LTC1779 データシートの表示（PDF） - Linear Technology

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LTC1779

250mA Current Mode Step-Down DC/DC Converter in ThinSOT

Linear Technology 

LTC1779

W

FU CTIO AL DIAGRA

SENSE– VIN

4

5

+

ICMP

–

SLOPE

OSC

COMP

OVERTEMP

DETECT

2Ω

RS1

VIN

R

Q

S

SWITCHING

LOGIC AND

BLANKING

CIRCUIT

1×

24×

SW

6

VIN

0.3V

GND

2

FREQ

FOLDBACK

SHORT-CIRCUIT

DETECT

0.3V

0.15V

BURST

+

CMP

SLEEP

–

0.5µA

VIN

VOLTAGE

REFERENCE

UNDERVOLTAGE

LOCKOUT

1 ITH/RUN

0.325V

VREF

0.8V

+

SHDN

CMP

–

SHDN

UV

OVP

EAMP

VREF

+

60mV

VREF

0.8V

VFB

3

VIN

1.2V

1779FD

U

OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC1779 is a constant frequency current mode switch-

ing regulator. During normal operation, the internal

P-channel power MOSFET is turned on each cycle when

the oscillator sets the RS latch (RS1) and turned off when

the current comparator (ICMP) resets the latch. The peak

inductor current at which ICMP resets the RS latch is

controlled by the voltage on the ITH/RUN pin, which is the

output of the error amplifier EAMP. An external resistive

divider connected between VOUT and ground allows the

EAMP to receive an output feedback voltage VFB. When the

load current increases, it causes a slight decrease in VFB

relative to the 0.8V reference, which in turn causes the

ITH/RUN voltage to increase until the average inductor

current matches the new load current.

The main control loop is shut down by pulling the ITH/RUN

pin low. Releasing ITH/RUN allows an internal 0.5µA

current source to charge up the external compensation

network. When the ITH/RUN pin reaches 325mV, the main

control loop is enabled with the ITH/RUN voltage then

pulled up to its zero current level of approximately 0.7V.

As the external compensation network continues to charge

4

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