RT8249 datasheet(4/23 Pages) RICHTEK

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RT8249A/B/C

DS8249A/B/C-02

June 2014

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Function Block Diagram

Operation

The RT8249A/B/C includes two constant on-time

synchronous step-down controllers and two linear

regulators.

Buck Controller

In normal operation, the high-side N-MOSFET is turned

on when the output is lower than VREF, and is turned off

after the internal one-shot timer expires. 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.

Soft-Start

For internal soft-start function, an internal current source

charges an internal capacitor to build the soft-start ramp

voltage. The output voltage will track the internal ramp

voltage during soft-start interval.

PGOOD

The power good output is an open-drain architecture. When

the two channels soft-start are both finished, the PGOOD

open-drain output will be high impedance.

Current Limit

The current limit circuit employs a unique

“valley” current

sensing algorithm. If the magnitude of the current sense

signal at PHASE is above the current limit threshold, the

PWM is not allowed to initiate a new cycle. Thus, the

current to the load exceeds the average output inductor

current, the output voltage falls and eventually crosses

the under-voltage protection threshold, inducing IC

shutdown.

Channel 2

Buck

Controller

BOOT2

UGATE2

PHASE2

LGATE2

GND

LDO5

FB2

CS2

PGOOD

Channel 1

Buck

Controller

BOOT1

UGATE1

PHASE1

LGATE1

LDO5

FB1

CS1

LDO5

REF

SW5 Threshold

VIN

LDO5

OSC

VCLK

(RT8249A/B)

LDO3

BYP1

Power-On

Sequence

Clear Fault Latch

BYP1

SKIPSEL

(RT8249C)

EN1

EN2

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