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LT3571

3571fa

TYPICAL APPLICATION

DESCRIPTION

75V DC/DC Converter

for APD Bias

The LT

3571 is a current mode step-up DC/DC converter

designed to bias avalanche photodiodes (APDs) in optical

receivers with an output voltage up to 75V. The LT3571

features a high side ﬁ xed voltage drop APD current moni-

tor with better than 10% relative accuracy over the entire

temperature range. The integrated power switch, Schottky

diode and APD current monitor allow a small solution

footprint and low solution cost. It combines a traditional

voltage loop and a unique current loop to operate as

a constant-current source or constant-voltage source.

The inductor-based topology ensures an input free from

switching noise. The integrated high side current monitor

produces a current proportional to the APD current with

better than 10% relative accuracy over four decades of

dynamic range in the input range of 250nA to 2.5mA. This

current can be used as a reference to provide a digitally

programmed output voltage via the CTRL pin. The LT3571

is available in the tiny footprint (3mm × 3mm) 16-lead

QFN package.

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.

45V Low Noise APD Bias Power Supply

FEATURES

APPLICATIONS

High APD Voltage: Up to 70V

Integrated Schottky Diode

75V, 370mA Internal Switch

High Side Fixed Voltage Drop APD Current Monitor

Adjustable Frequency: 250kHz to 2MHz

Frequency Synchronization

Wide V

Range: 2.7V to 20V

Constant-Voltage and Constant-Current Regulation

Programmable Current Limit Protection

Surface Mount Components

Low Shutdown Current <1µA

Internal Soft-Start

Internal Compensation

CTRL Pin Allows Output Adjustment with

No Polarity Inversion

3mm × 3mm 16-Lead QFN Package

APD Bias

PIN Diode Bias

Optical Receivers and Modules

Fiber Optic Network Equipment

APD Bias Ripple

1µF

MONIN

REF

SHDN

CTRL

GND

SYNC

OUT

LT3571

MON APD

OFF ON

20.5k

49.9

45V

12.1k

1MHz

3571 TA01a

0.1µF

10k

10nF

50V

0.1µF

10µH

20

10nF

APD

= 1mA 500ns/DIV

3571 TA01b

500µV/DIV

Summary of content (16 pages)

PAGE 1
LT3571 75V DC/DC Converter for APD Bias FEATURES DESCRIPTION n The LT®3571 is a current mode step-up DC/DC converter designed to bias avalanche photodiodes (APDs) in optical receivers with an output voltage up to 75V. The LT3571 features a high side ﬁxed voltage drop APD current monitor with better than 10% relative accuracy over the entire temperature range. The integrated power switch, Schottky diode and APD current monitor allow a small solution footprint and low solution cost.
PAGE 2
LT3571 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VREF CTRL FB MON TOP VIEW 16 15 14 13 12 SHDN NC 1 APD 2 11 VIN 17 MONIN 3 10 GND VOUT 4 6 7 8 SW GND SYNC 9 5 SW Input Voltage (VIN), SHDN ........................................20V VOUT, MONIN, APD....................................................75V MON..........................................................................12V SW ............................................................................
PAGE 3
LT3571 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C, VIN = 3V, VSHDN = 3V, unless otherwise noted. PARAMETER CONDITIONS Switching Frequency RT = 12.1k RT = 4.22k RT = 56.2k Maximum Duty Cycle RT = 56.2k (250kHz) SYNC = 300kHz Clock Signal, RT = 56.2k RT = 12.1k (1MHz) RT = 4.
PAGE 4
LT3571 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit vs Temperature Switch Saturation Voltage (VCESAT) 500 500 450 450 400 400 350 400 350 0 20 60 40 DUTY CYCLE (%) 250 –50 100 80 –25 50 25 0 75 TEMPERATURE (°C) 100 0 125 0 50 100 150 200 250 300 350 400 ISW (mA) 3571 G03 3571 G02 Schottky Forward Drop Oscillator Frequency vs Temperature Oscillator Frequency vs RT 10000 950 1100 850 800 750 700 OSCILLATOR FREQUENCY (kHz) RT = 12.
PAGE 5
LT3571 TYPICAL PERFORMANCE CHARACTERISTICS VOUT – VMONIN Threshold vs Temperature Current Monitor Output vs VMONIN 22 VMONIN = 50V IAPD = 100μA 204 VMONIN = 75V 2 21 202 1 ERROR (%) 200 20 0 –1 198 –2 19 196 –3 194 –50 –25 –4 18 50 25 0 75 TEMPERATURE (°C) 100 125 10 20 30 40 50 VMONIN (V) 60 70 Current Monitor Accuracy vs Temperature 2 10000 10 100 1000 INPUT CURRENT (μA) VMONIN – VAPD vs Temperature 5.1 VMONIN = 75V 2.
PAGE 6
LT3571 PIN FUNCTIONS NC (Pin 1): No Connect. APD (Pin 2): APD Cathode Pin. Connect APD cathode to this pin. MONIN (Pin 3): Current Monitor Power Supply Pin. An external lowpass ﬁlter can be included here to further reduce supply voltage ripple. This pin also serves as the inverting input of the current sense ampliﬁer. Put a sense resistor between the MONIN pin and the VOUT pin to set the boost converter output current limit as 200mV/RSENSE .
PAGE 7
LT3571 BLOCK DIAGRAM RSENSE L1 VIN CPL C1 R1 CIN FB R2 3 – APD A1 R4 1V 14 15 EAMP CTRL FB + + – D1 1V MAIN SWITCH DRIVER – A3 C3 FB + MON + C2 16 x5 + 2 5, 6 SW – APD CURRENT MIRROR R3 4 MONIN VOUT + RC CC A2 R A4 S Q1 MAIN SWITCH Q + + PWM COMPARATOR SOFT-START EXTERNAL CONTROL BLOCK A5 – RAMP GENERATOR 8 SYNC 7, 10 GND 2MHz TO 250kHz OSCILLATOR 1V + A6 – VREF 1.
PAGE 8
LT3571 OPERATION The LT3571 boost converter uses a constant-frequency current mode control scheme to provide excellent line and load regulation. Operation can be 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 Q1 power switch. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A4.
PAGE 9
LT3571 APPLICATIONS INFORMATION Switching Frequency There are two methods to set the switching frequency of the LT3571. Both methods require a resistor connected at the RT pin. Do not leave the RT pin open. Also, do not load this pin with a capacitor. A resistor must always be connected for proper operation. One way to set the frequency is simply connecting an external resistor between the RT pin and GND.
PAGE 10
LT3571 APPLICATIONS INFORMATION Inductor Selection The inductors used with the LT3571 should have a saturation current rating of 0.4A, or greater. If the device is used in an application where the input supply will be hot-plugged, the saturation current rating should be equal to, or greater than, the peak inrush current. For best loop stability, the inductor value selected should provide a ripple current of 80mA or more.
PAGE 11
LT3571 APPLICATIONS INFORMATION APD Current Monitor Transient Response Measurement APD Bias Voltage Temperature Compensation The transient response of the APD current monitor is a key performance characteristic. It is essentially a function of the input step-signal levels, since the small signal bandwidth increases with the input signal. At greater than 10μA, the LT3571 APD current mirror typically has several hundred nanoseconds response time.
PAGE 12
LT3571 APPLICATIONS INFORMATION Setting APD Current Limit The LT3571 has a unique current loop to limit the APD current. Choose the sense resistor RSENSE across VOUT and MONIN pins to set the APD current limit by using the following formula: RSENSE = 200mV 1.2 × IAPD (mA) + 0.3mA close together 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.
PAGE 13
LT3571 TYPICAL APPLICATIONS 5V to 69V APD Bias Supply with Soft-Start L1 22μH VIN 5V R5 20k C1 1μF VIN SHDN C5 47nF SW VOUT RSENSE 20Ω VREF CTRL LT3571 MONIN 74V C3 10nF EXTERNAL CONTROL BLOCK FB MON GND SYNC RT C5 10nF R3 10k R1 1M R2 13.7k APD RT 33.2k 400kHz R4 49.9Ω C2 0.22μF 69V, 2mA C4 0.
PAGE 14
LT3571 TYPICAL APPLICATIONS 5V to 50V APD Bias Power Supply with Temperature Compensation L1 15μH VIN 5V R5 30.1k Q2 R7 49.9k OFF ON Q1 R6 100k R8 36.5k R9 20k SHDN VREF SW VIN VOUT RSENSE 50Ω LT3571 MONIN CTRL RT SYNC C6 0.1μF C3 10nF R4 49.9Ω C5 10nF L1: TDK VLF4012AT – 150MR63 C1: TDK X7R C1608X7R1C105KT C2: MURATA X7R GRM21AR72A224KAC5L C3: AVX X7R 06031C103K C4: MURATA X7R GRM188R72A104KA35 C5: MURATA X7R GRM155R71H103K C6: MURATA X7R GRM155R71A104KA01D R2 15k APD RT 33.
PAGE 15
LT3571 PACKAGE DESCRIPTION UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.70 p0.05 3.50 p 0.05 1.45 p 0.05 2.10 p 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 p0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 s 45o CHAMFER R = 0.115 TYP 0.75 p 0.05 15 16 PIN 1 TOP MARK (NOTE 6) 0.40 p 0.10 1 1.45 p 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 NOTE: 1.
PAGE 16
LT3571 TYPICAL APPLICATIONS 3.3V to 50V APD Bias Power Supply Input Power vs APD Current 450 L1 10μH OFF ON VIN SHDN 400 350 SW VOUT RSENSE 20Ω VREF C1 1μF INPUT POWER (mW) VIN 3.3V LT3571 CTRL MONIN 55V R1 1M RT SYNC C2 0.1μF FB MON GND RT 26.1k 500kHz 300 250 200 150 100 50 R2 18.2k APD R3 10k 50V 0 C3 0.1μF 3571 TA05a 0 1 1.5 0.