RT9270GF datasheet(2/13 Pages) RICHTEK

2 / 13 page

RT9270

Preliminary

DS9270-09 March 2007

www.richtek.com

Function Block Diagram

Operation

The RT9270 is a high efficiency step-up Boost converter

with a fixed-frequency, current-mode PWM architecture.

It performs fast transient response and low noise operation

with appropriate component selection. The output voltage

is regulated through a feedback control consisting of an

error amplifier, a summing comparator, and several control

signal generators (as shown in function block diagram).

The feedback reference voltage is 1.24V. The error

amplifier varies the COMP voltage by sensing the FB pin.

The slope compensation signal summed with the current

-sense signal will be compared with the COMP voltage

through the summing comparator to determine the current

trip point and duty cycle. When driving light loads, the

RT9270 will perform the pulse-skipping mode to prevent

overcharging the output voltage. In this mode, the

switching frequency will be reduced to perform a higher

efficiency.

Soft-Start

The RT9270 provides soft-start function. When the EN pin

is connected to high, a 4uA constant current is sourced to

charge an external capacitor. The voltage rate of rise on

the COMP pin is limited during the charging period, and

so is the peak inductor current. When the EN pin is

connected to GND, the external capacitor will be

discharged to ground for the next time soft-start.

Current Limitation

The switch current is monitored to limit the value not to

exceed 1.9A typically. When the switch current reaches

1.9A, the output voltage will be pulled down to limit the

total output power to protect the power switch and external

components.

Shutdown

Connect the EN to GND to turn the RT9270 off and reduce

the supply current to 0.1uA. In this operation, the output

voltage is the value of VIN to subtract the forward voltage

of catch diode.

Frequency Selection

The switching frequency of RT9270 can be selected to

operate at either 670kHz or 1.3MHz. When the FREQ

pin is connected to GND for 670kHz operation, and

connected to VIN for 1.3MHz operation. FREQ is preset

to 670kHz operation for allowing the FREQ pin

unconnected.

Control

and

Driver

Logic

Current

Sense

Soft-

Start

Slope

Compen-

sation

Oscillator

Enable

Summing

Comparator

Clock

Error

Amplifier

1.24V

5uA

4uA

VIN

GND

COMP

FREQ

Protection

1.24

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