MAX15031

19-4299; Rev 2; 6/09 KIT ATION EVALU BLE AVAILA 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications General Description The MAX15031 consists of a constant-frequency pulsewidth modulating (PWM) step-up DC-DC converter with an internal switch and a high-side current monitor with high-speed adjustable current limiting. This device can generate output voltages up to 76V and provides current monitoring up to 4mA (up to 300mW). The MAX15031 can be used for a wide variety of applications such as avalanche photodiode biasing, PIN biasing, or varactor biasing, and LCD displays. The MAX15031 operates from 2.7V to 11V. The constant-frequency (400kHz), current-mode PWM architecture provides low-noise output voltage that is easy to filter. A high-voltage, internal power switch allows this device to boost output voltages up to 76V. Internal soft-start circuitry limits the input current when the boost converter starts. The MAX15031 features a shutdown mode to save power. The MAX15031 includes a current monitor with more than three decades of dynamic range and monitors current ranging from 500nA to 4mA with high accuracy. Resistor-adjustable current limiting protects the APD from optical power transients. A clamp diode protects the monitor’s output from overvoltage conditions. Other protection features include cycle-by-cycle current limiting of the boost converter switch, undervoltage lockout, and thermal shutdown if the die temperature reaches +160°C. The MAX15031 is available in a thermally enhanced 4mm x 4mm, 16-pin TQFN package and operates over the -40°C to +125°C automotive temperature range. Features ♦ Input Voltage Range +2.7V to +5.5V (Using Internal Charge Pump) or +5.5V to +11V ♦ Wide Output-Voltage Range from (VIN + 1V) to 76V ♦ Internal 1Ω (typ) 80V Switch ♦ 300mW Boost Converter Output Power ♦ Accurate ±10% (500nA to 1mA) and ±3.5% (1mA to 4mA) High-Side Current Monitor ♦ Resistor-Adjustable Ultra-Fast APD Current Limit (1µs Response Time) ♦ Open-Drain Current-Limit Indicator Flag ♦ 400kHz Fixed Switching Frequency ♦ Constant PWM Frequency Provides Easy Filtering in Low-Noise Applications ♦ Internal Soft-Start ♦ 2µA (max) Shutdown Current ♦ -40°C to +125°C Temperature Range ♦ Small Thermally Enhanced, 4mm x 4mm, 16-Pin TQFN Package MAX15031 Ordering Information PART M AX15031ATE+ TEMP RANGE -40°C to +125°C PIN-PACKAGE 16 TQFN-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. Applications CLAMP Pin Configuration RLIM 9 APD Avalanche Photodiode Biasing and Monitoring PIN Diode Bias Supplies Low-Noise Varactor Diode Bias Supplies FBON Modules GPON Modules LCD Displays TOP VIEW 12 11 MOUT 10 BIAS 13 SHDN 14 PGND 15 LX 16 8 7 ILIM CNTRL FB SGND MAX15031 6 *EP + 1 PWR 2 CP 3 CN 5 Typical Operating Circuits appear at end of data sheet. 4 IN *EXPOSED PAD THIN QFN (4mm × 4mm) ________________________________________________________________ Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 ABSOLUTE MAXIMUM RATINGS PWR, IN to SGND ...................................................-0.3V to +12V LX to PGND ............................................................-0.3V to +80V BIAS, APD to SGND ...............................................-0.3V to +80V SHDN to SGND............................................-0.3V to (VIN + 0.3V) CLAMP to SGND ......................................-0.3V to (VBIAS + 0.3V) FB, ILIM, RLIM, CP, CN, CNTRL to SGND .............-0.3V to +12V PGND to SGND .....................................................-0.3V to +0.3V MOUT to SGND ....................................-0.3V to (VCLAMP + 0.3V) Continuous Power Dissipation 16-Pin TQFN (derate 25mW/°C above +70°C) ............2000mW Thermal Resistance (Note 1) θJA ....................................................................................40°C/W θJC ......................................................................................6°C/W Operating Temperature Range .........................-40°C to +125°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = VPWR = 3.3V. VSHDN = 3.3V. CIN = CPWR = 10μF. CCP = 10nF, VCNTRL = VIN. VRLIM = 0. VPGND = VSGND = 0. VBIAS = 40V. APD = unconnected. CLAMP = unconnected. ILIM = unconnected, MOUT = unconnected. TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2) PARAMETER Supply Voltage Range SYMBOL VIN, VPWR CONDITIONS CP connected to IN, CCP = open VFB = 1.4V, no switching Supply Current Undervoltage Lockout Threshold Undervoltage Lockout Hysteresis Shutdown Current BIAS Current During Shutdown BOOST CONVERTER Output-Voltage Adjustment Range Switching Frequency Maximum Duty Cycle F B Set-Point Voltage F B Input Bias Current f SW DCLK VFB IFB VPWR = VIN = 2.9V, VCP = 5.5V VPWR = VIN = 5.5V, VCP = 1 0V VPWR = VIN = VCP = 5.5V VPWR = VIN = VCP = 11V 0.8 VLX = 3 6V 2.9V VPWR 11V, VPWR = VIN, ILOAD = 4.5mA 0 ILOAD 4.5mA 0.2 1 1 1 1 1 1.2 VIN = VPWR = 5V 2.9V 2.9V VPWR VPWR 11V, VIN = V PWR 11V, VIN = V PWR VIN + 1V 360 352 86 1.2201 400 400 90 1.245 76 440 448 94 1.2699 100 2 2 2 2 1.6 1 A μA % % V kHz % V nA I SUPPLY VUVLO VUVLO_HYS I IN_SHDN SHDN pulled low IBIAS_SHDN VBIAS = 3.3V, VSHDN = 0 VIN = 11V, VFB = 1.4V (no switching), CCP = open, CP = IN VIN rising 2.375 MIN 2.7 5.5 1 1.2 2.5 100 2 30 TYP MAX 5.5 11 2 3 2.675 mA V mV μA μA UNITS V ILX = 100mA Internal Switch On-Resistance R ON ILX = 100mA, VCP = VIN Peak Switch Current Limit LX Leakage Current L ine Regulation Load Regulation ILIM_LX 2 _______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications ELECTRICAL CHARACTERISTICS (continued) (VIN = VPWR = 3.3V. VSHDN = 3.3V. CIN = CPWR = 10μF. CCP = 10nF, VCNTRL = VIN. VRLIM = 0. VPGND = VSGND = 0. VBIAS = 40V. APD = unconnected. CLAMP = unconnected. ILIM = unconnected, MOUT = unconnected. TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2) PARAMETER Soft-Start Duration Soft-Start Steps CONTROL INPUT (CNTRL) M aximum Control Input Voltage Range CURRENT MONITOR B ias Voltage Range B ias Quiescent Current Voltage Drop D ynamic Output Resistance at MOUT MOUT Output Leakage Output Clamp Voltage Output Clamp Leakage Current Output-Voltage Range Current Gain VMOUT IMOUT/IAPD VMOUT VCLAMP VBIAS IBIAS VDROP RMOUT IAPD = 5 00nA IAPD = 2 mA IAPD = 2 mA, VDROP = VBIAS - VAPD IAPD = 5 00nA IAPD = 2.5mA APD is unconnected Forward diode current = 1mA VBIAS = VCLAMP = 76V 10V VBIAS 7 6V, 0 IAPD CLAMP is unconnected IAPD = 5 00nA IAPD = 2 mA ( IMOUT/IMOUT)/ VBIAS, VBIAS = 1 0V to 76V VAPD = 35V, RLIM = 3 .3k 12.45k tS RLIM 2.5k IAPD = 5 00nA IAPD = 2.5mA IMOUT settles to within 0.1%, 10nF connected from APD to ground IAPD = 5 00nA IAPD = 5 μA to 1mA 1mA, VBIAS 1V 0.095 0.0965 -1000 -250 3.15 1 7.5 90 0.1 0.1 +300 +24 3.75 0.11 0.1035 +1500 ppm/V +250 4.35 5 mA mA ms μs 0.5 1 890 1 0.73 1 0.95 10 76 100 3.2 1 V μA mA V G M nA V nA V FB set point is regulated to VCNTRL 1.25 V (0.25 x ILIM_LX) to ILIM_LX SYMBOL CONDITIONS MIN TYP 8 32 MAX UNITS ms Steps MAX15031 Power-Supply Rejection Ratio (Note 3) APD Input Current Limit Current-Limit Adjustment Range Power-Up Settling Time LOGIC INPUTS/OUTPUTS SHDN Input-Voltage Low SHDN Input-Voltage High ILIM Output-Voltage Low ILIM Output Leakage Current THERMAL PROTECTION Thermal Shutdown Thermal Shutdown Hysteresis PSRR ILIM_APD VIL VIH VOL I OH ILIM = 2 mA VILIM = 11V Temperature rising +160 10 2.4 0.8 0.3 1 V V V μA °C °C Note 2: All minimum/maximum parameters are tested at TA = +125°C. Limits over temperature are guaranteed by design. Note 3: Guaranteed by design and not production tested. _______________________________________________________________________________________ 3 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 Typical Operating Characteristics (VPWR = VIN = 3.3V, VOUT = 70V, circuit of Figure 3 (Figure 4 for VIN > 5.5V), unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT MAX15031 toc01 EFFICIENCY vs. LOAD CURRENT VOUT = 30V MAX15031 toc02 EFFICIENCY vs. LOAD CURRENT MAX15031 toc03 70 60 50 EFFICIENCY (%) VOUT = 30V 70 60 50 EFFICIENCY (%) 70 60 50 EFFICIENCY (%) 40 30 20 10 VIN = 3.3V VIN = 5V VIN = 8V 40 30 20 10 VIN = 3.3V 0 0 1 2 LOAD CURRENT (mA) 3 4 VOUT = 70V VOUT = 55V 40 30 VOUT = 70V 20 10 VIN = 5V 0 0 1 2 LOAD CURRENT (mA) 3 4 VOUT = 55V VOUT = 70V 0 0 1 2 LOAD CURRENT (mA) 3 4 MINIMUM STARTUP VOLTAGE vs. LOAD CURRENT MAX15031 toc04 SUPPLY CURRENT vs. SUPPLY VOLTAGE 1.8 1.6 SUPPLY CURRENT (mA) 1.4 1.2 1.0 0.8 0.6 0.4 TA = +125°C TA = +85°C TA = -40°C TA = +25°C MAX15031 toc05 NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE XMAX15031 toc06 2.55 MINIMUM STARTUP VOLTAGE (V) 2.54 2.53 2.52 2.51 2.50 2.49 2.48 0 1 2 LOAD CURRENT (mA) 3 4 2.0 60 NO-LOAD SUPPLY CURRENT (mA) 50 TA = +85°C 40 TA = +25°C 30 TA = -40°C 20 10 0 3 4 5 6 7 8 9 10 0.2 0 0 1 2 3 4 5 6 7 8 SUPPLY VOLTAGE (V) VFB = 1.4V 9 10 11 11 SUPPLY VOLTAGE (V) EXITING SHUTDOWN MAX15031 toc07 ENTERING SHUTDOWN MAX15031 toc08 VOUT 50V/div 3V 70V OUTPUT VOLTAGE 50V/div 3V INDUCTOR CURRENT 500mA/div 0mA IL 500mA/div 0mA 0V 1ms/div IOUT = 1mA VSHDN 2V/div ILOAD = 1mA 4ms/div SHUTDOWN VOLTAGE 2V/div 0V 4 _______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications Typical Operating Characteristics (continued) (VPWR = VIN = 3.3V, VOUT = 70V, circuit of Figure 3 (Figure 4 for VIN > 5.5V), unless otherwise noted.) MAX15031 LIGHT-LOAD SWITCHING WAVEFORM WITH RC FILTER MAX15031 toc09 HEAVY-LOAD SWITCHING WAVEFORM WITH RC FILTER MAX15031 toc10 IOUT = 0.1mA, VBIAS = 70V VBIAS AC-COUPLED 1mV/div VLX 50V/div 0V IL 500mA/div 0mA IOUT = 4mA, VBIAS = 70V VBIAS AC-COUPLED 1mV/div VLX 50V/div 0V IL 500mA/div 0mA 1μs/div 1μs/div LOAD-TRANSIENT RESPONSE MAX15031 toc11 LINE-TRANSIENT RESPONSE MAX15031 toc12 LX LEAKAGE CURRENT vs. TEMPERATURE VIN 2V/div 3.3V 180 LX LEAKAGE CURRENT (nA) 160 140 120 100 80 60 40 20 0 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) CURRENT INTO LX PIN MAX15031 toc13 200 VOUT AC-COUPLED 200mV/div ILOAD 5mA/div 0mA VOUT = 70V VIN = 3.3V 100ms/div 100ms/div VOUT = 70V IOUT = 1mA tRISE = 50μs VOUT AC-COUPLED 100mV/div LOAD REGULATION MAX15031 toc14 MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE MAX15031 toc15 BIAS CURRENT vs. BIAS VOLTAGE MAX15031 toc16 70.0 69.8 69.6 OUTPUT VOLTAGE (V) 69.4 69.2 69.0 68.8 68.6 68.4 68.2 68.0 0 1 2 3 4 5 LOAD CURRENT (mA) 110 100 MAXIMUM LOAD CURRENT (mA) 90 80 70 60 50 40 30 20 10 0 3 E F A: VOUT = 30V, B: VOUT = 35V, C: VOUT = 45V, D: VOUT = 55V, E: VOUT = 60V, F: VOUT = 72V 4 5 6 7 8 9 10 A B C D 10 BIAS CURRENT (mA) 1 IAPD = 2mA 0.1 IAPD = 500nA 0.01 0 10 20 30 40 50 60 70 80 BIAS VOLTAGE (V) 11 INPUT VOLTAGE (V) _______________________________________________________________________________________ 5 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 Typical Operating Characteristics (continued) (VPWR = VIN = 3.3V, VOUT = 70V, circuit of Figure 3 (Figure 4 for VIN > 5.5V), unless otherwise noted.) BIAS CURRENT vs. APD CURRENT VBIAS = 70V MAX15031 toc17 BIAS CURRENT vs. TEMPERATURE MAX15031 toc18 GAIN ERROR vs. APD CURRENT 4 3 GAIN ERROR (%) 2 1 0 -1 -2 -3 -4 VBIAS = 70V MAX15031 toc19 10 10 IAPD = 2mA BIAS CURRENT (mA) 5 BIAS CURRENT (mA) 1 1 0.1 0.1 IAPD = 500nA 0.01 0.0001 0.01 0.001 0.01 0.1 1 10 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) APD CURRENT (mA) -5 0.1 1 10 100 1000 10,000 IAPD (μA) GAIN ERROR vs. TEMPERATURE IAPD = 2mA IAPD = 500μA MAX15031 toc20 GAIN ERROR vs. BIAS VOLTAGE 0.60 0.40 IAPD = 5μA IAPD = 50μA MAX15031 toc21 1.0 0.5 0 GAIN ERROR (%) -0.5 -1.0 -1.5 -2.0 -2.5 0.80 IAPD = 5μA IAPD = 50μA IAPD = 500nA GAIN ERROR (%) 0.20 0 -0.20 -0.40 -0.60 IAPD = 500nA IAPD = 500μA IAPD = 2mA 10 20 30 40 50 60 70 80 VBIAS = 70V -3.0 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) -0.80 BIAS VOLTAGE (V) APD TRANSIENT RESPONSE MAX15031 toc22 STARTUP DELAY MAX15031 toc23 VAPD AC-COUPLED 70V 2V/div VBIAS 20V/div IAPD 2.5mA/div 0mA IMOUT 0.25mA/div 0mA 20μs/div 200μs/div 3V IAPD = 500nA IMOUT 20nA/div 0nA 6 _______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications Typical Operating Characteristics (continued) (VPWR = VIN = 3.3V, VOUT = 70V, circuit of Figure 3 (Figure 4 for VIN > 5.5V), unless otherwise noted.) MAX15031 STARTUP DELAY MAX15031 toc24 STARTUP DELAY MAX15031 toc25 VAPD 20V/div VAPD 2V/div 0V 3V IMOUT 50μA/div IAPD = 2mA 100μs/div IAPD = 500nA VBIAS = 5V 100μs/div IMOUT 20nA/div 0nA 0nA STARTUP DELAY MAX15031 toc26 SHORT-CIRCUIT RESPONSE MAX15031 toc27 VBIAS 2V/div ILIM 2V/div 0V 0V IBIAS 2mA/div IMOUT 50μA/div 0nA 40ms/div VBIAS = 70V TA = +85°C RLIM = 2kΩ IAPD = 2mA VBIAS = 5V 40μs/div 0mA VOLTAGE DROP vs. APD CURRENT MAX15031 toc28 SWITCHING FREQUENCY vs. TEMPERATURE 480 SWITCHING FREQUENCY (kHz) 460 440 420 400 380 360 340 320 300 10,000 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) MAX15031 toc29 1.40 1.20 1.00 VBIAS - VAPD (V) 0.80 0.60 0.40 0.20 TA = +85°C 0 0.1 1 10 100 1000 IAPD (μA) TA = +125°C TA = -40°C TA = +25°C 500 _______________________________________________________________________________________ 7 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 Typical Operating Characteristics (continued) (VPWR = VIN = 3.3V, VOUT = 70V, circuit of Figure 3 (Figure 4 for VIN > 5.5V), unless otherwise noted.) SWITCHING FREQUENCY vs. INPUT VOLTAGE 480 SWITCHING FREQUENCY (kHz) 460 440 420 400 380 360 340 320 300 2 4 6 8 10 12 INPUT VOLTAGE (V) MAX15031 toc30 SWITCHING FREQUENCY AND DUTY CYCLE vs. LOAD CURRENT 420 415 SWITCHING FREQUENCY (kHz) 410 405 400 395 390 385 MEASURED AT CN 380 0 1 2 LOAD CURRENT (mA) 3 4 0 10 SWITCHING FREQUENCY 30 DUTY CYCLE 40 DUTY CYCLE MAX15031 toc31 500 60 50 20 FB SET-POINT VARIATION vs. TEMPERATURE FB SET-POINT VOLTAGE VARIATION (V) MAX15031 toc32 APD OUTPUT RIPPLE VOLTAGE MAX15031 toc33 1.277 VIN = 2.9V 1.257 1.247 1.237 1.227 1.217 1.207 VIN = 2.9V VIN = 5.5V FB RISING VIN = 5.5V VAPD AC-COUPLED, 55V 200μV/div FB FALLING -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) 2μs/div APD OUTPUT RIPPLE VOLTAGE MAX15031 toc34 APD OUTPUT RIPPLE VOLTAGE MAX15031 toc35 VAPD AC-COUPLED, 55V 100μV/div VAPD AC-COUPLED, 70V 500μV/div 0.1μF CAPACITOR CONNECTED FROM APD TO GND. 2μs/div 0.1μF CAPACITOR CONNECTED FROM APD TO GND. 2μs/div 8 _______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications Pin Description PIN 1 2 3 4 5 NAME PWR CP CN IN SGND FUNCTION Boost Converter Input Voltage. PWR powers the switch driver and charge pump. Bypass PWR to PGND with a ceramic capacitor of 1μF minimum value. Positive Terminal of the Charge-Pump Flying Capacitor for 2.7V to 5.5V Supply Voltage Operation. Connect CP to IN when the input voltage is in the 5.5V to 11V range. Negative Terminal of the Charge-Pump Flying Capacitor for 2.7V to 5.5V Supply Voltage Operation. Leave CN unconnected when the input voltage is in the 5.5V to 11V range. Input Supply Voltage. IN powers all blocks of the MAX15031 except the switch driver and charge pump. Bypass IN to PGND with a ceramic capacitor of 1μF minimum value. Signal Ground. Connect directly to the local ground plane. Connect SGND to PGND at a single point, typically near the return terminal of the output capacitor. Feedback Regulation Input. Connect FB to the center tap of a resistive voltage-divider from the output (VOUT) to SGND to set the output voltage. The FB voltage regulates to 1.245V (typ) when VCNTRL is above 1.5V (typ) and to VCNTRL voltage when VCNTRL is below 1.245V (typ). Control Input for Boost Converter Output-Voltage Programmability. Allows the feedback set-point voltage to be set externally by CNTRL when VCNTRL is less than 1.245V. Pull CNTRL above 1.5V (typ) to use the internal 1.245V (typ) feedback set-point voltage. Open-Drain Current-Limit Indicator. ILIM asserts low when the APD current limit has been exceeded. Current-Limit Resistor Connection. Connect a resistor from RLIM to SGND to program the APD current-limit threshold. Current-Monitor Output. MOUT sources a current 1/10th of IAPD. Clamp Voltage Input. CLAMP is the external potential used for voltage clamping of MOUT. Reference Current Output. APD provides the source current to the cathode of the photodiode. Bias Voltage Input. Connect BIAS to the boost converter output (VOUT) either directly or through a lowpass filter for ripple attenuation. BIAS provides the voltage bias for the current monitor and is the current source for APD. Active-Low Shutdown Control Input. Apply a logic-low voltage to SHDN to shut down the device and reduce the supply current to 2μA (max). Connect SHDN to IN for normal operation. Ensure that VSHDN is not greater than the input voltage, VIN. Power Ground. Connect the negative terminals of the input and output capacitors to PGND. Connect PGND externally to SGND at a single point, typically at the return terminal of the output capacitor. Drain of Internal 80V n-Channel DMOS. Connect inductor and diode to LX. Minimize the trace area at LX to reduce switching noise emission. Exposed Pad. Connect EP to a large contiguous copper plane at SGND potential to improve thermal dissipation. Do not use as the main SGND connection. MAX15031 6 FB 7 8 9 10 11 12 13 CNTRL ILIM RLIM MOUT CLAMP APD BIAS 14 SHDN 15 16 — PGND LX EP _______________________________________________________________________________________ 9 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 Functional Diagram PWR OUTPUT ERROR AND CURRENT COMPARATOR FB VREF CNTRL SGND MUX -A +A LX -C +C SOFTSTART PEAK CURRENT-LIMIT COMPARATOR SWITCH CONTROL LOGIC 80V DMOS PGND SWITCH CURRENT SENSE CLAMP VREF REFERENCE COMPARATOR CN CP CHARGE PUMP (DOUBLER) MAX15031 THERMAL SHUTDOWN VREF CURRENT MONITOR BIAS AND REFERENCE 1x CURRENTLIMIT ADJUSTMENT MOUT RLIM IN UVLO CLK 10x CURRENT LIMIT APD ILIM OSCILLATOR 400kHz SHDN BIAS Detailed Description The MAX15031 constant-frequency, current-mode, PWM boost converter is intended for low-voltage systems that require a locally generated high voltage. This device can generate a low-noise, high output voltage required for PIN and varactor diode biasing and LCD displays. The MAX15031 operates either from +2.7V to +5.5V or from +5.5V to +11V. For 2.7V to 5.5V operation, an internal charge pump with an external 10nF ceramic capacitor is used. For 5.5V to 11V operation, connect CP to IN and leave CN unconnected. The MAX15031 operates in discontinuous mode in order to reduce the switching noise caused by reversevoltage recovery charge of the rectifier diode. Other continuous mode boost converters generate large voltage spikes at the output when the LX switch turns on 10 because there is a conduction path between the output, diode, and switch to ground during the time needed for the diode to turn off and reverse its bias voltage. To reduce the output noise even further, the LX switch turns off by taking 10ns typically to transition from ON to OFF. As a consequence, the positive slew rate of the LX node is reduced and the current from the inductor does not “force” the output voltage as hard as would be the case if the LX switch were to turn off faster. The constant-frequency (400kHz) PWM architecture generates an output voltage ripple that is easy to filter. An 80V vertical DMOS device used as the internal power switch is ideal for boost converters with output voltages up to 76V. The MAX15031 can also be used in other topologies where the PWM switch is grounded, like SEPIC and flyback converters. ______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications The MAX15031 includes a versatile current monitor intended for monitoring the APD, PIN, or varactor diode DC current in fiber and other applications. The MAX15031 features more than three decades of dynamic current ranging from 500nA to 4mA and provides an output current accurately proportional to the APD current at MOUT. The MAX15031 also features a shutdown logic input to disable the device and reduce its standby current to 2μA (max). The APD current-monitor range is from 500nA to 4mA, and the MOUT current-mirror output accuracy is ±10% from 500nA to 1mA of APD current and ±3.5% from 1mA to 4mA of APD current. MAX15031 Clamping the Monitor Output Voltage (CLAMP) CLAMP provides a means for diode clamping the voltage at MOUT; thus V MOUT is limited to (V CLAMP + 0.6V). CLAMP can be connected to either an external supply or BIAS. CLAMP can be left unconnected if voltage clamping is not required. Fixed-Frequency PWM Controller The heart of the MAX15031 current-mode PWM controller is a BiCMOS multiple-input comparator that simultaneously processes the output-error signal and switch current signal. The main PWM comparator uses direct summing, lacking a traditional error amplifier and its associated phase shift. The direct summing configuration approaches ideal cycle-by-cycle control over the output voltage since there is no conventional error amplifier in the feedback path. The device operates in PWM mode using a fixed-frequency, current-mode operation. The current-mode frequency loop regulates the peak inductor current as a function of the output error signal. The current-mode PWM controller is intended for DCM (discontinuous conduction mode) operation. No internal slope compensation is added to the current signal. Adjusting the Boost Converter Output Voltage (FB/CNTRL) The boost converter output voltage can be set by connecting FB to a resistor-divider from VOUT to ground. The set-point feedback reference is the 1.245V (typ) internal reference voltage when VCNTRL > 1.5V and is equal to the CNTRL voltage when VCNTRL < 1.25V. To change the converter output on the fly, apply a voltage lower than 1.25V (typ) to the CNTRL input and adjust the CNTRL voltage, which is the reference input of the error amplifier when VCNTRL < 1.25V (see the Functional Diagram). This feature can be used to adjust the APD voltage based on the APD mirror current, which compensates for the APD avalanche gain variation with temperature and manufacturing process. As shown in Figure 4, the voltage signal proportional to the MOUT current is connected to the ADC (analog to digital) input of the APD module, which then controls the reference voltage of the boost converter error amplifier through a DAC (digital to analog) block connected to the CNTRL input. The BIAS voltage and, therefore, the APD current, are controlled based on the MOUT mirror current, forming a negative feedback loop. Charge Pump At low supply voltages (2.7V to 5.5V), internal chargepump circuitry and an external 10nF ceramic capacitor connected between CP and CN double the available internal supply voltage to drive the internal switch efficiently. In the 5.5V to 11V supply voltage range, the charge pump is not required. In this configuration, disable the charge pump by connecting CP to IN and leaving CN unconnected. Shutdown (SHDN) The MAX15031 features an active-low shutdown input (SHDN). Pull SHDN low to enter shutdown. During shutdown, the supply current drops to 2μA (30μA from BIAS) (max). However, the output remains connected to the input through the inductor and the output diode, holding the output voltage to one diode drop below PWR when the MAX15031 shuts down. Connect SHDN to IN for always-on operation. Monitor Current Limit (RLIM) The current limit of the current monitor is programmable from 1mA to 5mA. Connect a resistor from RLIM to ground to program the current-limit threshold up to 5mA. The current monitor mirrors the current out of APD with a 1:10 ratio, and the MOUT current can be converted to a voltage signal by connecting a resistor from MOUT to SGND. ______________________________________________________________________________________ 11 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 VOUT output current in amperes, L is the inductor value in microhenrys, and η is the efficiency of the boost converter (see the Typical Operating Characteristics). Determining the Inductor Value R2 FB MAX15031 R1 Three key inductor parameters must be specified for operation with the MAX15031: inductance value (L), inductor saturation current (ISAT), and DC resistance (DCR). In general, the inductor should have a saturation current rating greater than the maximum switch peak current-limit value (ILIM_LX = 1.6A). Choose an inductor with a low-DCR resistance for reasonable efficiency. Use the following formula to calculate the lower bound of the inductor value at different output voltages and output currents. This is the minimum inductance value for discontinuous mode operation for supplying full 300mW of output power. Figure 1. Adjustable Output Voltage LMIN[μH] = 2 × TS × IOUT × (VOUT − VIN_MIN ) 2 η × ILIM_LX Design Procedure Setting the Output Voltage Set the MAX15031 output voltage by connecting a resistive divider from the output to FB to SGND (Figure 1). Select R1 (FB to SGND resistor) between 200kΩ and 400kΩ. Calculate R2 (VOUT to FB resistor) using the following equation: ⎡⎛ V ⎞⎤ R2 = R1 ⎢⎜ OUT ⎟ − 1⎥ ⎝ VREF ⎠ ⎦ ⎢ ⎥ ⎣ where VOUT can range from (VIN + 1V) to 76V and VREF = 1.245V or VCNTRL depending on the VCNTRL value. For VCNTRL > 1.5V, the internal 1.245V (typ) reference voltage is used as the feedback set point (V REF = 1.245V) and for VCNTRL < 1.25V, VREF = VCNTRL. where V IN_MIN , V OUT (both in volts), and I OUT ( in amperes) are typical values (so that efficiency is optimum for typical conditions), TS (in microseconds) is the period, η is the efficiency, and I LIM_LX is the peak switch current in amperes (see the E lectrical Characteristics table). Calculate the optimum value of L (LOPTIMUM) to ensure the full output power without reaching the boundary between continuous conduction mode (CCM) and DCM using the following formula: L [μH] LOPTIMUM[μH] = MAX 2.25 where LMAX [μH] = 2 VIN_MIN (VOUT − VIN_MIN ) × TS × η 2 2 × IOUT × VOUT Determining Peak Inductor Current If the boost converter remains in the discontinuous mode of operation, then the approximate peak inductor current, ILPEAK (in amperes), is represented by the formula below: ILPEAK = 2 × TS × (VOUT − VIN_MIN ) × IOUT_MAX η×L where T S is the switching period in microseconds, VOUT is the output voltage in volts, VIN_MIN is the minimum input voltage in volts, IOUT_MAX is the maximum For a design in which VIN = 3.3V, VOUT = 70V, IOUT= 3mA, η = 45%, ILIM_LX = 1.3A, and TS = 2.5μs: LMIN = 1.3μH and LMAX = 23μH. For a worse-case scenario in which VIN = 2.9V, VOUT = 70V, IOUT= 4mA, η = 43%, ILIM_LX= 1.3A, and TS = 2.5μs: LMIN = 1.8μH and LMAX = 15μH. The choice of 4.7μH is reasonable given the worst-case scenario above. In general, the higher the inductance, the lower the switching noise. Load regulation is also better with higher inductance. 12 ______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications Diode Selection The MAX15031’s high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward-voltage drop. Ensure that the diode’s peak current rating is greater than the peak inductor current. Also the diode reverse-breakdown voltage must be greater than VOUT. The output voltage of the boost converter. For very low output ripple applications, the output of the boost converter can be followed by an RC filter to further reduce the ripple. Figure 2 shows a 100Ω (RF), 0.1μF (CF) filter used to reduce the switching output ripple to 1mVP-P with a 0.1mA load or 2mVP-P with a 4mA load. The output-voltage regulation resistor-divider must remain connected to the diode and output capacitor node. Use X7R ceramic capacitors for more stability over the full temperature range. Use an X5R capacitor for -40°C to +85°C applications. MAX15031 Output Filter Capacitor Selection For most applications, use a small output capacitor of 0.1μF or greater. To achieve low output ripple, a capacitor with low ESR, low ESL, and high capacitance value should be selected. If tantalum or electrolytic capacitors are used to achieve high capacitance values, always add a smaller ceramic capacitor in parallel to bypass the high-frequency components of the diode current. The higher ESR and ESL of electrolytic capacitors increase the output ripple and peak-to-peak transient voltage. Assuming the contribution from the ESR and capacitor discharge equals 50% (proportions may vary), calculate the output capacitance and ESR required for a specified ripple using the following equations: COUT[μF] = IOUT 0.5 x ΔVOUT ⎡ ILPEAK x LOPTIMUM ⎤ ⎢ TS − ⎥ (VOUT − VIN_MIN ) ⎥ ⎢ ⎣ ⎦ 0.5 x ΔVOUT ESR [m Ω ] = IOUT Input Capacitor Selection Bypass PWR to PGND with a 1μF (min) ceramic capacitor and bypass IN to PGND with a 1μF (min) ceramic capacitor. Depending on the supply source impedance, higher values may be needed. Make sure that the input capacitors are close enough to the IC to provide adequate decoupling at IN and PWR as well. If the layout cannot achieve this, add another 0.1μF ceramic capacitor between IN and PGND (or PWR and PGND) in the immediate vicinity of the IC. Bulk aluminum electrolytic capacitors may be needed to avoid chattering at low input voltage. In case of aluminum electrolytic capacitors, calculate the capacitor value and ESR of the input capacitor using the following equations: CIN[μF] = ⎡ VOUT x IOUT ILPEAK x LOPTIMUM x VOUT ⎤ ⎢T − ⎥ η x VIN_MIN x 0.5 x ΔVIN ⎣ S VIN_MIN (VOUT − VIN_MIN ) ⎦ ⎢ ⎥ ESR [m Ω ] = 0.5 x ΔVIN x η x VIN_MIN VOUT x IOUT L1 CIN RF 100Ω VOUT R2 MAX15031 CP CPWR CCP CN PGND SGND BIAS R1 FB COUT1 CF 0.1μF IN VIN = 2.7V TO 5.5V PWR CNTRL SHDN LX D1 Figure 2. Typical Operating Circuit with RC Filter ______________________________________________________________________________________ 13 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 Determining Monitor Current Limit Calculate the value of the monitor current-limit resistor, RLIM, for a given APD current limit, ILIMIT, using the following equation: RLIM = 10 × 1.245V ILIMIT(mA) protection is less critical when MOUT is connected directly to subsequent transimpedance amplifiers (linear or logarithmic) that have low-impedance, near-groundreferenced inputs. If a transimpedance amplfier is used on the low side of the photodiode, its voltage drop must also be considered. Leakage from the clamping diode is most often insignificant over nominal operating conditions, but grows with temperature. To maintain low levels of wideband noise, lowpass filtering the output signal is suggested in applications where only DC measurements are required. Connect the filter capacitor at MOUT. Determining the required filtering components is straightforward, as the MAX15031 exhibits a very high output impedance of 890MΩ. In some applications where pilot tones are used to identify specific fiber channels, higher bandwidths are desired at MOUT to detect these tones. Consider the minimum and maximum currents to be detected, then consult the frequency response and noise typical operating curves. If the minimum current is too small, insufficient bandwidth could result, while too high a current could result in excessive noise across the desired bandwidth. The RLIM resistor, RLIM, ranges from 12.45kΩ to 2.5Ω for APD currents from 1mA to 5mA. Applications Information Using APD or PIN Photodiodes in Fiber Applications When using the MAX15031 to monitor APD or PIN photodiode currents in fiber applications, several issues must be addressed. In applications where the photodiode must be fully depleted, keep track of voltages budgeted for each component with respect to the available supply voltage(s). The current monitors require as much as 1.1V between BIAS and APD, which must be considered part of the overall voltage budget. Additional voltage margin can be created if a negative supply is used in place of a ground connection, as long as the overall voltage drop experienced by the MAX15031 is less than or equal to 76V. For this type of application, the MAX15031 is suggested so the output can be referenced to “true” ground and not the negative supply. The MAX15031’s output current can be referenced as desired with either a resistor to ground or a transimpedance amplifier. Take care to ensure that output voltage excursions do not interfere with the required margin between BIAS and MOUT. In many fiber applications, MOUT is connected directly to an ADC that operates from a supply voltage that is less than the voltage at BIAS. Connecting the MAX15031’s clamping diode output, CLAMP, to the ADC power supply helps avoid damage to the ADC. Without this protection, voltages can develop at MOUT that might destroy the ADC. This Layout Considerations Careful PCB layout is critical to achieve low switching losses and clean and stable operation. Protect sensitive analog grounds by using a star ground configuration. Connect SGND and PGND together close to the device at the return terminal of the output bypass capacitor. Do not connect them together anywhere else. Keep all PCB traces as short as possible to reduce stray capacitance, trace resistance, and radiated noise. Ensure that the feedback connection to FB is short and direct. Route high-speed switching nodes away from the sensitive analog areas. Use an internal PCB layer for SGND as an EMI shield to keep radiated noise away from the device, feedback dividers, and analog bypass capacitors. Refer to the MAX15031 evaluation kit data sheet for a layout example. 14 ______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications Typical Operating Circuits L1 4.7μH VIN COUT 0.1μF RF 100Ω R2 348kΩ RADJ MAX15031 D1 VOUT (70V MAX) CPWR 1μF PWR CNTRL IN CIN 1μF MAX15031 CP CCP 10nF CN LX PGND CF 0.1μF R1 6.34kΩ BIAS FB SHDN ILIM VDD CLAMP RLIM RLIM 2.87kΩ SGND APD MOUT APD RMOUT 10kΩ CMOUT (OPTIONAL) ADC DAC VDD μC GPIO GPIO Figure 3. Typical Operating Circuit for VIN = 2.7V to 5.5V ______________________________________________________________________________________ 15 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15031 Typical Operating Circuits (continued) L1 4.7μH VIN = 5.5V TO 11V D1 VOUT (70V MAX) COUT 0.1μF RF 100Ω R2 348kΩ CPWR 1μF PWR CNTRL IN CIN 1μF CP MAX15031 LX PGND CF 0.1μF R1 634kΩ BIAS FB SHDN CN ILIM VDD CLAMP RLIM RLIM 2.87kΩ SGND APD MOUT APD RMOUT 10kΩ CMOUT (OPTIONAL) ADC DAC VDD μC GPIO GPIO Figure 4. Typical Operating Circuit for VIN = 5.5V to 11V Chip Information PROCESS: BiCMOS Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 16 TQFN PACKAGE CODE T1644-4 DOCUMENT NO. 21-0139 16 ______________________________________________________________________________________ 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications Revision History REVISION REVISION NUMBER DATE 0 1 10/08 3/09 Initial release. Updated E lectrical Characteristics a nd added new Note 3. • Changed “Shutdown Input Bias Current” to “BIAS Current During Shutdown” in the E lectrical Characteristics t able. • Changed minimum value for the Current Gain (IAPD = 2 mA) specification to 0.0965 in the E lectrical Characteristics t able. DESCRIPTION PAGES CHANGED — 3 MAX15031 2 6/09 2, 3 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 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