Datasheet

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LT3482

3482fa

90V Boost DC/DC

Converter with APD

Current Monitor

The LT

3482 is a ﬁ xed frequency current mode step-up

DC/DC converter with voltage doubler designed to bias

avalanche photodiodes (APDs) in optical receivers. It can

provide up to 90V output. The LT3482 features high side

APD current monitoring with better than 10% relative ac-

curacy over the entire temperature range. The integrated

power switch, Schottky diodes and APD current monitor

allow a small converter footprint and low solution cost.

Constant switching frequency results in predictable output

noise that is easy to ﬁ lter. The inductor-based topology

ensures an input free from switching noise. An integrated

high side current monitor produces a current proportional

to 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 LT3482 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

■

High Output Voltage: Up to 90V

■

Integrated Schottky Diodes

■

48V, 280mA Internal Switch

■

High Side APD Current Monitor

■

Adjustable Switching Frequency: 650kHz or 1.1MHz

■

Wide V

Range: 2.5V to 16V

■

Surface Mount Components

■

Low Shutdown Current: <1µA

■

Soft-Start

■

Internal Compensation

■

CTRL Pin Allows Output Adjustment with No Polarity

Inversion

■

3mm × 3mm 16-Lead QFN Package

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

1µF

0.1µF

MONINV

SET

SHDN

CTRL

GND

OUT2

OUT1

SW PUMP

LT3482

MON APD

OFF ON

14k

85V

AT 2.5mA

3482 TA01a

0.47µF

0.1µF

10k

10nF

0.1µF

10µH

APD

RIPPLE

100 V/DIV

500ns/DIV

3482 TA01b

Output Voltage Ripple

Summary of content (12 pages)

PAGE 1
LT3482 90V Boost DC/DC Converter with APD Current Monitor U FEATURES DESCRIPTIO ■ The LT®3482 is a ﬁxed frequency current mode step-up DC/DC converter with voltage doubler designed to bias avalanche photodiodes (APDs) in optical receivers. It can provide up to 90V output. The LT3482 features high side APD current monitoring with better than 10% relative accuracy over the entire temperature range.
PAGE 2
LT3482 W W W AXI U U ABSOLUTE RATI GS PIN CONFIGURATION (Note 1) CTRL FB fSET MON TOP VIEW 16 15 14 13 NC 1 12 SHDN APD 2 11 VIN 17 MONIN 3 10 GND VOUT2 4 6 7 8 VOUT1 SW SW 9 5 PUMP Input Voltage (VIN) ....................................................16V VOUT1, SW Voltage ....................................................48V VOUT2, PUMP, MONIN, APD Voltage..........................90V FB Voltage ...................................................................
PAGE 3
LT3482 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3V, V⎯S⎯H⎯D⎯N = 3V unless otherwise noted. PARAMETER CONDITIONS MIN TYP SHDN Voltage Low SHDN Pin Bias Current 35 fSET Voltage High MAX 0.4 V 50 µA 1.5 V fSET Voltage Low fSET Bias Current fSET = 2V CTRL to FB Offset CTRL = 0.5V APD Current Monitor Gain IAPD = 250nA, 10V ≤ MONIN ≤ 90V IAPD = 2.
PAGE 4
LT3482 U W TYPICAL PERFOR A CE CHARACTERISTICS (TA = 25°C unless otherwise speciﬁed) Oscillator Frequency vs Temperature SWITCH CURRENT LIMIT (mA) 1100 FREQUENCY (kHz) fSET = 2V 1000 900 800 fSET = 0V 600 400 360 350 340 SWITCH CURRENT LIMIT (mA) 1200 700 300 250 200 150 100 50 25 75 0 TEMPERATURE (°C) 100 0 125 20 0 40 60 DUTY CYCLE (%) 3482 G01 1.0E-02 80 70 80 90 –2 1.0E-06 –8 1.0E-06 1.
PAGE 5
LT3482 U U U PI FU CTIO S APD (Pin 2): 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. VOUT2 (Pin 4): Voltage Doubler Output Pin. Put a 50V rated capacitor between this pin and VOUT1. Tie a resistor divider to the FB pin and GND. VOUT1 (Pin 5): Boost Output Pin. Put a capacitor between this pin and the GND plane. Minimize the length of the trace to the capacitor.
PAGE 6
LT3482 W FU CTIO AL DIAGRA U U CFLY L1 VIN 11 7 VIN SW 8 6 SW 9 GND D1 PUMP D3 D2 VOUT2 VOUT1 10 GND 1.235V REFERENCE + + – MONIN – A1 RC EAMP + CC 4 Q1 R S Q A2 PWM COMPARATOR + CS 13 CTRL 14 FB R2 FB APD CURRENT MIRROR APD 2 C3 CURRENT SENSE AMPLIFIER 650kHz/1.
PAGE 7
LT3482 U W U U APPLICATIO S I FOR ATIO Table 1. Inrush Peak Current Switching Frequency The LT3482 can operate at either 650kHz nominal or 1.1MHz nominal; the voltage at the fSET pin selects which frequency is used. At 1.1MHz, a physically smaller inductor and capacitor can be used in a given application, but higher frequencies will slightly decrease efﬁciency and maximum duty cycle.
PAGE 8
LT3482 U W U U APPLICATIO S I FOR ATIO Inductor Selection The inductors used with the LT3482 should have a saturation current rating of 0.3A 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 60mA or more.
PAGE 9
LT3482 U W U U APPLICATIO S I FOR ATIO In some applications, a long cable or wire is used to connect the LT3482 to APD. When APD is shorted to GND, APD pin voltage might ring below ground and damage the internal circuitry. To prevent damage during short-circuit event, a 20Ω resistor must be added in series with the APD. Layout Hints The high speed operation of the LT3842 demands careful attention to board layout. You will not get advertised performance with careless layout.
PAGE 10
LT3482 U TYPICAL APPLICATIO S 5V to 85V APD Bias Power Supply 60 C2 0.1µF L1 10µH VIN 5V Efﬁciency 50 SW VIN fSET OFF ON PUMP MONIN EFFICIENCY (%) C1 1µF LT3482 SHDN VOUT2 CTRL VOUT1 GND C4 0.47µF C5 0.47µF FB MON R3 10k 30 20 10 R2 14k 85V AT 2.5mA APD C7 10nF R1 1M C6 0.1µF 40 0 0 C3 0.1µF 0.5 1 2 1.5 2.5 IMONIN (mA) 3 3482 TA02a 3.
PAGE 11
LT3482 U PACKAGE DESCRIPTIO UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 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 16 PIN 1 TOP MARK (NOTE 6) 0.40 ± 0.10 1 1.45 ± 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 NOTE: 1.
PAGE 12
LT3482 U TYPICAL APPLICATIO 3.3V to 40V APD Bias Power Supply Efﬁciency L1 5.6mH VIN 3.3V 70 60 SW VIN fSET OFF ON PUMP MONIN 50 R3 100W LT3482 VOUT2 SHDN CTRL VOUT1 GND C2 1mF FB MON R4 10k C3 0.1mF R1 1M C4 0.1mF 40 30 20 10 R2 28k APD EFFICIENCY (%) C1 1mF 0 40V AT 3mA 0 3482 TA04a 0.5 1 1.5 2 2.5 3 3.