LT1576CS8-SYNC データシートの表示（PDF） - Linear Technology

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LT1576CS8-SYNC

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1576/LT1576-5

BLOCK DIAGRAM

INPUT

BIAS

2.9V BIAS

REGULATOR

INTERNAL

VCC

0.025Ω

+–

CURRENT

SENSE

AMPLIFIER

VOLTAGE GAIN = 35

SLOPE COMP

SYNC

SHUTDOWN

COMPARATOR

200kHz

OSCILLATOR

SHDN

0.4V

3.5µA

+

–

2.44V

LOCKOUT

COMPARATOR

Σ

0.8V

CURRENT

COMPARATOR

+

S

RS

FLIP-FLOP

R

–

FOLDBACK

CURRENT Q2

LIMIT

CLAMP

FREQUENCY

SHIFT CIRCUIT

ERROR

VC

AMPLIFIER

gm = 1000µMho

Figure 1. Block Diagram

BOOST

DRIVER

CIRCUITRY

Q1

POWER

SWITCH

VSW

1.21V

FB

GND

1576 BD

APPLICATIONS INFORMATION

FEEDBACK PIN FUNCTIONS

The feedback (FB) pin on the LT1576 is used to set output

voltage and provide several overload protection features.

The first part of this section deals with selecting resistors

to set output voltage and the remaining part talks about

foldback frequency and current limiting created by the FB

pin. Please read both parts before committing to a final

design. The fixed 5V LT1576-5 has internal divider resis-

tors and the FB pin is renamed SENSE, connected directly

to the output.

The suggested value for the output divider resistor (see

Figure 2) from FB to ground (R2) is 5k or less, and a

formula for R1 is shown below. The output voltage error

caused by ignoring the input bias current on the FB pin is

less than 0.25% with R2 = 5k. A table of standard 1%

values is shown in Table 1 for common output voltages.

Please read the following if divider resistors are increased

above the suggested values.

( ) R2 VOUT −1.21

R1 =

1.21

8

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