LT1767EMS8 데이터 시트보기 (PDF) - Linear Technology

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LT1767EMS8

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 oscilla-

tor 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 system 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 capaci-

tor and diode. A comparator connected to the shutdown

pin disables the internal regulator, reducing supply

current.

VIN 2

2.5V BIAS

REGULATOR

SYNC 8

SHUTDOWN

COMPARATOR

+–

1.33V

INTERNAL

VCC

0.01Ω

+–

CURRENT

SENSE

AMPLIFIER

VOLTAGE GAIN = 40

SLOPE COMP

1.25MHz

OSCILLATOR

7µA

Σ

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

6

7 VC

ERROR

AMPLIFIER

gm = 850µMho

Figure 1. Block Diagram

1.2V

6 FB

4 GND

1767 F01

sn1767 1767fas

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