LT3681 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT3681

36V, 2A, 2.8MHz Step-Down Switching Regulator with Integrated Power Schottky Diode

Linear Technology 

LT3681

BLOCK DIAGRA

VIN

13 VIN

C1

BIAS

2

INTERNAL 1.265V REF

–

+

RUN/SS

14

5

SLOPE COMP

RT

6

RT

SOFT-START

PG

1

1.12V

ERROR AMP

+

–

OSCILLATOR

300kHz–2.8MHz

GND GND GND

FB

4 7 15

3

R2

R1

SWITCH

LATCH

R

Q

S

DISABLE

BurstMode

DETECT

VC CLAMP

BD

10

BOOST

11

SW

12

DC

9

DC 16

DA 8

VC 5

C3

L1

C2

CC

CF

RC

VOUT

3681 BD

OPERATION

The LT3681 is a constant frequency, current mode step-

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

sets 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 cur-

rent 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 cir-

cuitry. The bias regulator normally draws power from the

VIN pin, but if the BIAS 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 LT3681 in shutdown, disconnecting the output and

reducing the input current to less than 1µA.

8

The switch driver operates from either the input or from

the BOOST pin. An external capacitor is 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 LT3681 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 55µA in a typical application.

The oscillator reduces the LT3681’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.

The LT3681 contains a power good comparator which trips

when the FB pin is at 91% of its regulated value. The PG

output is an open-collector transistor that is off when the

output is in regulation, allowing an external resistor to pull

the PG pin high. Power good is valid when the LT3681 is

enabled and VIN is above 3.6V.

The LT3681 integrates a high quality, 36V, 2A power

Schottky diode to reduce the overall solution size.

3681f

Share Link: