3.3 V Dual-Loop, 50 Mbps to 3.3 Gbps Laser Diode Driver ADN2870 FEATURES GENERAL DESCRIPTION SFP/SFF and SFF-8472 MSA-compliant SFP reference design available 50 Mbps to 3.3 Gbps operation Multirate 155 Mbps to 3.3 Gbps operation Dual-loop control of average power and extinction ratio Typical rise/fall time 60 ps Bias current range 2 mA to 100 mA Modulation current range 5 mA to 90 mA Laser fail alarm and automatic laser shutdown (ALS) Bias and modulation current monitoring 3.3 V operation 4 mm × 4 mm LFCSP package Voltage setpoint control Resistor setpoint control The ADN2870 laser diode driver is designed for advanced SFP and SFF modules, using SFF-8472 digital diagnostics. The device features dual-loop control of the average power and extinction ratio, which automatically compensates for variations in laser characteristics over temperature and aging. The laser need only be calibrated at 25°C, eliminating the need for expensive and time consuming temperature calibration. The ADN2870 supports single-rate operation from 50 Mbps to 3.3 Gbps or multirate from 155 Mbps to 3.3 Gbps. Average power and extinction ratio can be set with a voltage provided by a microcontroller DAC or by a trimmable resistor. The part provides bias and modulation current monitoring as well as fail alarms and automatic laser shutdown. The device interfaces easily with the ADI ADuC70xx family of microconverters and with the ADN289x family of limiting amplifiers to make a complete SFP/SFF transceiver solution. An SFP reference design is available. The product is available in a spacesaving 4 mm ×4 mm LFCSP package specified over the −40°C to +85°C temperature range. APPLICATIONS Multirate OC3 to OC48-FEC SFP/SFF modules 1×/2×/4× Fibre channel SFP/SFF modules LX-4 modules DWDM/CWDM SFP modules 1GE SFP/SFF transceiver modules VCC VCC VCC Tx_FAULT VCC L Tx_FAIL VCC FAIL ALS IMODN R MPD LASER IMODP DATAP PAVSET ADI MICROCONTROLLER PAVREF DAC IBIAS CONTROL ADC IBIAS RPAV 1kΩ DAC DATAN 100Ω IMOD CCBIAS GND ERREF ADN2870 1kΩ ERSET GND IBMON IMMON PAVCAP ERCAP VCC GND 1kΩ GND GND 470Ω GND 04510-001 GND Figure 1. Application Diagram Showing Microcontroller Interface Protected by US patent: US6414974 Rev. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.326.8703 © 2004 Analog Devices, Inc. All rights reserved. ADN2870 TABLE OF CONTENTS Specifications..................................................................................... 3 Voltage Setpoint Calibration..................................................... 12 SFP Timing Specifications............................................................... 5 Resistor Setpoint Calibration.................................................... 14 Absolute Maximum Ratings............................................................ 6 IMPD Monitoring ...................................................................... 14 ESD Caution.................................................................................. 6 Loop Bandwidth Selection ........................................................ 15 Pin Configuration and Function Descriptions............................. 7 Power Consumption .................................................................. 15 Typical Operating Characteristics.................................................. 8 Automatic Laser Shutdown (TX_Disable).............................. 15 Optical Waveforms Showing Multirate Performance Using Low Cost Fabry Perot Tosa NEC NX7315UA .......................... 8 Bias and Modulation Monitor Currents.................................. 15 Data Inputs.................................................................................. 15 Optical Waveforms Showing Dual-Loop Performance Over Temperature Using DFB Tosa SUMITOMO SLT2486............ 8 Laser Diode Interfacing............................................................. 16 Performance Characteristics....................................................... 9 Alarms.......................................................................................... 17 Theory of Operation ...................................................................... 11 Outline Dimensions ....................................................................... 18 Dual-Loop Control .................................................................... 11 Ordering Guide .......................................................................... 18 Control......................................................................................... 12 REVISION HISTORY 8/04—Revision 0: Initial Version Rev. 0 | Page 2 of 20 ADN2870 SPECIFICATIONS VCC = 3.0 V to 3.6 V. All specifications TMIN to TMAX,1 unless otherwise noted. Typical values as specified at 25°C. Table 1. Parameter LASER BIAS CURRENT (IBIAS) Output Current IBIAS Compliance Voltage IBIAS when ALS is High CCBIAS Compliance Voltage MODULATION CURRENT (IMODP, IMODN)2 Output Current IMOD Compliance Voltage IMOD when ALS is High Rise Time2, 3 Fall Time2, 3 Random Jitter2, 3 Deterministic Jitter2, 3 Pulse-Width Distortion2, 3 AVERAGE POWER SET (PAVSET) Pin Capacitance Voltage Photodiode Monitor Current (Average Current) EXTINCTION RATIO SET INPUT (ERSET) Resistance Range Voltage AVERAGE POWER REFERENCE VOLTAGE INPUT (PAVREF) Voltage Range Photodiode Monitor Current (Average Current) EXTINCTION RATIO REFERENCE VOLTAGE INPUT (ERREF) Voltage Range DATA INPUTS (DATAP, DATAN)4 V p-p (Differential) Input Impedance (Single-Ended) LOGIC INPUTS (ALS) VIH VIL ALARM OUTPUT (FAIL)5 VOFF VON Min Typ Max Unit 100 VCC 0.2 mA V mA V 90 VCC 0.05 104 96 1.1 35 30 mA V mA ps ps ps ps ps rms 20 mA < IMOD < 90 mA 20 mA < IMOD < 90 mA 80 1.35 1200 pF V µA Resistor setpoint mode 25 1.35 kΩ V Resistor setpoint mode Resistor setpoint mode 0.12 1 V 120 1000 µA Voltage setpoint mode (RPAV fixed at 1 kΩ) Voltage setpoint mode (RPAV fixed at 1 kΩ) 0.1 1 V Voltage setpoint mode (RERSET fixed at 1 kΩ) 2.4 V Ω AC-coupled 2 1.2 1.2 5 1.5 60 60 0.8 1.1 50 1.2 1.1 1.2 1.2 0.4 50 2 0.8 V V > 1.8 V < 1.3 V Rev. 0 | Page 3 of 20 Conditions/Comments Voltage required at FAIL for Ibias and Imod to turn off when FAIL asserted Voltage required at FAIL for Ibias and Imod to stay on when FAIL asserted ADN2870 Parameter IBMON, IMMON DIVISION RATIO IBIAS/IBMON3 IBIAS/IBMON3 IBIAS/IBMON STABILITY3, 6 IMOD/IMMON IBMON Compliance Voltage SUPPLY ICC7 VCC (w.r.t. GND)8 Min Typ Max Unit Conditions/Comments 85 92 100 100 115 108 ±5 A/A A/A % A/A V 11 mA < IBIAS < 50 mA 50 mA < IBIAS < 100 mA 10 mA < IBIAS < 100 mA mA V When IBIAS = IMOD = 0 50 0 1.3 30 3.3 3.0 3.6 1 Temperature range: –40°C to +85°C. Measured into a 15 Ω load (22 Ω resistor in parallel with digital scope 50 Ω input) using a 11110000 pattern at 2.5 Gbps, shown in Figure 2. 3 Guaranteed by design and characterization. Not production tested. 4 When the voltage on DATAP is greater than the voltage on DATAN, the modulation current flows in the IMODP pin. 5 Guaranteed by design. Not production tested. 6 IBIAS/IBMON ratio stability is defined in SFF-8472 revision 9 over temperature and supply variation. 7 ICC min for power calculation in the Power Consumption section. 8 All VCC pins should be shorted together. 2 ADN2870 R 22Ω VCC L C IMODP BIAS TEE 80kHz → 27GHz TO HIGH SPEED DIGITAL OSCILLOSCOPE 50Ω INPUT Figure 2. High Speed Electrical Test Output Circuit Rev. 0 | Page 4 of 20 04510-034 VCC ADN2870 SFP TIMING SPECIFICATIONS Table 2. Parameter ALS Assert Time Symbol t_off ALS Negate Time1 Time to Initialize, Including Reset of FAIL1 FAIL Assert Time ALS to Reset time Typ 1 Max 5 Unit µs t_on 0.83 0.95 ms t_init 25 275 ms Conditions/Comments Time for the rising edge of ALS (TX_DISABLE) to when the bias current falls below 10% of nominal. Time for the falling edge of ALS to when the modulation current rises above 90% of nominal. From power-on or negation of FAIL using ALS. 100 5 µs µs Time to fault to FAIL on. Time TX_DISABLE must be held high to reset TX_FAULT. t_fault t_reset Guaranteed by design and characterization. Not production tested. VSE SFP MODULE 1µH VCC_Tx DATAN 3.3V 0.1µF 0.1µF 10µF SFP HOST BOARD DATAP–DATAN 0V V p-p DIFF = 2 × VSE Figure 3. Signal Level Definition Rev. 0 | Page 5 of 20 Figure 4. Recommended SFP Supply 04510-003 DATAP 04510-002 1 Min ADN2870 ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. Table 3. Parameter VCC to GND IMODN, IMODP PAVCAP ERCAP PAVSET PAVREF ERREF IBIAS IBMON IMMON ALS CCBIAS RPAV ERSET FAIL DATAP, DATAN (single-ended differential) TEMPERATURE SPECIFICATIONS Operating Temperature Range Industrial Storage Temperature Range Junction Temperature (TJ max) LFCSP Package Power Dissipation1 θJA Thermal Impedance2 θJCThermal Impedance Lead Temperature (Soldering 10 s) Rating 4. 2 V –0.3 V to +4.8 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V –0.3 V to +3.9 V 1.5 V Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. −40°C to +85°C –65°C to +150°C 150°C (TJ max – TA)/θJA W 30°C/W 29.5°C/W 300°C ___________________ 1 Power consumption equations are provided in the Power Consumption section. 2 θJA is defined when part is soldered on a 4-layer board. ESD CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. Rev. 0 | Page 6 of 20 ADN2870 ERSET IMMON ERREF VCC IBMON 18 FAIL PIN CONFIGURATION AND FUNCTION DESCRIPTIONS 13 12 19 GND ALS VCC DATAN IMODP DATAP ADN2870 IMODN GND PAVCAP GND ERCAP IBIAS 7 6 04510-004 RPAV VCC PAVREF GND CCBIAS 1 PAVSET 24 Figure 5. Pin Configuration Table 4. Pin Fuction Descriptions Pin No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Mnemonic CCBIAS PAVSET GND VCC PAVREF RPAV ERCAP PAVCAP GND DATAP DATAN ALS ERSET IMMON ERREF VCC IBMON FAIL GND VCC IMODP IMODN GND IBIAS Description Control Output Current Average Optical Power Set Pin Supply Ground Supply Voltage Reference Voltage Input for Average Optical Power Control Average Power Resistor when Using PAVREF Extinction Ratio Loop Capacitor Average Power Loop Capacitor Supply Ground Data, Positive Differential Input Data, Negative Differential Input Automatic Laser Shutdown Extinction Ratio Set Pin Modulation Current Monitor Current Source Reference Voltage Input for Extinction Ratio Control Supply Voltage Bias Current Monitor Current Source FAIL Alarm Output Supply Ground Supply Voltage Modulation Current Positive Output, Connect to Laser Diode Modulation Current Negative Output Supply Ground Laser Diode Bias (Current Sink to Ground) Note: The LFCSP package has an exposed paddle that must be connected to ground. Rev. 0 | Page 7 of 20 ADN2870 TYPICAL OPERATING CHARACTERISTICS VCC = 3.3 V and TA = 25°C, unless otherwise noted. OPTICAL WAVEFORMS SHOWING MULTIRATE PERFORMANCE USING LOW COST FABRY PEROT TOSA NEC NX7315UA OPTICAL WAVEFORMS SHOWING DUAL-LOOP PERFORMANCE OVER TEMPERATURE USING DFB TOSA SUMITOMO SLT2486 Note: No change to PAVCAP and ERCAP values (ACQ LIMIT TEST) WAVEFORMS 1001 04510-016 04510-047 (ACQ LIMIT TEST) WAVEFORMS 1000 Figure 9. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 231-1 PAV = 0 dBm, ER = 9 dB, Mask Margin 22%, TA = 25°C Figure 6. Optical Eye 2.488 Gbps,65 ps/div, PRBS 231-1 PAV = −4.5 dBm, ER = 9 dB, Mask Margin 25% (ACQ LIMIT TEST) WAVEFORMS 1000 04510-048 04510-017 (ACQ LIMIT TEST) WAVEFORMS 1001 Figure 7. Optical Eye 622 Mbps, 264 ps/div, PRBS 231-1 PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50% Figure 10. Optical Eye 2.488 Gbps, 65 ps/div, PRBS 231-1 PAV = −0.2 dBm, ER = 8.96 dB, Mask Margin 21%, TA = 85°C 04510-020 (ACQ LIMIT TEST) WAVEFORMS 1000 Figure 8. Optical Eye 155 Mbps,1.078 ns/div, PRBS 231-1 PAV = −4.5 dBm, ER = 9 dB, Mask Margin 50% Rev. 0 | Page 8 of 20 ADN2870 PERFORMANCE CHARACTERISTICS 1.2 90 1.0 0.8 JITTER (rms) RISE TIME (ps) 60 0.6 0.4 30 0 0 20 40 60 MODULATION CURRENT (mA) 80 04510-037 04510-022 0.2 0 0 100 Figure 11. Rise Time vs. Modulation Current, Ibias = 20 mA 20 40 60 MODULATION CURRENT (mA) 80 100 Figure 14. Random Jitter vs. Modulation Current, Ibias = 20 mA 250 80 TOTAL SUPPLY CURRENT (mA) 220 0 0 20 40 60 MODULATION CURRENT (mA) 80 IBIAS = 20mA 100 40 0 100 Figure 12. Fall Time vs. Modulation Current, Ibias = 20 mA 45 60 40 55 SUPPLY CURRENT (mA) 35 30 25 20 15 10 5 0 20 40 60 80 MODULATION CURRENT (mA) 20 40 60 MODULATION CURRENT (mA) 80 100 Figure 15. Total Supply Current vs. Modulation Current Total Supply Current = ICC + Ibias + Imod 04510-042 DETERMINISTIC JITTER (ps) 130 70 04510-025 20 IBIAS = 40mA 160 04510-038 40 IBIAS = 80mA 190 100 Figure 13. Deterministic Jitter vs. Modulation Current, Ibias = 20 mA Rev. 0 | Page 9 of 20 50 45 40 35 30 25 20 –50 04510-027 FALL TIME (ps) 60 –30 –10 10 30 50 TEMPERATURE (°C) 70 90 110 Figure 16. Supply Current (ICC) vs. Temperature with ALS Asserted, Ibias = 20 mA ADN2870 60 120 58 115 56 IMOD/IMMON RATIO 105 100 95 54 52 50 48 46 90 04510-028 80 –50 –30 –10 10 30 50 TEMPERATURE (°C) 70 90 110 Figure 17. IBIAS/IBMON Gain vs. Temperature, Ibias = 20 mA 04510-031 44 85 42 40 –50 –30 –10 10 30 50 TEMPERATURE (°C) 70 90 110 Figure 20. IMOD/IMMON Gain vs. Temperature, Imod = 30 mA OC48 PRBS31 DATA TRANSMISSION t_OFF LESS THAN 1µs FAIL ASSERTED FAULT FORCED ON PAVSET 04510-045 04510-029 ALS Figure 18. ALS Assert Time, 5 µs/div Figure 21. FAIL Assert Time,1 µs/div OC48 PRBS31 DATA TRANSMISSION TRANSMISSION ON t_ON ALS 04510-046 POWER SUPPLY TURN ON 04510-032 IBIAS/IBMON RATIO 110 Figure 19. ALS Negate Time, 200 µs/div Figure 22. Time to Initialize, Including Reset, 40 ms/div Rev. 0 | Page 10 of 20 ADN2870 THEORY OF OPERATION Gm Φ2 BIAS SHA Φ1 VCC BIAS CURRENT 1.2V VBGAP IPA HIGH SPEED SWITCH ERSET P1 ER = PO P1 + PO PAV = 2 PAVSET Φ2 MOD SHA Φ2 MOD CURRENT 100 ∆P PAV LI = PO Ith 2 Figure 24. Dual-Loop Control of Average Power and Extinction Ratio ∆I ∆P ∆I A dual loop is made up of an APCL (average power control loop) and the ERCL (extinction ratio control loop), which are separated into two time states. During time Φ1, the APC loop is operating, and during time Φ2, the ER loop is operating. 04510-005 OPTICAL POWER IEX P1 OPTICAL COUPLING MPD INPUT 04510-039 Laser diodes have a current-in to light-out transfer function as shown in Figure 23. Two key characteristics of this transfer function are the threshold current, Ith, and slope in the linear region beyond the threshold current, referred to as slope efficiency, LI. CURRENT Figure 23. Laser Transfer Function Average Power Control Loop DUAL-LOOP CONTROL Typically laser threshold current and slope efficiency are both functions of temperature. For FP and DFB type lasers the threshold current increases and the slope efficiency decreases with increasing temperature. In addition, these parameters vary as the laser ages. To maintain a constant optical average power and a constant optical extinction ratio over temperature and laser lifetime, it is necessary to vary the applied electrical bias current and modulation current to compensate for the lasers changing LI characteristics. The APCL compensates for changes in Ith and LI by varying Ibias. APC control is performed by measuring MPD current, Impd. This current is bandwidth-limited by the MPD. This is not a problem because the APCL must be low frequency since the APCL must respond to the average current from the MPD. The APCL compares Impd × Rpavset to the BGAP voltage, Vbgap. If Impd falls, the bias current is increased until Impd × Rpavset equals Vbgap. Conversely, if the Impd increases, Ibias is decreased. Modulation Control Loop Single-loop compensation schemes use the average monitor photodiode current to measure and maintain the average optical output power over temperature and laser aging. The ADN2870 is a dual-loop device, implementing both this primary average power control loop and, additionally, a secondary control loop, which maintains constant optical extinction ratio. The dual-loop control of average power and extinction ratio implemented in the ADN2870 can be used successfully both with lasers that maintain good linearity of LI transfer characteristics over temperature and with those that exhibit increasing nonlinearity of the LI characteristics over temperature. Dual Loop The ADN2870 uses a proprietary patented method to control both average power and extinction ratio. The ADN2870 is constantly sending a test signal on the modulation current signal and reading the resulting change in the MPD current as a means of detecting the slope of the laser in real time. This information is used in a servo to control the ER of the laser, which is done in a time-multiplexed manner at a low frequency, typically 80 Hz. Figure 24 shows the dual-loop control implementation on the ADN2870. The ERCL measures the slope efficiency, LI, of the LD, and changes Imod as LI changes. During the ERCL, Imod is temporarily increased by ∆Imod. The ratio between Imod and ∆Imod is a fixed ratio of 50:1, but during startup, this ratio is increased in order to decrease settling time. During ERCL, switching in ∆Imod causes a temporary increase in average optical power, ∆Pav. However the APC loop is disabled during ERCL, and the increase is kept small enough so as not to disturb the optical eye. When ∆Imod is switched into the laser circuit, an equal current, Iex, is switched into the PAVSET resistor. The user sets the value of Iex; this is the ERSET setpoint. If ∆Impd is too small, the control loop knows that LI has decreased and increases Imod and, therefore, ∆Imod accordingly until ∆Impd is equal to Iex. The previous time state values of the bias and mod settings are stored on the hold capacitors PAVCAP and ERCAP. The ERCL is constantly measuring the actual LI curve, therefore it compensates for the effects of temperature and for changes in the LI curve due to laser aging. Thus the laser may be calibrated once at 25°C and can then automatically control the laser over temperature. This eliminates expensive and time consuming temperature calibration of the laser. Rev. 0 | Page 11 of 20 ADN2870 Operation with Lasers with Temperature-Dependent Nonlinearity of Laser LI Curve The ADN2870 ERCL extracts information from the monitor photodiode signal relating to the slope of the LI characteristics at the optical 1 level (P1). For lasers with good linearity over temperature, the slope measured by the ADN2870 at the optical 1 level is representative of the slope anywhere on the LI curve. This slope information is used to set the required modulation current to achieve the required optical extinction ratio. 4.0 OPTICAL POWER (mW) RELATIVELY LINEAR LI CURVE AT 25°C The ER correction scheme, while using the average nonlinearity for the laser population, in fact, supplies a corrective measurement based on each laser’s actual performance as measured during operation. The ER correction scheme corrects for errors due to laser nonlinearity while the dual loop continues to adjust for changes in the Laser LI. For more details on maintaining average optical power and extinction ratio over temperature when working with lasers displaying a temperature dependant nonlinearity of LI curve, see Application Note AN-743. 3.5 CONTROL 3.0 The ADN2870 has two methods for setting the average power (PAV) and extinction ratio (ER). The average power and extinction ratio can be voltage-set using a microcontroller’s voltage DACs outputs to provide controlled reference voltages PAVREF and ERREF. Alternatively, the average power and extinction ratio can be resistor-set using potentiometers at the PAVSET and ERSET pins, respectively. 2.5 2.0 1.5 1.0 NONLINEAR LI CURVE AT 80°C VOLTAGE SETPOINT CALIBRATION 04510-008 0.5 0 0 20 40 60 CURRENT (mA) 80 100 Figure 25. Measurement of a Laser LI Curve Showing Laser Nonlinearity at High Temperatures Some types of laser have LI curves that become progressively more nonlinear with increasing temperature (see Figure 25). At temperatures where the LI curve shows significant nonlinearity, the LI curve slope measured by the ADN2870 at the optical 1 level is no longer representative of the overall LI curve. It is evident that applying a modulation current based on this slope information cannot maintain a constant extinction ratio over temperature. However, the ADN2870 can be configured to maintain near constant optical bias and extinction ratio with a laser exhibiting a monotonic temperature-dependant nonlinearity. To implement this correction, it is necessary to characterize a small sample of lasers for their typical nonlinearity by measuring them at two temperature points, typically 25°C and 85°C. The measured nonlinearity is used to determine the amount of feedback to apply. Typically one must characterize 5 to 10 lasers of a particular model to get a good number. Then the product can be calibrated at 25°C only, avoiding the expense of temperature calibration. Typically the microcontroller supervisor is used to measure the laser and apply the feedback. This scheme is particularly suitable for circuits that already use a microcontroller for control and digital diagnostic monitoring. The ADN2870 allows interface to a microcontroller for both control and monitoring (see Figure 26). The average power at the PAVSET pin and extinction ratio at the ERSET pin can be set using the microcontroller’s DACs to provide controlled reference voltages PAVREF and ERREF. Note that during power up, there is an internal sequence that allows 25 ms before enabling the alarms; therefore the customer must ensure that the voltage for PAVREF and ERREF are active within 20 ms. PAVREF = PAV × RSP × RPAV (Volts) ERREF = R ERSET × I MPD _ CW PCW × ER − 1 × PAV (Volts) ER + 1 where: RSP is the monitor photodiode responsivity. PCW is the dc optical power specified on the laser data sheet. IMPD_CW is MPD current at that specified PCW. PAV is the average power required. ER is the desired extinction ratio (ER = P1/P0). In voltage setpoint, RPAV and RERSET must be 1 kΩ resistors with a 1% tolerance and a temperature coefficient of 50 ppm/°C. Rev. 0 | Page 12 of 20 ADN2870 VCC VCC VCC Tx_FAULT VCC L Tx_FAIL VCC FAIL ALS IMODN LASER R MPD IMODP DATAP PAVSET ADI MICROCONTROLLER DATAN 100Ω IMOD PAVREF DAC IBIAS CONTROL ADC IBIAS RPAV 1kΩ CCBIAS GND ERREF DAC ADN2870 1kΩ ERSET GND IBMON IMMON PAVCAP ERCAP VCC GND GND 470Ω GND 04510-009 1kΩ GND GND Figure 26. ADN2870 Using Microconverter Calibration and Monitoring VCC VCC VCC VCC L FAIL VCC ALS IMODN R LASER VCC IMODP PAVREF DATAP MPD RPAV PAVSET DATAN 100Ω IMOD IBIAS CONTROL IBIAS GND CCBIAS ERSET GND ADN2870 VCC ERREF IMMON 1kΩ 470Ω GND PAVCAP GND ERCAP GND GND Figure 27. ADN2870 Using Resistor Setpoint Calibration of Average Power and Energy Ratio Rev. 0 | Page 13 of 20 04510-010 VCC GND IBMON ADN2870 RESISTOR SETPOINT CALIBRATION Method 2: Measuring IMPD Across a Sense Resistor In resistor setpoint calibration. PAVREF, ERREF, and RPAV must all be tied to VCC. Average power and extinction ratio can be set using the PAVSET and ERSET pins, respectively. A resistor is placed between the pin and GND to set the current flowing in each pin as shown in Figure 27. The ADN2870 ensures that both PAVSET and ERSET are kept 1.2 V above GND. The PAVSET and ERSET resistors are given by the following: The second method has the advantage of providing a valid IMPD reading at all times, but has the disadvantage of requiring a differential measurement across a sense resistor directly in series with the IMPD. As shown in Figure 29, a small resistor, Rx, is placed in series with the IMPD. If the laser used in the design has a pinout where the monitor photodiode cathode and the lasers anode are not connected, a sense resistor can be placed in series with the photodiode cathode and VCC as shown in Figure 30. When choosing the value of the resistor, the user must take into account the expected IMPD value in normal operation. The resistor must be large enough to make a significant signal for the buffered A/Ds to read, but small enough so as not to cause a significant voltage reduction across the IMPD. The voltage across the sense resistor should not exceed 250 mV when the laser is in normal operation. It is recommended that a 10 pF capacitor be placed in parallel with the sense resistor. R ERSET = 1.23 V PAV × RSP (Ω) 1.23 V I MPD _ CW ER − 1 + PAV × P CW ER + 1 (Ω) where: RSP is the monitor photodiode responsivity. PCW is the dc optical power specified on the laser data sheet. IMPD_CW is MPD current at that specified PCW. PAV is the average power required. ER is the desired extinction ratio (ER = P1/P0). VCC PHOTODIODE µC ADC DIFFERENTIAL INPUT IMPD MONITORING IMPD monitoring can be implemented for voltage setpoint and resistor setpoint as follows. LD 200Ω RESISTOR 10pF PAVSET 04510-011 RPAVSET = ADN2870 Voltage Setpoint In voltage setpoint calibration, the following methods may be used for IMPD monitoring. Figure 29. Differential Measurement of IMPD Across a Sense Resistor VCC VCC Method 1: Measuring Voltage at RPAV 200Ω RESISTOR The IMPD current is equal to the voltage at RPAV divided by the value of RPAV (see Figure 28) as long as the laser is on and is being controlled by the control loop. This method does not provide a valid IMPD reading when the laser is in shut-down or fail mode. A microconverter buffered A/D input may be connected to RPAV to make this measurement. No decoupling or filter capacitors should be placed on the RPAV node because this can disturb the control loop. VCC PAVSET ADN2870 RPAV 04510-043 INPUT R 1kΩ INPUT PHOTODIODE ADN2870 04510-011 PAVSET Figure 30. Single Measurement of IMPD Across a Sense Resistor Resistor Setpoint PHOTODIODE µC ADC LD µC ADC Figure 28. Single Measurement of IMPD RPAV in Voltage Setpoint Mode In resistor setpoint calibration, the current through the resistor from PAVSET to ground is the IMPD current. The recommended method for measuring the IMPD current is to place a small resistor in series with PAVSET resistor (or potentiometer) and measure the voltage across this resistor as shown in Figure 31. The IMPD current is then equal to this voltage divided by the value of resistor used. In resistor setpoint, PAVSET is held to 1.2 V nominal; it is recommended that the sense resistor should be selected so that the voltage across the sense resistor does not exceed 250 mV. Rev. 0 | Page 14 of 20 ADN2870 VCC POWER CONSUMPTION PHOTODIODE The ADN2870 die temperature must be kept below 125°C. The LFCSP package has an exposed paddle, which should be connected such that is at the same potential as the ADN2870 ground pins. Power consumption can be calculated as follows: PAVSET ADN2870 µC ADC INPUT ICC = ICC min + 0.3 IMOD 04510-040 R P = VCC × ICC + (IBIAS × VBIAS_PIN) + IMOD (VMODP_PIN + VMODN_PIN)/2 Figure 31. Single Measurement of IMPD Across a Sense Resistor in Resistor Setpoint IMPD Monitoring TDIE = TAMBIENT + θJA × P LOOP BANDWIDTH SELECTION To ensure that the ADN2870 control loops have sufficient bandwidth, the average power loop capacitor (PAVCAP) and the extinction ratio loop capacitor (ERCAP) are calculated using the lasers slope efficiency (watts/amps) and the average power required. For resistor point control: PAVCAP = 3.2 E − 6 × ERCAP = LI (Farad) PAV PAVCAP (Farad) 2 ERCAP = ICC min = 30 mA, the typical value of ICC provided in the Specifications with IBIAS = IMOD = 0. TDIE is the die temperature. TAMBIENT is the ambient temperature. VBIAS_PIN is the voltage at the IBIAS pin. VMODP_PIN is the voltage at the IMODP pin. VMODN_PIN is the voltage at the IMODN pin. ALS (TX disable) is an input that is used to shut down the transmitter optical output. The ALS pin is pulled up internally with a 6 kΩ resistor, and conforms to SFP MSA specification. When ALS is logic high or when open, both the bias and modulation currents are turned off. LI (Farad) PAV PAVCAP (Farad) 2 BIAS AND MODULATION MONITOR CURRENTS where PAV is the average power required and LI (mW/mA) is the typical slope efficiency at 25°C of a batch of lasers that are used in a design. The capacitor value equation is used to get a centered value for the particular type of laser that is used in a design and average power setting. The laser LI can vary by a factor of 7 between different physical lasers of the same type and across temperature without the need to recalculate the PAVCAP and ERCAP values. In ac coupling configuration the LI can be calculated as follows: LI = where: AUTOMATIC LASER SHUTDOWN (TX_DISABLE) For voltage setpoint control: PAVCAP = 1.28 E − 6 × Thus, the maximum combination of IBIAS + IMOD must be calculated. P1 − P0 (mW/mA) Imod IBMON and IMMON are current-controlled current sources that mirror a ratio of the bias and modulation current. The monitor bias current, IBMON, and the monitor modulation current, IMMON, should both be connected to ground through a resistor to provide a voltage proportional to the bias current and modulation current, respectively. When using a microcontroller, the voltage developed across these resistors can be connected to two of the ADC channels, making available a digital representation of the bias and modulation current. DATA INPUTS where P1 is the optical power (mW) at the one level, and P0 is the optical power (mW) at the zero level. These capacitors are placed between the PAVCAP and ERCAP pins and ground. It is important that these capacitors are low leakage multilayer ceramics with an insulation resistance greater than 100 GΩ or a time constant of 1000 sec, whichever is less. The capacitor tolerance may be ±30% from the calculated value to the available off the shelf value including the capacitors own tolerance. Data inputs should be ac-coupled (10 nF capacitors are recommended) and are terminated via a 100 Ω internal resistor between the DATAP and DATAN pins. A high impedance circuit sets the common-mode voltage and is designed to allow maximum input voltage headroom over temperature. It is necessary to use ac coupling to eliminate the need for matching between common-mode voltages. Rev. 0 | Page 15 of 20 ADN2870 LASER DIODE INTERFACING The schematic in Figure 32 describes the recommended circuit for interfacing the ADN2870 to most TO-Can or Coax lasers. These lasers typically have impedances of 5 Ω to 7 Ω, and have axial leads. The circuit shown works over the full range of data rates from 155 Mbps to 3.3 Gbps including multirate operation (with no change to PAVCAP and ERCAP values); see the Typical Operating Characteristics for multirate performance examples. Coax lasers have special characteristics that make them difficult to interface to. They tend to have higher inductance, and their impedance is not well controlled. The circuit in Figure 32 operates by deliberately misterminating the transmission line on the laser side, while providing a very high quality matching network on the driver side. The impedance of the driver side matching network is very flat versus frequency and enables multirate operation. A series damping resistor should not be used. VCC L (0.5nH) VCC C 100nF IMODP IBIAS CCBIAS Tx LINE 30Ω Tx LINE 30Ω R 24Ω C 2.2pF L 04510-014 ADN2870 BLMI8HG60ISN1D Figure 32. Recommended Interface for ADN2870 AC Coupling Care should be taken to mount the laser as close as possible to the PC board, minimizing the exposed lead length between the laser can and the edge of the board. The axial lead of a coax laser are very inductive (approximately 1 nH per mm). Long exposed leads result in slower edge rates and reduced eye margin. Recommended component layouts and gerber files are available by contacting the factory. Note that the circuit in Figure 32 can supply up to 56 mA of modulation current to the laser, sufficient for most lasers available today. Higher currents can be accommodated by changing transmission lines and backmatch values; contact factory for recommendations. This interface circuit is not recommended for butterfly-style lasers or other lasers with 25 Ω characteristic impedance. Instead, a 25 Ω transmission line and inductive (instead of resistive) pull-up is recommended; contact the factory for recommendations. The ADN2870 also supports differential drive schemes. These can be particularly useful when driving VCSELs or other lasers with slow fall times. Differential drive can be implemented by adding a few extra components. A possible implementation is shown in Figure 33. VCC L4 = BLM18HG601SN1 L1 = 0.5nH R1 = 15Ω L3 = 4.7nH C1 = C2 = 100nF TOCAN/VCSEL IMODN 20Ω TRANMISSION LINES ADN2870 R3 C3 SNUBBER LIGHT IMODP CCBIAS IBIAS R1 = 15Ω (12 TO 24Ω) L5 = 4.7nH L2 = 0.5nH L6 = BLM18HG601SN1 VCC SNUBBER SETTINGS: 40Ω AND 1.5pF, NOT OPTIMIZED, OPTIMIZATION SHOULD CONSIDER PARASITIC. Figure 33. Recommended Differential Drive Circuit Rev. 0 | Page 16 of 20 04510-041 RP 24Ω The 30 Ω transmission line used is a compromise between drive current required and total power consumed. Other transmission line values can be used, with some modification of the component values. The R and C snubber values in Figure 32, 24 Ω and 2.2 pF, respectively, represent a starting point and must be tuned for the particular model of laser being used. RP, the pull-up resistor is in series with a very small (0.5 nH) inductor. In some cases, an inductor is not required or can be accommodated with deliberate parasitic inductance, such as a thin trace or a via, placed on the PC board. ADN2870 • ALARMS The ADN2870 has a latched active high monitoring alarm (FAIL). The FAIL alarm output is an open drain in conformance to SFP MSA specification requirements. The ADN2870 has a 3-fold alarm system that covers • Use of a bias current higher than expected, probably as a result of laser aging. • Out-of-bounds average voltage at the monitor photodiode (MPD) input, indicating an indicating an excessive amount of laser power or a broken loop. Undervoltage in IBIAS node (laser diode cathode) that would increase the laser power. The bias current alarm trip point is set by selecting the value of resistor on the IBMON pin to GND. The alarm is triggered when the voltage on the IBMON pin goes above 1.2 V. FAIL is activated when the single-point faults in Table 5 occur. Table 5. ADN2870 Single-Point Alarms Alarm Type 1. Bias Current 2. MPD Current 3. Crucial Nodes Pin Name IBMON PAVSET ERREF IBIAS Over Voltage or Short to VCC Condition Alarm if > 1.2 V Alarm if > 1.7 V Alarm if shorted to VCC Ignore Under Voltage or Short to GND Condition Ignore Alarm if < 0.9 V Alarm if shorted to GND Alarm if < 600 mV Table 6. ADN2870 Response to Various Single-Point Faults in AC-Coupled Configuration as Shown in Figure 32 Pin CCBIAS PAVSET PAVREF Short to VCC Fault state occurs Fault state occurs Voltage mode: Fault state occurs Resistor mode: Tied to VCC Voltage mode: Fault state occurs Resistor mode: Tied to VCC Short to GND Fault state occurs Fault state occurs Fault state occurs Open Does not increase laser average power Fault state occurs Fault state occurs Fault state occurs ERCAP PAVCAP DATAP DATAN ALS ERSET IMMON ERREF Does not increase laser average power Fault state occurs Does not increase laser average power Does not increase laser average power Output currents shut off Does not increase laser average power Does not affect laser power Voltage mode: Fault state occurs Resistor mode: Tied to VCC IBMON FAIL IMODP IMODN IBIAS Fault state occurs Fault state occurs Does not increase laser average power Does not increase laser average power Fault state occurs Does not increase laser average power Fault state occurs Does not increase laser average power Does not increase laser average power Normal currents Does not increase laser average power Does not increase laser average power Voltage mode: Does not increase average power Resistor mode: Fault state occurs Does not increase laser average power Does not increase laser average power Does not increase laser average power Does not increase laser average power Fault state occurs Voltage mode: Fault state occurs Resistor mode: Does not increase average power Does not increase laser average power Fault state occurs Does not increase laser average power Does not increase laser average power Output currents shut off Does not increase laser average power Does not increase laser average power Does not increase laser average power RPAV Rev. 0 | Page 17 of 20 Does not increase laser average power Does not increase laser average power Does not increase laser average power Does not increase laser power Fault state occurs ADN2870 OUTLINE DIMENSIONS 0.60 MAX 4.00 BSC SQ PIN 1 INDICATOR 0.60 MAX TOP VIEW 3.75 BSC SQ 0.50 BSC 0.50 0.40 0.30 1.00 0.85 0.80 12° MAX PIN 1 INDICATOR 19 18 24 1 2.25 2.10 SQ 1.95 BOTTOM VIEW 13 12 7 6 0.25 MIN 2.50 REF 0.80 MAX 0.65 TYP 0.05 MAX 0.02 NOM SEATING PLANE 0.30 0.23 0.18 0.20 REF COPLANARITY 0.08 COMPLIANT TO JEDEC STANDARDS MO-220-VGGD-2 Figure 34. 24-Lead Lead Frame Chip Scale Package [LFCSP] (CP-24) Dimensions shown in millimeters Note: The LFCSP package has an exposed paddle that must be connected to ground. ORDERING GUIDE Model ADN2870ACPZ1 ADN2870ACPZ-RL1 ADN2870ACPZ-RL71 1 Temperature Range −40°C to +85°C −40°C to +85°C −40°C to +85°C Package Description 24-Lead Lead Frame Chip Scale Package 24-Lead Lead Frame Chip Scale Package 24-Lead Lead Frame Chip Scale Package Z = Pb-free part. Rev. 0 | Page 18 of 20 Package Option CP-24 CP-24 CP-24 Preliminary Technical Data ADN2870 NOTES Rev. 0 | Page 19 of 20 ADN2870 NOTES © 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D04510–0–8/04(0) Rev. 0 | Page 20 of 20