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LTC1872

FUNCTIONAL DIAGRA

OPERATIO

(Refer to Functional Diagram)

Main Control Loop

The LTC1872 is a constant frequency current mode switch-

ing regulator. During normal operation, the external

N-channel power MOSFET is turned on each cycle by the

oscillator and turned off when the current comparator

(ICMP) resets the RS 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 in-

creases, it causes a slight decrease in VFB relative to the

0.8V

reference,

which

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 0.35V, 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

up, the corresponding output current trip level follows,

allowing normal operation.

SWITCHING

LOGIC AND

BLANKING

CIRCUIT

–

0.15V

0.5

µA

0.3V

SLEEP

OVP

BURST

CMP

SHDN

1.2V

1872FD

VREF

60mV

VREF

0.8V

VIN

VOLTAGE

REFERENCE

SLOPE

COMP

ICMP

FREQ

FOLDBACK

OSC

SENSE–

VIN

–

EAMP

VFB

–

NGATE

VIN

ITH/RUN

VIN

0.35V

VREF

0.8V

–

SHDN

CMP

0.3V

GND

–

UNDERVOLTAGE

LOCKOUT

VIN

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