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19-5132; Rev 1; 3/10
TION KIT EVALUA BLE VAILA A
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications
General Description
The MAX15059 constant-frequency pulse-width modulating (PWM) step-up DC-DC converter features an internal switch and a high-side current monitor with highspeed adjustable current limiting. This device is capable of generating output voltages up to 76V (300mW for the MAX15059A and 200mW for the MAX15059B) and provides current monitoring up to 4mA. The MAX15059 operates from 2.8V to 5.5V. The constant-frequency (400kHz) current-mode PWM architecture provides low-noise-output voltage that is easy to filter. A high-voltage internal power MOSFET allows this device to boost output voltages up to 76V. Internal soft-start circuitry limits the input current when the boost converter starts. The MAX15059 features a shutdown mode to save power. The MAX15059 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 (UVLO), and thermal shutdown if the die temperature reaches +150NC. The MAX15059 is available in a thermally enhanced, lead-free, 16-pin TQFN-EP package and operates over the -40NC to +85NC temperature range.
S Input Voltage Range: +2.8V to +5.5V S Wide Output-Voltage Range from (VIN + 5V) to 76V S Internal 1I (typ) 80V MOSFET S Boost Converter Output Power: 300mW S 200mW Version Available for Smaller Inductor S Accurate Q5% (1:1 and 5:1) High-Side Current
Features
MAX15059
Monitor
S Resistor-Adjustable Ultra-Fast APD Current Limit S S S S S S S
(1s 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 2A (max) Shutdown Current -40NC to +85NC Temperature Range Small, Thermally Enhanced, 3mm x 3mm, LeadFree, 16-Pin TQFN-EP Package
Ordering Information
PART MAX15059AETE+ MAX15059BETE+ MAXIMUM POWER (mW) 300 200 IAPD: IMOUT 1:1 5:1 PINPACKAGE 16 TQFN-EP* 16 TQFN-EP*
Applications
Avalanche Photodiode Biasing and Monitoring PIN Diode Bias Supply Low-Noise Varactor Diode Bias Supply FBON Modules GPON Modules
Note: All devices operate over the -40C to +85C temperature range. +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad.
Typical Operating Circuit
VIN = 2.8V TO 5.5V L1 4.7H CIN 1F IN CNTRL LX PGND BIAS R1 6.34kI D1 VOUT R2 348kI RADJ (76V MAX)
COUT 0.1F
MAX15059
RLIM RLIM 2.87kI SGND APD APD
FB SHDN ILIM VDD CLAMP MOUT RMOUT 1kI GPIO GPIO VDD DAC
C
ADC CMOUT OPTIONAL (10nF)
TIA
_______________________________________________________________ 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.
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
ABSOLUTE MAXIMUM RATINGS
IN, SHDN, FB, ILIM, RLIM, CNTRL to SGND ..........-0.3V to +6V LX to PGND ...........................................................-0.3V to +80V BIAS to SGND ......................................................-0.3V to +79V APD, CLAMP to SGND ........................... -0.3V to (VBIAS + 0.3V) PGND to SGND ....................................................-0.3V to +0.3V MOUT to SGND .................................. -0.3V to (VCLAMP + 0.3V) Continuous Power Dissipation (TA = +70NC) 16-Pin TQFN-EP (derate 20.8mW/NC above +70NC) .........................................................1666.7mW Junction-to-Case Thermal Resistance (BJC) (Note 1) 16-Pin TQFN-EP ......................................................... +7NC/W Junction-to-Ambient Thermal Resistance (BJA) (Note 1) 16-Pin TQFN-EP ....................................................... +48NC/W Operating Temperature Range .......................... -40NC to +85NC Maximum Junction Temperature.....................................+150NC Storage Temperature Range............................ -65NC to +150NC Lead Temperature (soldering, 10s) ................................+300NC Soldering Temperature (reflow) ......................................+260NC
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 = VSHDN = VCNTRL = 3.3V, CIN = 1FF, VPGND = VSGND = 0V, VBIAS = 40V, LX = APD = CLAMP = ILIM = unconnected, VMOUT = VRLIM = 0V, TA = -40NC to +85NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER INPUT SUPPLY Supply Voltage Range Supply Current Undervoltage-Lockout Threshold Shutdown Current Shutdown BIAS Current BOOST CONVERTER Output-Voltage Adjustment Range Switching Frequency Maximum Duty Cycle FB Set-Point Voltage FB Input-Bias Current Internal Switch On-Resistance Peak Switch Current Limit Peak Current-Limit Response LX Leakage Current Line Regulation Load Regulation Soft-Start Duration Soft-Start Steps CONTROL INPUT (CNTRL) Maximum Control Input Voltage Range FB set point is controlled to VCNTRL 1.2 V VLX = 76V, TA = +25NC 2.8V P VIN P 5.5V, ILOAD = 4.5mA 0 P ILOAD P 4.5mA 0.2 1 8 32 fSW DCLK VFB_SET IFB RON ILIM_LX VFB = VFB_SET, TA = +25NC ILX = 100mA, VIN = 2.8V MAX15059A MAX15059B 1.1 0.825 1 1.2 0.9 100 1 VIN = 5V VIN = 2.8V VIN + 5 380 88 1.2054 400 90 1.23 76 420 92 1.2546 500 2 1.3 0.975 V kHz % V nA I A ns FA % % ms Steps VIN ISUPPLY VUVLO ISHDN VFB = 1.4V, no switching VIN rising VSHDN = 0V 2.475 2.8 1 2.6 200 2 20 5.5 1.2 2.775 V mA V mV FA FA SYMBOL CONDITIONS MIN TYP MAX UNITS
Undervoltage-Lockout Hysteresis VUVLO_HYS IBIAS_SHDN VBIAS = 3.3V, VSHDN = 0V
2
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76V, 300mW Boost Converter and Current Monitor for APD Bias Applications
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSHDN = VCNTRL = 3.3V, CIN = 1FF, VPGND = VSGND = 0V, VBIAS = 40V, LX = APD = CLAMP = ILIM = unconnected, VMOUT = 0V, TA = -40NC to +85NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 2) PARAMETER CNTRL-to-REF Transition Threshold CNTRL Input-Bias Current CURRENT MONITOR Bias Voltage Range VBIAS IAPD = 500nA Bias Quiescent Current IBIAS IAPD = 2mA Voltage Drop Dynamic Output Resistance at MOUT APD Current-Step Response MOUT Output Leakage Output Clamp Voltage Output Clamp Leakage Current MOUT Voltage Range VMOUT VMOUT VCLAMP Forward diode current = 500FA VBIAS = VCLAMP = 76V 10V P VBIAS P 76V, 0 P IAPD P 1mA, CLAMP is unconnected IAPD = 500nA Current Gain IMOUT/IAPD IAPD = 2mA Power-Supply Rejection Ratio APD Input Current Limit Current-Limit Adjustment Range Power-Up Settling Time LOGIC I/O SHDN Input Voltage Low SHDN Input Voltage High ILIM Output Voltage Low ILIM Output Leakage Current THERMAL PROTECTION Thermal-Shutdown Temperature Thermal-Shutdown Hysteresis VIL VIH VOL IOH ILIM = 2mA TA = +25NC Temperature rising +150 15 2.1 0.1 1 0.8 V V V FA NC NC tS PSRR ILIM_APD 9.75kI R RLIM R 0 IMOUT settles to within 0.1%, IAPD = 500nA 10nF connected from APD to IAPD = 2.5mA ground MAX15059A MAX15059B MAX15059A MAX15059B VBIAS 2.7 0. 95 0.19 0.965 0.193 35 35 4 0.9 7.5 90 1 0.2 1 0.2 300 300 4.6 1.1 0.22 1.035 0.207 610 700 5.2 5.2 ppm/V mA mA ms Fs mA/mA 0.45 VDROP RMOUT MAX15059A MAX15059B MAX15059A MAX15059B 10 150 150 4 3 2.7 5 25 1 0.7 1 0.95 76 250 250 6 4 3.5 V FA mA V GI ns nA V nA V SYMBOL CONDITIONS VFB = VREF above this voltage VCNTRL = VFB_SET, TA = +25NC MIN TYP 1.3 500 MAX UNITS V nA
MAX15059
IAPD = 2mA, VDROP = VBIAS - VAPD RMOUT = DVMOUT/DIMOUT, IAPD = 2.5mA MAX15059A
Step load on IAPD = 20FA to 1mA
(DIMOUT/IMOUT)/DVBIAS, MAX15059A VBIAS = 10V to 76V and IAPD MAX15059B = 5FA to 1mA (Note 3)
Note 2: All MIN/MAX parameters are tested at TA = +25NC. Limits overtemperature are guaranteed by design. Note 3: Guaranteed by design and not production tested.
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3
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
Typical Operating Characteristics
(VIN = 3.3V, VOUT = 70V, TA = +25C, unless otherwise noted.)
MINIMUM STARTUP VOLTAGE vs. LOAD CURRENT
MAX15059 toc02
EFFICIENCY vs. LOAD CURRENT
MAX15059 toc01
EFFICIENCY vs. LOAD CURRENT
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 VOUT = 70V VOUT = 50V VOUT = 30V VIN = 5V 2.65 2.64 MINIMUM STARTUP VOLTAGE (V) 2.63 2.62 2.61 2.60 2.59 2.58 2.57 2.56 2.55 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 0.5 1.0 1.5 2.0 2.5 VOUT = 70V VOUT = 30V
VIN = 3.3V
VOUT = 50V
3.0
3.5
4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX15059 toc04
NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE
45 40 SUPPLY CURRENT (mA) 35 30 25 20 15 10 5 0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
MAX15059 toc05
2.0 1.8 1.6
50
VFB = 1.4V
SUPPLY CURRENT (mA)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
TA = +25C TA = +85C
TA = +25C
TA = +85C
TA = -40C
TA = -40C
SUPPLY VOLTAGE (V)
EXITING SHUTDOWN
MAX15059 toc06
ENTERING SHUTDOWN
MAX15059 toc07
SHDN 2V/div
SHDN 2V/div
INDUCTOR CURRENT 500mA/div VOUT 50V/div
INDUCTOR CURRENT 500mA/div
VOUT 50V/div 1ms/div 4ms/div
4
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MAX15059 toc03
100
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications
Typical Operating Characteristics (continued)
(VIN = 3.3V, VOUT = 70V, TA = +25C, unless otherwise noted.)
LIGHT-LOAD SWITCHING WAVEFORMS WITH RC FILTER HEAVY-LOAD SWITCHING WAVEFORMS WITH RC FILTER
MAX15059 toc09
MAX15059
MAX15059 toc08
VBIAS (AC-COUPLED) 20mV/div
VBIAS (AC-COUPLED) 50mV/div
VLX 50V/div IL 500mA/div 1s/div 1s/div
VLX 50V/div IL 1A/div
LOAD-TRANSIENT RESPONSE
MAX15059 toc10
LINE-TRANSIENT RESPONSE
MAX15059 toc11
VIN 2V/div IAPD 2mA/div 0mA VBIAS (AC-COUPLED) 500mV/div 3.3V
VBIAS (AC-COUPLED) 50mV/div
100s/div
100s/div
LX LEAKAGE CURRENT vs. TEMPERATURE
MAX15059 toc12
LOAD REGULATION
MAX15059 toc13
MAXIMUM LOAD CURRENT vs. SUPPLY VOLTAGE
90 80 70 IOUT(MAX) (mA) 60 50 40 30 20 10 0 D 2.5 3.0 3.5 4.0 4.5 E 5.0 F 5.5 C B A A: VOUT = 30V, B: VOUT = 35V, C: VOUT = 45V, D: VOUT = 55V, E: VOUT = 60V, F: VOUT = 70V
MAX15059 toc14
10 9 LX LEAKAGE CURRENT (nA) 8 7 6 5 4 3 2 1 0 -40 -15 10 35 60
0.5 0.4 0.3 REGULATION (%) 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
100
CURRENT INTO LX PINS VLX = 70V
85
4.0
TEMPERATURE (C)
LOAD CURRENT (mA)
SUPPLY VOLTAGE (V)
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5
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
Typical Operating Characteristics (continued)
(VIN = 3.3V, VOUT = 70V, TA = +25C, unless otherwise noted.)
BIAS CURRENT vs. BIAS VOLTAGE
MAX15059 toc15
BIAS CURRENT vs. APD CURRENT
MAX15059 toc16
BIAS CURRENT vs. TEMPERATURE
MAX15059 toc17
10
10
10
BIAS CURRENT (mA)
BIAS CURRENT (mA)
BIAS CURRENT (mA)
1
IAPD = 2mA
1
IAPD = 2mA
1
0.1
0.1
IAPD = 500nA VBIAS = 70V
0.01 0 10 20 30 40 50 60 70 80 BIAS VOLTAGE (V) 0.01 0.0001 0.1 0.001 0.01 0.1 1 10 -40 -15
IAPD = 500nA
10
35
60
85
APD CURRENT (mA)
TEMPERATURE (C)
GAIN ERROR vs. APD CURRENT
MAX15059 toc18
GAIN ERROR vs. APD CURRENT
MAX15059 toc18b
GAIN ERROR vs. TEMPERATURE
1.6 1.2 GAIN ERROR (%) 0.8 0.4 0 -0.4 -0.8 -1.2 -1.6 -2.0 -40 -15 10 35 60 85 TEMPERATURE (C)
4 3 GAIN ERROR (%) 2 1 0 -1 -2 -3 -4 -5 0.1 1 10 100 1000
4 3 GAIN ERROR (%) 2 1 0 -1 -2 -3
VBIAS = 70V
IAPD = 0.5A
IAPD = 5A
IAPD = 50A
IAPD = 500A IAPD = 2mA
VBIAS = 70V
10,000
-4 -5 0.1 1 10 100 1000 10,000
IAPD (A)
IAPD (A)
GAIN ERROR vs. TEMPERATURE
MAX15059 toc19b
GAIN ERROR vs. BIAS VOLTAGE
1.6 1.2 GAIN ERROR (%) 0.8 0.4 0 -0.4 -0.8 -1.2 -1.6 -2.0 IAPD = 500A IAPD = 0.5A IAPD = 2mA
MAX15059 toc20
2.0 1.6 1.2 GAIN ERROR (%) 0.8 0.4 0 -0.4 -0.8 -1.2 -1.6 -2.0 -40 -15 10 35 60 IAPD = 500A IAPD = 50A IAPD = 5A IAPD = 2mA IAPD = 0.5A
2.0 IAPD = 50A IAPD = 5A
85
10
20
30
40
50
60
70
80
TEMPERATURE (C)
BIAS VOLTAGE (V)
6
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MAX15059 toc19
5
5
2.0
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications
Typical Operating Characteristics (continued)
(VIN = 3.3V, VOUT = 70V, TA = +25C, unless otherwise noted.)
MAX15059
GAIN ERROR vs. BIAS VOLTAGE
1.6 1.2 GAIN ERROR (%) 0.8 0.4 0 -0.4 -0.8 -1.2 -1.6 -2.0 10 20 30 40 50 60 70 80 IAPD = 50A IAPD = 0.5A IAPD = 500A IAPD = 5A IAPD = 2mA
MAX15059 toc20b
APD TRANSIENT RESPONSE
MAX15059 toc21
2.0
IAPD 2mA/div 0mA IMOUT 2mA/div 0mA VAPD (AC-COUPLED) 2V/div
20s/div
BIAS VOLTAGE (V)
STARTUP DELAY
MAX15059 toc22
STARTUP DELAY
MAX15059 toc23
SHDN 5V/div VBIAS 50V/div
SHDN 5V/div VBIAS 50V/div
IMOUT 500nA/div VBIAS = 70V, IAPD = 500nA 2ms/div 1ms/div
IMOUT 1mA/div VBIAS = 70V IAPD = 2mA
STARTUP DELAY
MAX15059 toc24
STARTUP DELAY
MAX15059 toc25
SHDN 5V/div VBIAS 5V/div
SHDN 5V/div
VBIAS 5V/div IMOUT 1mA/div
IMOUT 500nA/div VBIAS = 10V, IAPD = 500nA 200s/div 400s/div
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7
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
Typical Operating Characteristics (continued)
(VIN = 3.3V, VOUT = 70V, TA = +25C, unless otherwise noted.)
SHORT-CIRCUIT RESPONSE
MAX15059 toc26
VOLTAGE DROP vs. APD CURRENT
MAX15059 toc27
SWITCHING FREQUENCY vs. TEMPERATURE
409 408 FREQUENCY (kHz) 407 406 405 404 403 402 401 400 -40 -15 10 35 60 85 TEMPERATURE (C)
MAX15059 toc28 MAX15059 toc30
3.0 2.5 VBIAS - VAPD (V) 2.0 1.5 1.0 0.5 0
410
RLIM = 3.16kI
VAPD 50V/div IMOUT 2mA/div
TA = -40C
TA = +25C TA = +85C
IILIM 5V/div 2s/div
0.1
1
10
100
1000
10,000
IAPD (A)
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX15059 toc29
SWITCHING FREQUENCY AND DUTY CYCLE vs. LOAD CURRENT
410 408 SWITCHING FREQUENCY (kHz) 406 404 402 400 398 396 394 392 390 50 45 40 30 25 DUTY CYCLE (%) 35
410 408 SWITCHING FREQUENCY (kHz) 406 404 402 400 398 396 394 392 390 2.5 3.0 3.5 4.0 4.5 5.0
DUTY CYCLE
SWITCHING FREQUENCY
20 15 10 5 0
5.5
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
FB SET POINT vs. TEMPERATURE
1.238 1.236 FB SET POINT (V) 1.234 1.232 1.230 1.228 1.226 1.224 1.222 1.220 -40 -15 10 35 60 85
MAX15059 toc31
1.240
APD OUTPUT RIPPLE VOLTAGE (0.1F FROM APD TO GROUND, VBIAS = 70V, LAPD = 1mA)
MAX15059 toc32
VAPD AC-COUPLED, 70V 1mV/div
VIN = 3.3V
1s/div
TEMPERATURE (C)
8
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76V, 300mW Boost Converter and Current Monitor for APD Bias Applications
Pin Configuration
MOUT RLIM APD
MAX15059
TOP VIEW
12
CLAMP 11
10
9
BIAS 13 LX 14 LX 15 PGND 16
8 7
SGND ILIM CNTRL FB
MAX15059 +
1 PGND 2 IN 3 SHDN
6
EP
5
4 SGND
TQFN
Pin Description
PIN 1, 16 2 3 NAME PGND IN SHDN FUNCTION 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. Input-Supply Voltage. Bypass IN to PGND with a ceramic capacitor of 1FF minimum value. Active-Low Shutdown Control Input. Apply a logic-low voltage to SHDN to shut down the device. Connect SHDN to IN for normal operation. Ensure that VSHDN is not greater than the input voltage, VIN. SHDN is internally pulled low. The converter is disabled when SHDN is left unconnected. 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 boost output to SGND to set the output voltage. The FB voltage regulates to 1.23V (typ) when VCNTRL is above 1.3V (typ) and to VCNTRL when VCNTRL is below 1.2V (typ). Control Input for Boost Converter Output-Voltage Programmability. CNTRL allows the feedback set-point voltage to be set externally by CNTRL when CNTRL is less than 1.2V. Pull CNTRL above 1.3V (typ) to use the internal 1.23V (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 currentlimit threshold. When RLIM is connected to SGND, the current limit is set to 4.6mA. Current-Monitor Output. For the MAX15059A, MOUT sources a current equal to IAPD. For the MAX15059B, MOUT sources a current equal to 1/5 of IAPD. Clamp Voltage Input. CLAMP is the external potential used for voltage clamping of MOUT.
4, 8
SGND
5
FB
6 7 9 10 11
CNTRL ILIM RLIM MOUT CLAMP
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9
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
Pin Description (continued)
PIN 12 13 NAME APD BIAS FUNCTION 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. Drain of Internal 80V n-Channel DMOS. Connect inductor to LX. Minimize the trace area at LX to reduce switching-noise emission. Exposed Paddle. Connect to a large copper plane at the SGND and PGND potential to improve thermal dissipation. Do not use as the only ground connection.
14, 15 --
LX EP
Functional Diagram
FB CNTRL SGND VREF MUX
-A +A -C +C
OUTPUT ERROR AND CURRENT COMPARATOR 80V DMOS
LX
SOFTSTART
PEAK CURRENT-LIMIT COMPARATOR
SWITCH CONTROL LOGIC
PGND
VREF
REFERENCE COMPARATOR
SWITCH CURRENT SENSE VREF CLAMP THERMAL SHUTDOWN CONTROL MONITOR 1X (A) 5X (B) 1X CURRENTLIMIT ADJUSTMENT MOUT RLIM
BIAS AND REF IN UVLO
CLK OSCILLATOR 400kHz
CURRENT LIMIT
APD ILIM
MAX15059
SHDN
BIAS
10
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76V, 300mW Boost Converter and Current Monitor for APD Bias Applications
Detailed Description
The MAX15059 constant-frequency, current-mode, PWM boost converters are intended for low-voltage systems that require a locally generated high voltage. These devices are capable of generating a low-noise, high output voltage required for PIN and varactor diode biasing. The MAX15059 operates from +2.8V to +5.5V. The MAX15059 operates in discontinuous mode in order to reduce the switching noise caused by reverse recovery charge of the rectifier diode and eliminates the need for external compensation components. Other continuous-mode boost converters generate large voltage spikes at the output when the LX switch turns on 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 lateral DMOS device used as the internal power switch is ideal for boost converters with output voltages up to 76V. The MAX15059 can also be used in other topologies where the PWM switch is grounded, like SEPIC and flyback converters. The MAX15059 includes a versatile current monitor intended for monitoring the APD, PIN, or varactor diode DC current in fiber and other applications. The MAX15059 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. MOUT output accuracy is Q10% from 500nA to 1mA and Q5% from 1mA to 2mA. The MAX15059 also features a shutdown logic input to disable the device and reduce its standby current to 2FA (max). The heart of the MAX15059 current-mode PWM controller is a BiCMOS multi-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 configura-
MAX15059
tion approaches ideal cycle-by-cycle control over the output voltage since there is no conventional error amplifier in the feedback path. The devices operate in PWM mode using a fixedfrequency, current-mode operation. The current-mode frequency loop regulates the peak inductor current as a function of the output-voltage error signal. The current-mode PWM controller is intended for DCM operation. No internal slope compensation is added to the current signal. The current limit of the current monitor is programmable from 1mA to 4.6mA (typ). Connect RLIM to SGND to get a default current-limit threshold of 4.6mA or connect a resistor from RLIM to SGND to program the current-limit threshold below the default setting of 4.6mA. Calculate the value of the external resistor, RLIM, for a given current limit, ILIM, using the following equation:
Current Limit
1.23V R LIM(k) = x 10 - 2.67(k) ILIM(mA)
CLAMP provides a means for diode clamping the voltage at MOUT; thus, VMOUT is limited to (VCLAMP + 0.6V). CLAMP can be connected to either an external supply or BIAS. Leave CLAMP unconnected if voltage clamping is not required. The MAX15059 features an active-low shutdown input (SHDN). Pull SHDN low or leave it unconnected to enter shutdown. During shutdown, the supply current drops to 2FA (max). The output remains connected to the input through the inductor and output rectifier, holding the output voltage to one diode drop below IN when the MAX15059 is in shutdown. Connect SHDN to IN for always-on operation.
Clamping the Monitor Output Voltage (MOUT)
Shutdown
Fixed-Frequency PWM Controller
Apply a voltage to the CNTRL input to set the feedback set-point reference voltage, VREF (see the Functional Diagram). For VCNTRL > 1.3V, the internal 1.23V (typ) reference voltage is used as the feedback set point and for VCNTRL < 1.2V, the CNTRL voltage is used as the reference voltage (VFB set equal to VCNTRL).
Adjusting the Feedback Set-Point/Reference Voltage
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11
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
Design Procedure
Set the MAX15059 output voltage by connecting a resistive divider from the output to FB to SGND (Figure 1). Select R1 (FB to SGND resistor) between 5kI and 10kI. Calculate R2 (VOUT to FB resistor) using the following equation: V R 2 = R1 OUT - 1 VREF where VOUT can range from (VIN + 5V) to 76V. Apply a voltage to the CNTRL input to set the feedback set-point reference voltage, VREF (see the Functional Diagram). For VCNTRL > 1.3V, the internal 1.23 (typ) reference voltage is used as the feedback set point and for VCNTRL < 1.2V, VREF = VCNTRL. See the Adjusting the Feedback Set-Point/Reference Voltage section for more information on adjusting the feedback reference voltage, VREF. If the boost converter remains in the discontinuous mode of operation, then the approximate peak inductor current, ILPEAK (in A), is represented by the formula below: ILPEAK = 2 x t S x (VOUT - VIN _MIN ) x I OUT_MAX xL
Setting the Output Voltage
Three key inductor parameters must be specified for operation with the MAX15059: 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 peak switch current-limit value (ILIM_LX = 1.3A). DC series resistance (DCR) should be be low 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: L MIN[H] = 2 x t S x I OUT x (VOUT - VIN_MIN )
2 x ILIM_LX
Determining the Inductor Value
Determining Peak Inductor Current
where VIN_MIN, VOUT (both in volts), and IOUT (in amps) are typical values (so that efficiency is optimum for typical conditions), tS (in Fs) is the period, E is the efficiency, and ILIM_LX is the peak switch current in amps (see the Electrical 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 discontinuous-conduction mode (DCM) using the following formula: L [H] L OPTIMUM [H] = MAX 2.25 where: V2 (VOUT - VIN_MIN ) x t S x L MAX [H] = IN_MIN 2 2 x I OUT x VOUT For a design in which VIN = 3.3V, VOUT = 70V, IOUT = 3mA, E = 45%, ILIM_LX = 1.2A, and tS = 2.5Fs: LMAX = 27FH and LMIN = 1.5FH. For a worse-case scenario in which VIN = 2.8V, VOUT = 70V, IOUT = 4mA, = 43%, ILIM_LX = 1.2A, and tS = 2.5Fs: LMAX = 15FH and LMIN = 2.2FH. The choice of 4.7FH 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.
where tS is the switching period in Fs, VOUT is the output voltage in volts, VIN_MIN is the minimum input voltage in volts, IOUT_MAX is the maximum output current in amps, L is the inductor value in FH, and E is the efficiency of the boost converter (see the Typical Operating Characteristics).
VOUT
MAX15059
FB
R2 VCNTRL > 1.3V, VFB = 1.23V VCNTRL < 1.2V, VFB = VCNTRL R1
Figure 1. Adjustable Output Voltage 12 _____________________________________________________________________________________
76V, 300mW Boost Converter and Current Monitor for APD Bias Applications
The MAX15059'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 breakdown voltage must be greater than VOUT. For most applications, use a small output capacitor of 0.1FF 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:
Diode Selection
MAX15059
VIN = 2.8V TO 5.5V
L1
CIN IN CNTRL SHDN CIN LX D1 RF VOUT
Output Filter Capacitor Selection
MAX15059
FB
R2 COUT CF R1
PGND SGND
BIAS
Figure 2. Typical Operating Circuit with RC Filter
C OUT[F] =
I OUT ILPEAK x L OPTIMUM t S - 0.5 x VOUT (VOUT - VIN_MIN ) ESR[m] = 0.5 x VOUT I OUT
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 100I, 0.1FF (RF CF) filter used to reduce the switching output ripple to 1mVPP with a 0.1mA load or 1mVP-P with a 4mA load. The output voltage regulation resistive divider must remain connected to the diode/output capacitor node. Use X7R ceramic capacitors for more stability over the full temperature range.
Bypass IN to PGND with a 1FF (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 as well. If the layout cannot achieve this, add another 0.1FF ceramic capacitor between IN 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:
VOUT x I OUT ILPEAK x L OPTIMUM x VOUT t S - x VIN_MIN x 0.5 x VIN VIN_MIN (VOUT - VIN_MIN ) ESR[m] = 0.5 x VIN x x VIN_MIN VOUT x IOUT
Input-Capacitor Selection
CIN[F] =
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76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
Applications Information
When using the MAX15059 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 3.5V 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 MAX15059 is less than or equal to 76V. For this type of application, the MAX15059 is suggested so the output can be referenced to "true" ground and not the negative supply. The MAX15059'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 MAX15059'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 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 amp 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 MAX15059 exhibits a very high output impedance of 5GI.
Using APD or PIN Photodiodes in Fiber Applications
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. 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 MAX15059 Evaluation Kit data sheet for a layout example.
Layout Considerations
Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a "+", "#", or "-" in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE 16 TQFN-EP PACKAGE CODE T1633-4 DOCUMENT NO. 21-0136
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76V, 300mW Boost Converter and Current Monitor for APD Bias Applications MAX15059
Revision History
REVISION NUMBER 0 1 REVISION DATE 1/10 3/10 Initial release Replaced five TOCs, added three TOCs, updated text DESCRIPTION PAGES CHANGED -- 1, 2, 3, 5-8, 11
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15
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