scaling sn5100 5100fs 43 LTC5100 U U REGISTER DEFI ITIO S Table 24. Register: USER_ADC—Reading (I2C Command Code 0x18) REGISTER .BITFIELD BIT .Reserved 15 .Adc_src RESET VALUE (BIN) FUNCTION AND VALUES 14 0 ADC Signal Source 13 0 12 0 Specifies the Signal Source of the Last User ADC Conversion. See Table 23 for the Definition of These Signal Sources. Adc_src Reflects the Last Signal Source Converted. It Does Not Necessarily Hold the Last Value Written to the ADC_src_sel Bit Field. .Reserved 11 .Valid 10 0 ADC Data Valid Indicates That the Result in the Data Bit Field (Defined Below) Contains Newly Converted Data Since the Last Time Adc_src_sel Was Written or This Register Was Read. Immediately After Power Up Valid is False. Valid Becomes True as Soon as the First User ADC Conversion is Completed. 0: The ADC Result is Not a Valid Conversion of the Most Recently Selected ADC Source. 1: The ADC Has Finished Conversion and the Result is Valid. .Data 9 0 ADC Data (10-Bit Unsigned Integer) 8 0 7 0 Contains the Result of the Last User ADC Conversion. See Table 23 for the Definition of the Available Signal Sources. 6 0 5 0 4 0 3 0 2 0 1 0 0 0 Table 25. Register: T_INT_ADC—Internal Temperature ADC (I2C Command Code 0x05) REGISTER .BITFIELD BIT RESET VALUE (BIN) .Reserved 15:10 .T_int_adc 9 0 ADC Reading of the Internal (Die) Temperature (10-Bit Unsigned Integer) 8 0 This Bit Field Contains the Result of the Last Conversion of the LTC5100’s Internal Die Temperature. 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 FUNCTION AND VALUES The Scaling is 512°K or 239°C Full Scale, Corresponding to 0.5°C/LSB. T = T_int_adc • 0.5°C – 273°C, Where T is the Internal Temperature in Degrees Celsius. sn5100 5100fs 44 LTC5100 U U REGISTER DEFI ITIO S Table 26. Register: IM_ADC—Modulation Current ADC (I2C Command Code 0x06) REGISTER .BITFIELD BIT .Reserved 15:10 .Im_adc RESET VALUE (BIN) FUNCTION AND VALUES 9 0 ADC Reading of the Modulation Current (10-Bit Unsigned Integer) 8 0 7 0 6 0 Im_adc Contains the Last ADC Conversion of the Average Modulation Current Delivered at the MODA and MODB Pins. (The MODA and MODB Pins are Tied Together On-Chip.) The Peak-to-Peak Current s Twice the Average Current for a Data Stream with 50% Duty Cycle. The Modulation Current Reaching the Laser Depends on its Resistance Relative to the Termination Resistor. 5 0 4 0 3 0 2 0 1 0 0 0 The Average Physical Current at the MODA and MODB Pins is Given By: IM = ADC_code • (Im_ rng + 1)• 9mA (typical) 1024 Table 27. Register: IS_ADC (IMD_ADC)—Source Current/Monitor Diode Current ADC (I2C Command Code 0x07) REGISTER .BITFIELD BIT .Reserved 15:10 .Is_adc (.Imd_adc in APC Mode) RESET VALUE (BIN) FUNCTION AND VALUES 9 0 ADC Reading of the SRC Pin Current or Monitor Diode Current 8 0 This Bit Field Has Different Functions Depending on Apc_en. Constant Current Control (CCC) Mode (Apc_en = 0): Is_adc Contains the Last ADC Conversion of the SRC Pin Current. The Physical SRC Pin Current is Given By: 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 IS = ADC_code • (Is _ rng + 1)• 9mA (typical) 1024 Automatic Power Control (APC) Mode (Apc_en = 1): Imd_adc Contains the Last ADC Conversion of the Monitor Diode Current. The Physical Monitor Diode Current is Given By: ADC _ code IMD = 4.25µA • 4Imd_rng • exp In(8)• 1024 Table 28. Register: IS_DAC—Souce Current DAC (I2C Command Code 0x01) REGISTER .BITFIELD BIT .Reserved 15:10 .Is_dac RESET VALUE (BIN) FUNCTION AND VALUES 9 0 DAC Setting for the Source Current (the SRC Pin Current) 8 0 Read Access to This DAC is Always Available. Write Access is Only Valid if LPC_en = 0. 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 IS = Is_dac • (Is _ rng + 1)• 9mA (typical) 1024 sn5100 5100fs 45 LTC5100 U U REGISTER DEFI ITIO S Table 29. Register: IM_DAC—Modulation Current DAC (I2C Command Code 0x02) REGISTER .BITFIELD BIT .Reserved 15:10 .Im_dac RESET VALUE (BIN) FUNCTION AND VALUES 9 0 DAC Setting for the Peak-to-Peak Modulation Current (the Combined MODA and MODB Pin Currents) 8 0 Read Access to This DAC is Always Available. Write Access is Only Valid if LPC_en = 0. 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 IM = Im_dac • (Im_ rng + 1)• 9mA (typical) 1024 Table 30. Register: PWR_LIMIT_DAC—Optical Power Limit DAC—Read Only (I2C Command Code 0x03) REGISTER .BITFIELD BIT .Reserved 15:7 .pwr_limit_dac Read Only RESET VALUE (BIN) FUNCTION AND VALUES 6 0 DAC Setting for the Over and Underpower Fault Detection Comparator (Read Only) 5 0 This Bit Field Has Different Functions Depending on Apc_en. 4 0 3 0 Constant Current Control (CCC) Mode (Apc_en = 0): Pwr_limit_dac Has No Function in This Mode. Its Contents are Undefined. 2 0 1 0 0 0 Automatic Power Control (APC) Mode (Apc_en = 1): Pwr_limit_dac Tracks the Value of the Monitor Diode Current. The Laser Power Controller Continuously Updates the PWR_LIMIT_DAC with the Most Recent ADC Reading of Imd. Reading the DAC Will Return the Value of Imd_adc Shifted Right by Three Bits. sn5100 5100fs 46 LTC5100 U W U U APPLICATIO S I FOR ATIO HIGH SPEED DESIGN AND LAYOUT and 12) have webs of copper connecting them to the central pad to reduce ground inductance. The laser modulation current returns to the ground plane primarily through the exposed pad. Any measures that reduce the inductance from the pad to the ground plane improve the modulation waveforms and reduce RFI. Figure 29 and Figure 30 show the schematic and layout of a minimum component count circuit for standalone operation. The exposed pad of the package is soldered to a copper pad on top of the board, and nine vias couple this pad to the ground plane. The four VSS pins (Pins 1, 4, 9, ENABLE L1 VDD + 3.3V 16 FERRITE BEAD VSS 1 ZO = 50Ω +TX_DATA 15 VDD –TX_DATA EN 13 14 SRC MD VSS VSS 2 IN + MODA 3 IN – 4 LTC5100 MODB VSS VSS FAULT SDA 5 FAULT 12 11 10 R1 50Ω C1 10nF ZO = 50Ω 9 SCL VDD(HS) 6 7 FIBER 8 C3 10nF NC SDA 5100 F29 VSS VCC SCL 24LC00 EEPROM PROGRAMMING SOT23 PACKAGE PADS SRC MD VCC EN SCL VDD Figure 29. Schematic of a Minimum Component Count Circuit 16 VSS 1 15 14 13 12 VSS C1 + 2 11 MODA IN – 3 10 MODB IN R1 VSS L1 7 SCL SDA 6 8 9 VSS VDD(HS) EEPROM 5 FAULT NC SDA VSS 4 C3 5100 F30 Figure 30. Layout of the Minimum Component Count Circuit Using 0402 Passive Components sn5100 5100fs 47 LTC5100 U W U U APPLICATIO S I FOR ATIO The termination resistor, R1, and its decoupling capacitor, C1, are placed as close as possible to the LTC5100 to reduce inductance. Inductance in these two components causes high frequency peaking and overshoot in the current delivered to the laser. R1 and C1 are folded against each other so that their mutual inductance and counterflowing current partially cancel their self-inductance. C1 has two vias to the ground plane and a trace directly to Pin 12. The layout shows the EEPROM placed next to the LTC5100. However, placement of the EEPROM is not critical. It can be placed several centimeters from the LTC5100 or on the back of the PC board if desired. The transmission line connecting the MODB pin to the laser has a short length of minimum width trace. The net inductance of this section of trace helps compensate onchip capacitance to further reduce reflections from the chip. ENABLE L1 VDD + 3.3V 16 FERRITE BEAD VSS VDD 1 ZO = 50Ω +TX_DATA –TX_DATA 15 EN 14 SRC C2 10nF 13 MD VSS VSS 2 IN + MODA 3 IN – 4 LTC5100 MODB VSS VSS FAULT 5 FAULT SDA 6 12 11 10 R1 50Ω C1 10nF ZO = 50Ω 9 SCL VDD(HS) 7 FIBER 8 C3 10nF NC SDA 5100 F29 VSS VCC SCL 24LC00 EEPROM PROGRAMMING SOT23 PACKAGE PADS SRC MD VCC EN SCL VDD Figure 31. Schematic of a Minimum Output Reflection Coefficient Circuit 16 VSS 1 15 14 13 12 VSS C1 + IN 2 11 MODA IN – 3 10 MODB R1 C2 VSS L1 7 SCL SDA 6 8 9 VSS VDD(HS) EEPROM 5 FAULT NC SDA VSS 4 C3 5100 F32 Figure 32. Layout of the Minimum Reflection Coeffieicnt Circuit Using 0402 Passive Components sn5100 5100fs 48 LTC5100 U W U U APPLICATIO S I FOR ATIO Figure 31 and Figure 32 show the schematic and layout of a minimum reflection coefficient, minimum peaking solution. Two capacitors, C1 and C2 are used to further reduce the inductance in the termination network. C2 has two vias to the ground plane. TEMPERATURE COMPENSATION The LTC5100 has first and second order digital temperature compensation for the laser bias current, laser modulation current, and monitor diode current. Recall that in constant current control mode, the LTC5100 provides direct temperature compensation of the laser bias current and the laser modulation current. In automatic power control mode, the laser bias current is under closed-loop control and the LTC5100 provides temperature compensation for the monitor diode current and the laser modulation current. The simplest procedure for determining the temperature coefficients (TC1 and TC2 in Equation 12, Equation 18, Equation 23, and Equation 29) is as follows: • Select a nominal or representative laser diode and assemble it into a transceiver module with the LTC5100. • Set all temperature coefficients to zero. • Place the transceiver module in a temperature chamber and find the values of Ib_nom, Im_nom, and Imd_nom that give constant average optical power and extinction ratio at several temperature points. • Record the LTC5100’s temperature reading, T_int, at each temperature point. • Select a convenient value for T_nom, the nominal temperature. (It is customary, but not mandatory, to use 25°C for the nominal temperature.) • Find the best values of TC1 and TC2 by fitting the quadratic temperature compensation formula (Equation 12) to the experimental values of Ib_nom, Im_nom, Imd_nom, and T_int. To configure the LTC5100 for normal operation, set the nominal current to the value found at the nominal temperature. Set TC1 and TC2 to the values determined by the best fit of the data. For standalone operation, store these values in the EEPROM. For microprocessor operation, store the values in the microprocessor’s internal nonvolatile memory or in another source of nonvolatile memory and load them into the LTC5100 after power-up. The above procedure not only corrects for the laser temperature drift, but also corrects the small temperature drift found in the LTC5100’s internal references. DEMONSTRATION BOARD Figure 33 shows the schematic of the DC499 demonstration board. Details of the use of this demo board and accompanying software can be found in the DC499 demo board manual. Figure 34 shows the layout of the demo board and Table 31 gives the bill of materials for the demo board. The core applications circuit for the LTC5100 VCSEL driver appears inside the box in Figure 33. This is the complete circuit for an optical transceiver module, including power supply filtering. It consists of the LTC5100 with EEPROM for storing setup parameters, L1 and C3 for power supply filtering, and R1, C1, and C2 for terminating the 50Ω modulation output. The circuitry outside the box in Figure 33 is for support of the demonstration. 5V power enters through 2-pin connector P2 and is regulated by U3 to 3.3V to power the LTC5100. Connector P1 sends 5V power and serial control signals to another board, allowing a personal computer to control the LTC5100. U4 produces 1.8VDC to bias the modulation output for electrical eye measurements. High speed data enters the LTC5100 through SMA connectors J1 and J2. The LTC5100 high speed inputs are internally AC coupled with rail-to-rail common mode input voltage range. The input signal swing can go as much as 300mV above VDD or below VSS without degrading performance or causing excessive current flow. The high speed inputs may be AC coupled, in which case the common mode voltage floats to mid-supply or 1.65V nominally. A common cathode VCSEL can be attached to the demo board via SMA connector J3. R1 establishes a precision, low reflection coefficient 50Ω modulation drive. By maintaining a wide band microwave quality 50Ω path, the length of the connection to the laser can be arbitrarily long. The LTC5100 generates 20% to 80% transition times of 60ps (80ps 10% to 90%), corresponding to an instantaneous transition filtered by a 4.4GHz Gaussian lowpass filter. At these speeds the primary limitation on line length is high frequency loss. For high grade, low loss laboratory cabling with silver coated center conductor and foamed PTFE dielectric, a practical limit is about 30cm. sn5100 5100fs 49 LTC5100 U W U U APPLICATIO S I FOR ATIO The laser’s monitor diode (if needed) can be attached to either pin of 2-pin header H2 (labeled MD) or to the test turret labeled MD. H2 is a 2mm, 2-pin header with 0.5mm square pins. The demo board includes an EEPROM that provides nonvolatile storage for the LTC5100’s configuration settings and parameters. For example, the EEPROM stores parameters for the laser bias and modulation levels as well as temperature coefficients and fault detection modes. The LTC5100 transfers the data in the EEPROM to its internal registers at power up. The LTC5100 is designed for hot plugging and can be configured to load the EEPROM and enable the transmitter as soon as power is applied. Be careful with this mode of operation! It is possible to leave the EEPROM in a state that automatically turns the laser on at power up. The LTC5100’s FAULT output is available at the test turret labeled “FAULT.” The FAULT pin can be software configured with several output pull-up options, including open drain. The demo board has three jumpers for enabling the transmitter, observing the electrical eye diagram, and measuring the LTC5100’s power supply current. Details of the use of these jumpers are given in the DC499 demo manual. 5V P2 5V GND U3 VDD1 LT1762EMS8-3.3 1 3.3V 8 1 OUT IN 2 7 + NC SENSE C4 3 6 10µF NC BYP NC 4 5 SHDN GND VCC 5V ±5% 150mA MAX VDD2 IDD JP3 R2 22.1k 2 C5 10µF + 3 R3 26.7k D3 5 + V+ 4 V– – 2 + C6 10µF EN SRC R4 10Ω 1 VOUT 3A SCHOTTKY D2 U4 LT1812 C7 0.1µF 1.8V R5 22.1k MD REMOVE JUMPER BEFORE ATTACHING A LASER DIODE! 3A SCHOTTKY H2 MD ENABLE TRANS 1 1 1 2 ELEC EYE 2 JP1 1 C1 10nF VDD SCL 50Ω IN – 4 J2 SMA P1 IN VSS U1 LTC5100 MD MODA MODB VSS 5 6 7 8 12 TERMINATION RESISTOR R1 49.9Ω C2 10nF (OPTIONAL) 11 J3 SMA 10 9 50Ω C3 10nF H3 GND GND VSS VCC SCL SDA GND SCL NC SDA 5V SDA SCL GND1 GND2 FAULT – VSS VDD(HS) GND IN + SCL SDA EN VDD 3 VSS SDA 2 FAULT 1 50Ω IN + SRC 16 15 14 13 J1 SMA JP2 L1 BLM15AG121PN1D U2 24LC00 EEPROM 5-LEAD SOT23 PACKAGE 128 BITS LTC5100 CORE APPLICATIONS CIRCUIT 5100 F33 Figure 33. Schematic Diagram of the DC499 Demo Board sn5100 5100fs 50 LTC5100 U W U U APPLICATIO S I FOR ATIO Figure 34 Layout of the DC499 Demo Board (Silkscreen and Top Layer Copper) Table 31. Bill of Materials for the DC499 Demo Board REFERENCE QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE Mill-Max (516) 922-6000 Murata (770) 436-1300 5V, VDD1, VDD2, SDA, SCL, FAULT, EN, SRC, MD, GND(3) 12 2501-2 1-Pin Terminal Turret Test Point C1, C2, C3 3 GRP155R71E103JA01 0.01µF 25V 5% X7R 0402 Capacitor C4, C5, C6 3 12066D106MAT 10µF 6.3V 20% X5R 1206 Capacitor AVX (843) 946-0362 C7 1 0603YC104KAT 0.1µF 16V 10% X7R 0603 Capacitor AVX (843) 946-0362 D2,D3 2 B320A 3A Schottky Rectifier Diode Diodes, Inc. (805) 446-4800 D4 0 Option (No Load) N/A (No Load) H2, JP1, JP2, JP3 4 2802S-02G2 2mm 2-Pin Header H3 1 2802S-03G2 2mm 3-Pin Header J1, J2, J3 3 142-0701-851 50Ω SMA Edge-Lanch Connector L1 1 BLM15AG121PN1D P1 1 P2 1 R1 1 N/A Comm Con (626) 301-4200 Comm Con (626) 301-4200 Johnson Components (800) 247-8256 0402 Ferrite Bead Murata (770) 436-1300 70553-0004 5-Pin Right Angle Header Molex (630) 969-4550 70553-0001 2-Pin Right Angle Header Molex (630) 969-4550 CR05-49R9FM 49.9Ω 1% 1/16W 0402 Resistor AAC (800) 508-1521 R2, R5 2 CR16-2212FM 22.1k 1% 1/16W 0603 Resistor AAC (800) 508-1521 R3 1 CR16-2672FM 26.7k 1% 1/16W 0603 Resistor AAC (800) 508-1521 R4 1 CR16-10R0FM 10Ω 1% 1/16W 0603 Resistor AAC (800) 508-1521 U1 1 LTC5100 QFN 4mm × 4mm IC LTC (408) 432-1900 U2 1 24LC00 128-Bit IC Bus Serial EEPROM 5-Pin SOT-23 U3 1 LT1762EMS8-3.3 Low Noise LDO Micropower Regulator IC U4 1 LT1812CS5 Op Amp with Shutdown IC H3 1 CCIJ2mm-138G 2-Pin 2mm Shunt Microchip LTC (408) 432-1900 LTC (408) 432-1900 Comm Con (626) 301-4200 sn5100 5100fs Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 51 LTC5100 U PACKAGE DESCRIPTIO UF Package 16-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1692) BOTTOM VIEW—EXPOSED PAD 4.00 ± 0.10 (4 SIDES) 0.72 ±0.05 0.75 ± 0.05 R = 0.115 TYP 0.55 ± 0.20 15 16 PIN 1 TOP MARK 1 4.35 ± 0.05 2.15 ± 0.05 2.90 ± 0.05 (4 SIDES) 2.15 ± 0.10 (4-SIDES) PACKAGE OUTLINE (UF) QFN 0802 0.30 ± 0.05 0.200 REF 0.30 ±0.05 0.65 BCS RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 2 0.00 – 0.05 0.65 BSC NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC) 2. ALL DIMENSIONS ARE IN MILLIMETERS 3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 4. EXPOSED PAD SHALL BE SOLDER PLATED RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1773 Current Mode Synchronous Buck Regulator Design Note 295 “High Efficiency Adaptable Power Supply for XENPAK 10Gbps Ethernet Transceivers” LTC1923 High Efficiency Thermoelectric Cooler Controller ® LT 1930A 2.2MHz Step-Up DC/DC Converter in 5-Lead SOT-23 Design Note 273 “Fiber Optic Communication Systems Benefit from Tiny, Low Noise Avalanche Photodiode Bias Supply” sn5100 5100fs 52 Linear Technology Corporation LT/TP 0903 1K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2003