LT1767 Просмотр технического описания (PDF) - Linear Technology

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LT1767

Monolithic 1.5A, 1.25MHz Step-Down Switching Regulators

Linear Technology 

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

Block diagram

The LT1767 is a constant frequency, current mode buck

converter. This means that there is an internal clock and

two feedback loops that control the duty cycle of the power

switch. In addition to the normal error amplifier, there is a

current sense amplifier that monitors switch current on a

cycle-by-cycle basis. A switch cycle starts with an oscillator

pulse which sets the RS flip-flop to turn the switch on. When

switch current reaches a level set by the inverting input of

the comparator, the flip-flop is reset and the switch turns

off. Output voltage control is obtained by using the output

of the error amplifier to set the switch current trip point.

This technique means that the error amplifier commands

current to be delivered to the output rather than voltage.

A voltage fed system will have low phase shift up to the

resonant frequency of the inductor and output capacitor,

then an abrupt 180° shift will occur. The current fed sys-

tem will have 90° phase shift at a much lower frequency,

but will not have the additional 90° shift until well beyond

the LC resonant frequency. This makes it much easier to

frequency compensate the feedback loop and also gives

much quicker transient response.

High switch efficiency is attained by using the BOOST pin

to provide a voltage to the switch driver which is higher

than the input voltage, allowing switch to be saturated.

This boosted voltage is generated with an external capac-

itor and diode. A comparator connected to the shutdown

pin disables the internal regulator, reducing supply

current.

VIN 2

2.5V BIAS

REGULATOR

INTERNAL

VCC

0.01Ω

+–

CURRENT

SENSE

AMPLIFIER

VOLTAGE GAIN = 40

SLOPE COMP

SYNC 8

1.25MHz

OSCILLATOR

SHUTDOWN

COMPARATOR

7µA

+–

1.33V

∑

0.35V

CURRENT

COMPARATOR

+

S

RS

FLIP-FLOP

R

–

1 BOOST

DRIVER

CIRCUITRY

PARASITIC DIODES

DO NOT

FORWARD BIAS

Q1

POWER

SWITCH

3 VSW

SHDN 5

3µA

7 VC

gm

ERROR

AMPLIFIER

gm = 850µMho

1.2V

Figure 1. Block Diagram

For more information www.linear.com/LT1767

6 FB

4 GND

1767 F01

1767fb

7

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