LT3905 Boost DC/DC Converter with APD Current Monitor Features Description 65V, 350mA Internal DMOS Switch n Integrated Schottky Diode n2% Accurate APD Current Monitoring Over the 3µA to 3mA Range n Single Resistor Programs Maximum APD Current n Fast APD Current Limiter with Indicator n Programmable Loss-of-Signal Indicator n CTRL Pin Adjusts Output Voltage n Programmable V Undervoltage Lockout IN n High Efficiency Step-Up Converter n Selectable 1MHz/2MHz Switching Frequency n Internally Compensated n Internal Soft-Start n2.7V to 12V V Range IN n Low Shutdown Current: <1µA The LT®3905 is a fixed frequency current-mode step-up converter designed to bias avalanche photodiodes (APD) in optical receivers. The LT3905 features high side APD current monitoring over four decades of dynamic range with better than 2% relative accuracy over the 3µA to 3mA range. n The maximum APD current is programmed with a single resistor, and a fast current limiter with indicator protects the APD during overload conditions. Adjustable output voltage provides dynamic bias control, and an adjustable loss-of-signal indicator flags low APD current. Coupled with the integrated DMOS power switch and Schottky rectifier, the LT3905 provides a compact total solution with few external components and low solution cost. Constant switching frequency results in predictable output noise that is easy to filter. Applications n n n n The LT3905 is available in the tiny footprint (3mm × 3mm) 16-Lead QFN Package. APD Bias PIN Diode Bias Optical Receivers and Modules Fiber Optic Network Equipment L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application Adjustable APD Bias Supply Input Current vs APD Current 10µH 1µF EN/UVLO VIN LOS_ADJ SW 634k LT3905 MONIN 220nF GND 100k LOS ILIM 1:10 LOS_MON APD 1:20 ILIM_MON 1M 12.5µA LOS 9.09k 2.75mA LIMIT 1:5 MON 4.99k 1mV/µA VAPD = 45V 100 15k CTRL VIN 100k UP TO 54V FB fSEL VOUT ADJUST VOUT 125 INPUT CURRENT (mA) VIN 2.7V to 12V APD RX POWER 3905 TA01a 75 VIN = 3.3V 50 VIN = 5V 25 0 0 0.625 1.875 1.25 IAPD (mA) 2.5 3905 TA01b 3905fa For more information www.linear.com/LT3905 1 LT3905 Absolute Maximum Ratings Pin Configuration (Note 1) LOS LOS_ADJ fSEL FB TOP VIEW VIN, FB, EN/UVLO, CTRL, fSEL, LOS_ADJ, LOS, ILIM..................................................12V VOUT, SW, MONIN, APD.............................................65V MON, LOS_MON, ILIM_MON...................................2.5V Operating Ambient Temperature Range (Note 2)....................................................... –40 to 125°C Operating Junction Temperature Range (Note 2)....................................................... –40 to 125°C Storage Temperature Range....................... –65 to 125°C 16 15 14 13 ILIM 1 12 CTRL LOS_MON 2 11 EN/UVLO 17 GND ILIM_MON 3 10 VIN 9 GND 5 6 7 8 APD MONIN SW VOUT MON 4 UD PACKAGE 16-LEAD (3mm × 3mm) PLASTIC QFN θJA = 68°C/W, θJC = 7.5°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3905EUD#PBF LT3905EUD#TRPBF LGGP 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C LT3905IUD#PBF LT3905IUD#TRPBF LGGP 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on nonstandard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 2 3905fa For more information www.linear.com/LT3905 LT3905 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = VENUVLO = VCTRL = 3.3V unless otherwise noted. PARAMETER CONDITIONS MIN Operating Voltage Range Supply Current VENUVLO = 0V, Device in Shutdown VENUVLO = 1.1V, Before Trip Point VENUVLO = 1.3V, VFB = 2V, Switcher Standby VENUVLO = 1.3V, VFB = 1.5V, Not Switching Feedback Voltage (VFB) VCTRL = 1.5V l Feedback Line Regulation 1.238 1.223 2.7V ≤ VIN ≤ 12V FB Pin Bias Current FB Standby Threshold TYP 2.7 l VFB Rising 1.65 FB Standby Threshold Hysteresis MAX V 0.1 6 250 650 1 10 350 750 µA µA µA µA 1.248 1.258 1.273 V V 0.005 0.025 %/V 15 75 nA 1.80 1.95 V 50 –13 UNITS 12 mV CTRL to FB Offset VCTRL = 0.5V l 0 13 mV CTRL Input Bias Current Current Out of Pin, VCTRL = 0.5V l 15 75 nA MON Pin Voltage Clamp IAPD = 300µA, RMON = Open l 2.35 2.5 V LOS_MON Pin Voltage Clamp IAPD = 300µA, RLOS_MON = Open l 2.35 2.5 V MON Pin Bias Current Current Out of Pin, IAPD = 0A, VMON = 1V LOS_MON Pin Bias Current Current Out of Pin, IAPD = 0A, VLOS_MON = 1V 10 50 nA ILIM_MON Pin Bias Current Current Out of Pin, IAPD = 0A, VILIM_MON = 1V 35 175 nA APD Monitor (MON) Current Gain 300nA ≤ IAPD ≤ 3µA, 12V ≤ MONIN ≤ 65V 3µA ≤ IAPD ≤ 3mA, 12V ≤ MONIN ≤ 65V l l 0.185 0.196 0.20 0.20 0.215 0.204 APD Loss of Signal Monitor (LOS_MON) Current Gain 3µA ≤ IAPD ≤ 30µA, 12V ≤ MONIN ≤ 65V 30µA ≤ IAPD ≤ 3mA, 12V ≤ MONIN ≤ 65V l l 0.092 0.096 0.10 0.10 0.108 0.104 APD Current Limit Monitor (ILIM_MON) Current Gain 30µA ≤ IAPD ≤ 300µA, 12V ≤ MONIN ≤ 65V 300µA ≤ IAPD ≤ 3mA, 12V ≤ MONIN ≤ 65V l l 0.045 0.048 0.05 0.05 0.055 0.052 APD Monitor Voltage Drop MONIN – APD at IAPD = 3mA, MONIN = 65V 3.7 3.95 4.2 V 0.9 1.8 1 2 1.1 2.2 MHz MHz 90 80 95 90 0 nA Boost Converter Switching Frequency fSEL = 0V fSEL = 2V Maximum Duty Cycle fSEL = 0V fSEL = 2V l l Switch Current Limit 350 400 Switch On-Resistance ISW = 150mA 0.75 Switch Leakage Current SW = 65V 0.1 Schottky Forward Voltage ISCHOTTKY = 150mA 780 Schottky Reverse Leakage VOUT – SW = 65V % % 450 mA Ω 3 µA mV 3 µA 1.310 V 1.050 V Loss Of Signal Comparator Loss of Signal Comparator Internal Threshold LOS_ADJ = 1.5V, LOS_MON Falling Loss of Signal Comparator External Threshold LOS_ADJ = 1.0V, LOS_MON Falling Loss of Signal Comparator Hysteresis LOS_MON Rising 1.185 l 1.248 0.950 20 mV 3905fa For more information www.linear.com/LT3905 3 LT3905 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = VENUVLO = VCTRL = 3.3V unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS 1.223 1.211 1.248 l 1.273 1.285 V V 1.323 1.311 1.348 l 1.373 1.385 V V Current Regulation Buffer Amplifier ILIM_MON Voltage Regulation Threshold (VIFB) VFB = 0.5V ILIM_MON APD Current Limit Threshold (VILIM) ILIM_MON APD Current Limit Line Regulation VAPD = 0V, ∆IAPD/∆VMONIN, 5V < VMONIN < 65V ILIM_MON Indicator Threshold VILIM_MON Rising 10 l VILIM – 30 ILIM_MON Indicator Hysteresis VILIM – 10 μA/V VILIM – 2 30 mV mV Inputs/Outputs EN/UVLO Threshold VENUVLO Falling l 1.128 1.104 1.200 1.200 1.272 1.296 V V EN/UVLO Internal Hysteresis VENUVLO Rising 25 mV EN/UVLO Hysteresis Current VENUVLO = 1.1V, Device in Shutdown VENUVLO = 1.3V, Device in Operation 3 0 µA µA fSEL Voltage Threshold fSEL Rising 0.700 fSEL Threshold Hysteresis fSEL Input Bias Current 0.900 1.100 V 50 mV 0 µA 100 Ω LOS Open Drain ON Resistance ILOS = 1mA LOS Output Low Voltage ILOS = 2mA LOS Off-State Leakage VIN = VLOS = 12V ILIM Open Drain ON Resistance IILIM = 1mA ILIM Output Low Voltage IILIM = 2mA 0.3 V ILIM Off-State Leakage VILIM = 12V 1 µA Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. 4 0.3 V 1 µA 100 Ω Note 2: The LT3905E is guaranteed to meet specified performance from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. The LT3905I is guaranteed to meet performance specifications over the –40°C to 125°C operating junction temperature range. 3905fa For more information www.linear.com/LT3905 LT3905 Typical Performance Characteristics APD Current Regulation vs Temperature 1.020 1.2525 1.015 1.2500 1.010 1.2475 1.005 VFB (V) IAPD (mA) 1.2550 1.2450 1.2425 1.2375 0.985 50 100 TEMPERATURE (°C) 2.0 0.995 0.990 0 0 950 250 900 200 850 750 50 700 0 650 300 350 400 2.00 10mA 50 100 150 200 250 ID (mA) 300 350 1.00 1.0 CTRL (V) 1.5 2.0 3905 G07 100 0 20 40 60 DUTY CYCLE (%) 80 100 3905 G06 1mA 100µA 1µA 150 Current Monitor Outputs vs APD Current LT3905 1k 10k 100k 1M RILIM_MON (Ω) MON 10µA ILIM_MON 1µA LOS_MON 100nA IDEAL 0.5 200 100µA 10µA 0 250 0 LT3905 Current Regulation vs RILIM_MON 0.25 0 300 3905 G05 0.75 0.50 350 400 MONITOR OUTPUT FB 400 50 1mA IAPD FB (V) 450 CTRL 1.50 1.25 150 50 100 TEMPERATURE (°C) 500 3905 G04 FB vs CTRL 1.75 0 Switch Current Limit vs Duty Cycle 800 100 200 250 ISW (mA) fSEL = 0V 3905 G03 SWITCH CURRENT LIMIT (mA) 300 VF (mV) VSW (mV) 1000 150 1.2 Diode Forward Drop vs Diode Current 350 100 1.4 3905 G02 Switch Resistive Drop vs Switch Current 50 1.6 0.8 –50 25 50 75 100 125 150 TEMPERATURE (°C) 3905 G01 150 fSEL = 2V 1.8 1.0 0.980 –50 –25 150 2.2 RILIM_MON = 24.9k 1.000 1.2400 1.2350 –50 Oscillator Frequency vs Temperature FREQUENCY (MHz) FB Voltage vs Temperature 10M 100M 3905 G08 10nA 300nA 3µA 30µA 300µA APD CURRENT 3mA 3905 G09 3905fa For more information www.linear.com/LT3905 5 LT3905 Typical Performance Characteristics LOS_MON Accuracy vs Temperature MON Accuracy vs Temperature 1 ILIM_MON Accuracy vs Temperature 4 8 3 0 6 2 –2 –3 IAPD = 300nA IAPD = 3µA IAPD = 30µA IAPD = 300µA IAPD = 3mA –4 –5 –50 –25 0 4 1 ERROR (%) ERROR (%) ERROR (%) –1 0 –1 0 –2 IAPD = 3µA IAPD = 30µA IAPD = 300µA IAPD = 3mA –3 25 50 75 100 125 150 TEMPERATURE (°C) –4 –50 –25 0 IAPD = 30µA IAPD = 300µA IAPD = 3mA –2 –4 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 3905 G10 Current Monitor Voltage Drop vs APD Current 2 0 25 50 75 100 125 150 TEMPERATURE (°C) 3905 G11 3905 G12 Current Monitor Transient Response (Falling Edge) Current Monitor Transient Response (Rising Edge) 4.5 IAPD = 1mA 3.5 MONIN – APD (V) IAPD = 10µA IAPD = 10µA 4.0 IAPD = 1mA 3.0 RESPONSE 2.5 2.0 1.5 1.0 TA = –40°C TA = 25°C TA = 125°C 0.5 0 100nA tLH ~ 120ns tHL ~ 120ns 1µA 1mA 10µA 100µA IAPD 3905 G14 100ns/DIV 3905 G13 MON Clamp Voltage vs Temperature 3.00 425 2.75 MON PIN VOLTAGE (V) SW CURRENT LIMIT (mA) 450 400 375 0 25 50 75 100 125 150 TEMPERATURE (°C) RMON = OPEN 2.50 2.25 2.00 –40°C 40°C 125°C 0 3905 G17 6 3905 G15 10mA SW Current Limit vs Temperature 350 –50 –25 100ns/DIV 0.5 1 1.5 2 APD CURRENT (mA) 2.5 3 3905 G16 3905fa For more information www.linear.com/LT3905 LT3905 Pin Functions ILIM (Pin 1): Open Drain Overload Indicator. ILIM pulls low when the voltage on ILIM_MON reaches the 1.348V threshold, and releases when ILIM_MON drops by 50mV. Tie this pin to desired logic high voltage with an external pull-up resistor. The maximum recommended sink current is 2mA to prevent excess power dissipation. Leave this pin open if unused. LOS_MON (Pin 2): Loss of Signal Monitor. This pin sources a current proportional to 10% of the APD current, and acts as the positive input for the Loss of Signal comparator. It is clamped to 2.25V through an internal diode. Tie a resistor from LOS_MON to GND to set the Loss of Signal gain. Tie this pin to GND if unused. ILIM_MON (Pin 3): APD Overload Monitor. This pin sources a current proportional to 5% of the APD current, and acts as the negative input for the current regulation loop and the APD current limiter. It is clamped to 2.25V through an internal diode. Tie a resistor from ILIM_MON to GND to set the gain. If ILIM_MON exceeds the 1.248V setpoint, the current regulation loop will first throttle back the boost converter output voltage. If ILIM_MON reaches the 1.348V threshold, the ILIM indicator is pulled low and the APD current is actively limited. This pin cannot be allowed to float, tie this pin to GND if unused. MON (Pin 4): Current Monitor Output Pin. This pin sources a current proportional to 20% of the APD current for external current monitoring. It is clamped to 2.25V through an internal diode. Tie a resistor from MON to GND to set the current monitor gain. APD (Pin 5): Connect APD cathode to this pin. The maximum current of this pin is 7mA (typ). MONIN (Pin 6): Power Supply for the APD and All Current Monitors. Connect to VOUT through an external lowpass filter for lowest noise operation. SW (Pin 7): Switch Pin. Drain of internal power FET and anode of internal power Schottky. Connect to switching side of inductor. Minimize the trace length on this pin to reduce EMI. GND (Pin 9, Exposed Pad Pin 17): Ground. Tie to the exposed pad. The exposed pad must be soldered to a large PCB copper area for proper functionality and heat sinking. VIN (Pin 10): Input Supply Pin. Bypass this pin with a capacitor to GND as close to the IC as possible. EN/UVLO (Pin 11): Master Enable and VIN Undervoltage Lockout. When low, the IC is put into shutdown mode and quiescent current is reduced to <1µA. This pin contains a 1.2V comparator with 25mV internal hysteresis and a 3µA hysteresis current source for programmable external hysteresis. Connect to a resistor divider between VIN and GND to program the enable/disable thresholds, or drive with a digital signal greater than 1.5V for simple ON/OFF control. CTRL (Pin 12): External Reference Input. When CTRL is lower than 1.248V, it acts as an external reference to the FB error amp. If unused, tie CTRL to VIN. LOS (Pin 13): Open Drain Loss of Signal Indicator. LOS goes high when the voltage on LOS_MON is below the voltage on LOS_ADJ. Tie this pin to desired logic high voltage with an external pull-up resistor. The maximum recommended sink current is 2mA to prevent excess power dissipation. Leave this pin open if unused. LOS_ADJ (Pin 14): External Reference Input for the Loss of Signal Indicator. This pin feeds the negative input of the loss of signal comparator, and provides 20mV internal hysteresis. For additional external hysteresis, use a resistor network between LOS and LOS_ADJ. Tie this pin to VIN for a fixed 1.248V threshold, or if unused. FB (Pin 15): Feedback Pin. Connect to a resistor divider between VOUT and GND to limit the output voltage. The internal reference for this pin is 1.248V. Pull this pin above 1.8V to disable the step-up converter and operate the Loss of Signal monitor standalone. fSEL (Pin 16): Frequency Selector. Tie to VIN for 2MHz operation, or tie to GND for 1MHz operation. VOUT (Pin 8): Boost Converter Output Pin. Cathode of internal power Schottky. Connect an output voltage smoothing capacitor from VOUT to GND. 3905fa For more information www.linear.com/LT3905 7 LT3905 Block Diagram EN/UVLO 1.2V fSEL – VIN SW SHDN + OSC 3µA – 1.8V STBY + FB – + CV EAMP + 1.248V – VC VOUT R + CTRL 10µA Q PWM COMPARATOR + 1.248V S + CSA CC EAMP – – ILIM = 400mA RC CC 1.348V – ILIM + MONIN ILIM 1:20 CURRENT MIRROR 1:10 1:5 1 LOS_ADJ APD LOS – – + 1k 1.248V 1k 1k GND ILIM_MON LOS_MON 2.25V UPPER CLAMPS MON 3905 F01 Figure 1. Block Diagram 8 3905fa For more information www.linear.com/LT3905 LT3905 Operation Overview The LT3905 combines a 65V current mode step-up DC/DC converter with a combination APD current monitor and regulator to provide accurate monitoring and efficient supply while offering additional flexibility and protection. The step-up converter utilizes a 65V, 0.75Ω DMOS power switch and integrated Schottky diode to convert a low input voltage to a higher voltage appropriate for powering the APD. The operation of the step-up converter is a fixed frequency, current mode topology with internal compensation and accurate current limit. Operation is best understood by referring to the Block Diagram in Figure 1. At the start of each oscillator cycle, the SR latch is set, which turns on the internal DMOS power switch. Current begins to rise in the inductor and power switch, and this switch current is sensed and added to a stabilizing ramp. The resulting sum is fed to the positive terminal of the PWM comparator. The level at the negative terminal of the PWM comparator is set by the integrated error signal, VC. When the current sense signal exceeds the VC threshold or the fixed 400mA current limit, the SR latch is reset and the power switch turns off for the remainder of the cycle. The stored energy in the inductor transfers through the Schottky diode to the output capacitor, raising the output voltage. This output voltage is externally filtered and returned to the current monitor through the MONIN pin. The avalanche photodiode is then biased from the APD pin, which allows the monitor to both sense the APD current as well as to limit it in an overload condition. The monitor provides three currents proportional to APD current on the MON (1:5), LOS_MON (1:10), and ILIM_MON (1:20) pins. Resistors at these pins convert the output currents to voltages suitable for monitoring and regulating the APD current. In addition, the LOS_MON and ILIM_MON have special functions used for loss-of-signal and overload cases. These functions are described in detail in the sections regarding these pins. The step-up converter output is controlled through either the FB or ILIM_MON voltages. A fixed or maximum output voltage is programmed using the FB pin, and an adaptive output voltage for fixed APD current is programmed using the ILIM_MON pin. The reference voltage for the FB pin can also be supplied externally through the CTRL pin, for external adjustment of output voltage. Applications Information Switching Frequency The LT3905 allows selection between 1MHz and 2MHz switching frequency. Tie the fSEL pin to a voltage greater than 0.9V to select 2MHz mode, or tie to GND to select 1MHz mode. The selection of inductor and capacitor should be chosen to keep the peak inrush current below 1A. The peak in-rush current is estimated as follows: Ip = Inrush Current Due to the Schottky diode in the boost converter, a DC path exists from VIN to VOUT through the power inductor. When supply voltage is applied to the VIN pin, an inrush current will flow through the inductor to charge the output capacitor. VIN – 0.9 L sqrt – 1 C –pi • e 2 • sqrt L – 1 C Where L is the power inductor value, and C is the output capacitor value. Table 1 gives inrush peak currents for some typical component values. 3905fa For more information www.linear.com/LT3905 9 LT3905 Applications Information Converter Standby Table 1. Inrush Peak Current VIN (V) L (μH) C (μF) Ip (A) 3.3 10 1 0.47 3.3 22 1 0.37 5 10 1 0.81 5 22 1 0.63 Voltage Feedback The LT3905 error amplifier is equipped with both a fixed internal reference of 1.248V and an adjustable external reference input (CTRL). This feature allows users to select between using the built-in reference and an external reference voltage when programming the output voltage. It is possible to adjust the voltage at the CTRL pin while the device is operating, in order to alter the output voltage of LT3905 and therefore the bias voltage of the APD. To choose the fixed internal reference, tie the CTRL pin to a voltage higher than 1.5V, such as the input voltage. The FB pin is regulated to 1.248V. When the CTRL pin is below 1.248V, the FB pin is regulated to the voltage at the CTRL pin. To set the output voltage, tie FB to a resistor divider between MONIN and GND. The R2 value can be selected to provide a minimum load to the device, to maintain a constant switching frequency when the APD current is very low. Preventing entry into pulse-skipping mode is an important consideration for post-filtering the regulator output. The value of R1 is determined using the desired output voltage and the reference voltage, VREF. Where VREF is either the internal or external reference voltage as defined by the state of the CTRL pin, and R2 is the resistor value previously selected for minimum load. V R1= R2 MONIN – 1 VREF It is also possible to put the switching converter in standby mode by driving FB to a voltage higher than 1.8V. In this mode, only the internal reference and the LOS_MON circuitry remain active, and quiescent current drops to 250μA. Inductor Selection The inductor used with LT3905 should have a saturation rating of 400mA or greater. If the device is used in an application where the input supply is hot-plugged, the saturation current should exceed the peak inrush current. For best loop stability, the inductance value selected should provide a ripple current of 20% of current limit, or 80mA. For a given VIN and VOUT, the inductor value to use in continuous conduction mode is calculated by: L =D• VIN f • 80mA where: D= VOUT +1– VIN VOUT +1 and f is the switching frequency in Hz. To force the LT3905 to operate in discontinuous conduction mode (DCM), select a smaller value inductor. The following inequality is true when the LT3905 is operating in discontinuous conduction mode. L< D• VIN f • 400mA Operating in DCM reduces the maximum output current and the conversion efficiency. Capacitor Selection Use low ESR capacitors at the output to minimize output voltage ripple. Use only X5R and X7R types, which retain their capacitance over wider voltage and temperature ranges than other types. High output voltages typically require less capacitance for loop stability. Typically use a 1µF capacitor for output voltage less than 25V and 0.22µF MONIN LT3905 CTRL R1 FB R2 3905 F02 Figure 2. Output Voltage Feedback 10 3905fa For more information www.linear.com/LT3905 LT3905 Applications Information capacitor for output voltage beyond 25V. Place the output capacitor as close as possible to the VOUT lead and to the GND of the IC. Either ceramic or solid tantalum capacitors may be used for the input decoupling capacitor, which should be placed as close as possible to the LT3905. A 1µF capacitor is sufficient for most applications. Phase Lead Capacitor A small value capacitor (i.e., 10pF to 22pF) can be added in parallel with the R1 resistor between the output and the FB pin to reduce output perturbation due to a load step and to improve transient response. This phase lead capacitor introduces a pole-zero pair to the feedback that boosts phase margin near the crossover frequency. The APD is very sensitive to a noisy bias supply. To lowpass filter noise from the internal reference and error amplifier, a 0.1µF phase lead capacitor can be used. The corner frequency of the noise filter is R1 • CPL. MONIN LT3905 CTRL R1 Loss of Signal Monitor The second largest output current is the LOS_MON output, at 10% of the APD current. This is typically used for detecting a loss of signal condition by sensing a drop in the APD current. The LOS_MON pin is an input to the comparator that drives the open-drain LOS pin. This comparator uses either a fixed internal threshold of 1.248V, activated by pulling the LOS_ADJ pin to VIN, or an adjustable external threshold, activated by driving the LOS_ADJ pin with a voltage lower than 1.248V. In either case, the open-drain LOS pin goes high when the voltage on LOS_MON falls below the threshold. To program the loss of signal monitor, set a resistor from LOS_MON to ground such that the LOS_MON voltage reaches the loss of signal threshold at the point the APD current has dropped to the desired loss of signal threshold, IAPDLOS. For the case of internal threshold: CPL RLOS _ MON = 10 •1.248V IAPDLOS For the case of external threshold, simply replace 1.248V in the above equation with the external threshold applied to the LOS_ADJ pin: FB R2 3905 F03 Figure 3. Phase Lead Capacitor APD Current Monitor The APD current Monitor is supplied from the MONIN pin. It provides a current-limited power supply for the avalanche photodiode on the APD pin, and outputs three monitor currents proportional to the APD current on the MON, LOS_MON, and ILIM_MON pins. Each of the monitor outputs is diode-clamped to a 2.25V internal voltage. The largest output current is the MON output, at 20% of the APD current. This output is typically used for precise system monitoring of the APD bias point, and is tied either to a resistor to GND or to the input of a transimpedance amplifier. In either case, it is important to keep the MON voltage lower than 2.1V, as forward-biasing the internal diode will clamp the MON voltage and compromise accuracy. RLOS _ MON = 10 • VLOS _ ADJ IAPDLOS Loss of Signal Hysteresis The LOS_MON comparator contains internal hysteresis of typically 20mV on the rising edge of the LOS_MON voltage. When using an external threshold, additional external hysteresis can be generated with the addition of the resistors R7 and R8 shown in Figure 4, where R10 is the standard pull-up resistor for the LOS signal. VIN R10 LOS LOS ADJUST LT3905 LOS R7 R8 LOS_ADJ 3905 F04 Figure 4. Additional LOS Hysteresis 3905fa For more information www.linear.com/LT3905 11 LT3905 Applications Information To compute the values of R7 and R8, start by setting R8 at least 10 times the value of the pull-up resistor R10. This will prevent undue loading of the LOS signal itself. Next, compute the total hysteresis using the voltage divider generated by R7/R8: R7 VH = VIN + 20mV R7 +R8 This hysteresis is composed of two parts, the hysteresis when LOS_MON is rising (leaving loss of signal) and that when LOS_MON is falling (entering loss of signal). Use the formulas below with the value of external threshold applied to LOS ADJUST to calculate the new rising and falling thresholds of LOS_MON when using external hysteresis. Adjust the LOS_MON resistor if necessary to achieve a desired threshold with respect to APD current. R8 VLOS _ MON(FALL) = VLOS _ ADJUST R7 +R8 VLOS _ MON(RISE) = VLOS _ ADJUST + 20mV + VIN – VLOS _ ADJUST R7 • R7 +R8 Current Limit Monitor The smallest output current is the ILIM_MON output, at 5% of the APD current. This output is used for regulating the boost converter to provide a fixed APD current, as well as detecting and limiting of overload conditions. When the voltage at ILIM_MON reaches the first threshold of 1.248V, an auxiliary error amplifier limits the boost converter output voltage. In this manner, the LT3905 can regulate a fixed APD current with a single resistor at the ILIM_MON pin. In this mode, use the FB function to set a maximum output voltage limit. The FB resistor divider should be set for an output voltage higher than the expected output voltage required for the APD and monitor drop. The ILIM_MON error amplifier is the dominant regulation path, and the FB error amplifier will only take over to limit the output voltage in the case of a disconnected APD. 12 For this strategy, the ILIM_MON resistor is selected so the ILIM_MON pin reaches 1.248V at the desired APD regulation current, IAPDREG. RILIM _ MON = 20 •1.248V IAPDREG Although this loop will effectively limit the output voltage to that which is needed to regulate the APD current, the fast overload case must be considered as well. The regulation of the boost converter output is comparatively slow with respect to an APD overload condition. Additionally, substantial energy is stored in the charged output capacitor that should be prevented from flowing into the APD during a condition of optical flooding. To protect against this type of overload, the ILIM_MON pin has a fast current regulator which limits the output current of the APD pin itself. This current limiting loop has a threshold 100mV above the 1.248V boost regulation threshold, and two actions are taken if this threshold is reached. First, the output current of the APD pin is limited to the value corresponding to 1.348V on ILIM_MON. For a shorted or optically flooded APD, the voltage at the APD pin will drop to any level necessary to limit the current to the programmed value. Even for the case of the APD pin shorted to GND, the current will remain limited to the programmed value and ILIM_MON will continue to indicate an accurate measurement of APD current. The second action that is taken in response to an overload condition is assertion of the open-drain ILIM flag to indicate the overload condition to the user. It is asserted once the ILIM_MON voltage exceeds 1.348V and enters active limiting, and will remain asserted until the ILIM_MON voltage has dropped by 50mV, indicating that the device is no longer in active limiting. The response of the APD current limiter is shown in Figure 5. When the overload is applied, the APD voltage drops abruptly while the output voltage decays slowly due to the regulated current load. As the output voltage reaches the new operating point, the ILIM indicator is released and the boost converter begins regulating to 1.248V on ILIM_MON. 3905fa For more information www.linear.com/LT3905 LT3905 Applications Information LT3905 VOUT VAPD MON 0.5pF 4.99k ILIM_MON ILIM 1ms/DIV APD 3905 F05 1k – OUT LT6210 4.99k + Figure 5. APD Current Limiter Response VL VH 2V 0.1µF Although this greatly reduces the energy dissipated in the APD during overload, care must be taken that any bypass capacitors directly on the APD do not cause the overload surge to exceed recommended values. In the case of a hard short on the APD pin, the APD pin current is regulated and the boost converter will not switch, but for high input voltages, the DC path from the input through the inductor and Schottky rectifier will continue to supply current to the shorted load. If this current is larger than the programmed current limit, the device will continue to regulate the output current of the APD pin, and to assert the ILIM flag until the short on the APD pin is removed. 3905 F06 Figure 6. Transient Response Measurement Setup and high frequency resonance problems, proper layout of the high frequency switching path is essential. Keep the output capacitor as close to the Schottky diode (VOUT pin) as possible. Minimize the length and area of all traces connected to the switch pin, and always use a ground plane under the switching regulator to minimize interplane coupling. The high speed switching current path is shown in Figure 8. The signal path including the switch, output diode and output capacitor contains nanosecond rise and fall times and should be kept as short as possible. APD Current Monitor Transient Response IAPD = 10µA IAPD = 1mA The transient response of the APD current monitor is a key performance characteristic. It is essentially a function of the signal levels, since the small signal bandwidth increases with the input signal. At greater than 10µA, the LT3905 APD current mirror typically has several hundred nanosecond response time. To measure such fast transient response, a wideband transimpedance amplifier is implemented using the LT6210 as shown in Figure 6. Operating in a shunt configuration, the amplifier buffers the MON output and dramatically reduces the effective output impedance. Note that there is an inversion and DC reference offset in the final measurement. RESPONSE tHL ~ 120ns 100ns/DIV Figure 7. Typical 10μA to 1mA Step Transient Response L1 VIN Layout Hints 3905 G14 VOUT SW HIGH FREQUENCY CIRCULATING PATH The high speed operation of the LT3905 demands careful attention to board layout. You will not get advertised performance with careless layout. To prevent radiation LOAD 3905 F08 Figure 8. High Frequency Path 3905fa For more information www.linear.com/LT3905 13 LT3905 Typical Applications Low Noise APD Bias with External LOS Threshold VIN 2.7V TO 12V L1 10µH 1µF EN/UVLO fSEL VOUT ADJUST OPTIONAL FOR EXTERNAL LOS HYSTERESIS R8 24.9k LOS ADJUST VIN R9 R7 R6 1M 100k 100k C1: AVX X7R 0805YC105KAT2A C2: AVX X7R 12061C224KAT2A C3: AVX X7R 08051C104K4T2A L1: COOPER SD3110-100-R VIN SW UP TO 54VOUT VOUT R1 634k FB CTRL LOS_ADJ LT3905 R2 15k MONIN C2 220nF GND LOS ILIM 1:10 LOS_MON APD 1:20 ILIM_MON R3 1M 12.5µA LOS 1:5 MON R4 12.4k 2mA LIMIT R10 49.9Ω C3 0.1µF RX POWER R5 4.99k 1mV/µA APD 3905 TA03a APD Bias Ripple 1mV/DIV VIN = 3.3V VAPD = 40V IAPD = 1mA 14 1µs/DIV 3905 TA03b 3905fa For more information www.linear.com/LT3905 LT3905 Typical Applications Adjustable APD Bias Supply L1 10µH VIN 2.7V TO 12V C1 1µF EN/UVLO VIN LOS_ADJ SW CTRL VIN R7 100k R6 100k C1: AVX X7R 0805YC105KAT2A C2: AVX X7R 12061C224KAT2A L1: COOPER SD3110-100-R R1 634k FB fSEL VOUT ADJUST UP TO 54V VOUT LT3905 R2 15k MONIN C2 220nF GND LOS ILIM 1:10 LOS_MON APD 1:20 ILIM_MON R3 1M 12.5µA LOS APD 1:5 MON R4 9.09k 2.75mA LIMIT R5 4.99k 1mV/µA RX POWER 3905 TA02a Output Voltage Adjustment Using CTRL 60 VIN = 3.3V IAPD = 1mA VOLTAGE (V) 50 40 30 20 10 0 MONIN APD 0 0.5 1 CTRL (V) 1.5 2 3905 TA02b 3905fa For more information www.linear.com/LT3905 15 LT3905 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691 Rev Ø) 0.70 ±0.05 3.50 ±0.05 1.45 ±0.05 2.10 ±0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 × 45° CHAMFER R = 0.115 TYP 0.75 ±0.05 15 PIN 1 TOP MARK (NOTE 6) 16 0.40 ±0.10 1 1.45 ± 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. 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 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 16 0.25 ±0.05 0.50 BSC 3905fa For more information www.linear.com/LT3905 LT3905 Revision History REV DATE DESCRIPTION A 08/14 Clarified Description PAGE NUMBER 1 Clarified Block Diagram 8 Clarified Applications Information 12 Clarified APD Voltage Adjustment graph 15 3905fa 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. For more information www.linear.com/LT3905 17 LT3905 Typical Application Wide Output Range Constant Current APD Driver APD Current Regulation vs VAPD L1 10µH C1 1µF 1.050 EN/UVLO VIN SW LOS_ADJ CTRL UP TO 62V VOUT R1 732k FB R2 15k fSEL VIN R7 100k LT3905 1.000 MONIN C2 220nF GND R6 100k LOS ILIM 1:10 LOS_MON C1: AVX X7R 0805YC105KAT2A C2: AVX X7R 12061C224KAT2A C3: AVX X7R 08051C104K4T2A C4: AVX X7R 0603YC332KAT2A L1: COOPER SD3110-100-R APD 1:20 ILIM_MON R3 49.9k 250µA LOS R5 3.1k C4 3.3nF 1:5 MON 1.025 APD CURRENT (mA) VIN 3.3V 0.975 R8 49.9Ω R4 4.99k 1mA OUT 1mA APD C3 0.1µF 0.950 3905 TA04a 10 20 40 30 VAPD (V) 50 60 3905 TA04b Related Parts PART NUMBER DESCRIPTION COMMENTS LT3482 0.3A (ISW), 650kHz/1.1MHz Step-Up DC/DC Converter with APD Current Monitor VIN: 2.5V to 16V, VOUT1(MAX) = 48V, VOUT2(MAX) = 90V, IQ = 3.3mA, ISD < 1µA, 3mm × 3mm QFN Package LT3571 0.37A (ISW), 75V DC/DC Converter for APD Bias VIN: 2.7V to 20V, VOUT(MAX) = 75V, IQ = 1.7mA, ISD < 1µA, 3mm × 3mm QFN Package 18 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT3905 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT3905 3905fa LT 0814 REV A • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2013