LT3480 Просмотр технического описания (PDF) - Linear Technology

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LT3480

36V, 2A, 2.4MHz Step-Down Switching Regulator with 70µA Quiescent Current

Linear Technology 

LT3480

BLOCK DIAGRAM

VIN

4 VIN

C1

INTERNAL 0.79V REF

RUN/SS

5

–

+

Σ

SLOPE COMP

RT

10

RT

SYNC

6

SOFT-START

PG

7

ERROR AMP

0.7V

+

–

OSCILLATOR

200kHz–2.4MHz

GND

FB

11

8

R2

R1

SWITCH

LATCH

R

Q

S

DISABLE

Burst Mode

DETECT

VC CLAMP

BD

1

BOOST

2

SW

3

C3

L1

D1

VOUT

C2

VC 9

CC

CF

RC

3480 BD

OPERATION

The LT3480 is a constant frequency, current mode step-

down regulator. An oscillator, with frequency set by RT,

enables an RS flip-flop, turning on the internal power

switch. An amplifier and comparator monitor the current

flowing between the VIN and SW pins, turning the switch

off when this current reaches a level determined by the

voltage at VC. An error amplifier measures the output

voltage through an external resistor divider tied to the FB

pin and servos the VC pin. If the error amplifier’s output

increases, more current is delivered to the output; if it

decreases, less current is delivered. An active clamp on the

VC pin provides current limit. The VC pin is also clamped to

the voltage on the RUN/SS pin; soft-start is implemented

by generating a voltage ramp at the RUN/SS pin using an

external resistor and capacitor.

An internal regulator provides power to the control circuitry.

The bias regulator normally draws power from the VIN pin,

but if the BD pin is connected to an external voltage higher

than 3V bias power will be drawn from the external source

(typically the regulated output voltage). This improves

efficiency. The RUN/SS pin is used to place the LT3480

in shutdown, disconnecting the output and reducing the

input current to less than 1µA.

The switch driver operates from either the input or from

the BOOST pin. An external capacitor and diode are used

to generate a voltage at the BOOST pin that is higher than

the input supply. This allows the driver to fully saturate the

internal bipolar NPN power switch for efficient operation.

To further optimize efficiency, the LT3480 automatically

switches to Burst Mode operation in light load situations.

Between bursts, all circuitry associated with controlling

the output switch is shut down, reducing the input supply

current to 70µA in a typical application.

The oscillator reduces the LT3480’s operating frequency when

the voltage at the FB pin is low. This frequency foldback helps

to control the output current during startup and overload.

3480fe

8

For more information www.linear.com/LT3480

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