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LT3482EUD-TRPBF

90V Boost DC/DC Converter with APD Current Monitor

Linear Technology 

LT3482

W

FU CTIO AL DIAGRA

9 GND

10 GND

L1

VIN

11 7

VIN SW

CFLY

8

SW

D1

6

PUMP

D3

D2

VOUT2 4

VOUT1

5

1.235V

REFERENCE

+

+ A1

–

EAMP

–

RC

A2

+

RSQ

CC

PWM

COMPARATOR

Σ

DRIVER

Q1

VIN

RS

12 SHDN

CS

RAMP

GENERATOR

CTRL

13

FB

14

650kHz/1.1MHz

OSCILLATOR

fSET

15

CURRENT

SENSE

AMPLIFIER

MON

16

CONTROL

BLOCK

R3

C4

MONIN

3

APD

CURRENT

MIRROR

APD

2

NC 1

3482 BD

C2 R1

CPL

FB

C1 R2

C3

U

OPERATIO

The LT3842 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 Functional Diagram. At the start of each

oscillator cycle, the SR latch is set, which turns on the

power switch, Q1. 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,

A2. When this voltage exceeds the level at the negative

input of A2, the SR latch is reset turning off the power

switch. The level at the negative input of A2 is set by the

error amplifier A1, and is simply an amplified version of the

difference between the feedback voltage and the reference

voltage of 1.235V, or externally provided CTRL voltage.

In this manner, the error amplifier sets the correct peak

current level to keep the output in regulation. If the error

amplifier’s output increases, more current is delivered to

the output; if it decreases, less current is delivered.

The LT3482 has an integrated high side APD current moni-

tor with a 5:1 ratio. The MONIN pin can accept a supply

voltage up to 90V, which is suitable for APD photodiode

applications. The MON pin has an open-circuit protection

feature and is internally clamped to 11.5V.

If an APD is tied to the APD pin, the current will be mir-

rored to the MON pin and converted to a voltage signal

by the resistor R3. This voltage signal can be used to

drive an external control block to adjust the APD voltage

by adjusting the feedback threshold of EAMP A1 through

the CTRL input.

3482fa

6

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