LTC1622 データシートの表示（PDF） - Linear Technology

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LTC1622

Low Input Voltage Current Mode Step-Down DC/DC Controller

Linear Technology 

W

FUNCTIONAL DIAGRA

VIN

Y = “0” ONLY WHEN X IS A CONSTANT “1”

BURST DEFEAT

OTHERWISE Y = “1”

VCC

Y

X

1µA

LTC1622

SYNC/

MODE

5

SLOPE

COMP

OSC

0.3V

–

VFB 3

+

VIN

0.8V

REFERENCE

VREF

0.8V

UVLO

TRIP = 1.97V

SHUTDOWN

FREQ

SHIFT

0.8V

VREF

VIN

2.5µA

RUN/SS 4

+

EA

– gm = 0.5mΩ

RUN/

SOFT-START

+

OV

6

VREF + 60mV

–

GND

0.36V

0.12V

2

ITH

EN

–

+

BURST

S

RQ

RS1

SENSE– 1

8 VIN

SLEEP

SWITCHING

LOGIC

AND

BLANKING

CIRCUIT

+–

ICOMP

VIN

PDRV

7

1622 BD

U

OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC1622 is a constant frequency current mode switch-

ing regulator. During normal operation, the external

P-channel power MOSFET is turned on each cycle when

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

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

inductor current at which ICOMP resets the RS latch is

controlled by the voltage on the ITH pin, which is the output

of the error amplifier EA. An external resistive divider

connected between VOUT and ground allows EA 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 voltage to

increase until the average inductor current matches the

new load current.

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

pin low. Releasing RUN/SS allows an internal 2.5µA

current source to charge up the soft-start capacitor CSS.

When CSS reaches 0.7V, the main control loop is enabled

with the ITH voltage clamped at approximately 5% of its

maximum value. As CSS continues to charge, ITH is gradu-

ally released allowing normal operation to resume.

Comparator OV guards against transient overshoots

> 7.5% by turning off the P-channel power MOSFET and

keeping it off until the fault is removed.

Burst Mode Operation

The LTC1622 can be enabled to go into Burst Mode

operation at low load currents simply by leaving the SYNC/

MODE pin open or connecting it to a voltage of at least 2V.

In this mode, the peak current of the inductor is set as if

VITH = 0.36V (at low duty cycles) even though the voltage

at the ITH pin is at lower value. If the inductor’s average

current is greater than the load requirement, the voltage at

5

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