VLF10045 datasheet(3/15 Pages) RICHTEK

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RT7272B

DS7272B-01

January 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Operation

The RT7272B is a constant frequency, current mode

synchronous step-down converter. In normal operation,

the high side N-MOSFET is turned on when the S-R latch

is set by the oscillator and is turned off when the current

comparator resets the S-R latch. While the high side

N-MOSFET is turned off, the low side N-MOSFET is turned

on to conduct the inductor current until next cycle begins.

Error Amplifier

The error amplifier adjusts its output voltage by comparing

the feedback signal (VFB) with the internal 0.8V reference.

When the load current increases, it causes a drop in the

feedback voltage relative to the reference. The error

amplifier's output voltage then rises to allow higher inductor

current to match the load current.

Oscillator

The internal oscillator runs at fixed frequency 500kHz. In

short circuit condition, the frequency is reduced to 75kHz

for low power consumption.

Internal Regulator

The regulator provides low voltage power to supply the

internal control circuits and the bootstrap power for high

side gate driver.

Enable

The converter is turned on when the EN pin is higher than

2V. When the EN pin is lower than 0.4V, the converter will

enter shutdown mode and reduce the supply current to

0.5

μA.

Soft-Start (SS)

An internal current source charges an internal capacitor

to build a soft-start ramp voltage. The FB voltage will track

the internal ramp voltage during soft-start interval. The

typical soft-start time is 2ms.

Current Setting

The current limit of high side MOSFET is adjustable by

an external resistor connected to the RLIM pin. The current

limit range is from 2A to 6A. When the inductor current

reaches the current limit threshold, the COMP voltage

will be clamped to limit the inductor current.

UV Comparator

If the feedback voltage (VFB) is lower than 0.4V, the UV

Comparator will go high to turn off the high side MOSFET.

The output under voltage protection is designed to operate

in Hiccup mode. When the UV condition is removed, the

converter will resume switching.

Thermal shutdown

The over temperature protection function will shut down

the switching operation when the junction temperature

exceeds 150

°C. Once the junction temperature cools

down by approximately 20

°C, the converter will

automatically resume switching.

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