RT9271

3

DS9271-13 March 2007

www.richtek.com

Function Block Diagram

Operation

The RT9271 is a constant frequency step-up converter with an internal switch. For excellent line and load regulation, the

current mode control is adopted. The operations of RT9271 can be understood from block diagram clearly. The oscillator

triggers the SET input of SR latch to turn on the power switch M1 at the start of each cycle. A current sense voltage sum

with a stabilizing ramp is connected to the positive terminal of the PWM comparator A2. When this voltage exceeds the

output voltage of the error amplifier A1, the SR latch is reset to turn off the power switch till next cycle starts. The output

voltage of the error amplifier A1 is amplified from the difference between the reference voltage 0.25V and the feedback

voltage. In this manner, if the error amplifiers voltage increases, more current is delivered to the output; if it decreases,

less current is delivered. A 15.5V Zener diode connects from OVP pin to FB pin internally to provide an optional protection

function which prevents LX pin from over-voltage damage. Especially when the case of the feedback loop broken due to

component wear-out or improper connection occurs. The behavior of OVP is to clamp the output voltage to 15.5V

typically. This function is suitable for the applications while driving white LEDs less than 4 in series.

+

-

RAMP

GENERATOR

1.1MHz

OSCILLATOR

OVP

FB

R

Q

S

0.25V

LX

DRIVER

COMPARATOR

GND

M1

A1

A2

+

-

EN

CHIP

ENABLE

VCC

15.5V

0.75Ω

4

μA

Functional Pin Description

Pin No.

−XB −XE

Pin Name Pin Function

1

1

LX

Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI.

2

2

GND

Ground Pin. Connect directly to local ground plane.

3

3

FB

Feedback Pin. Reference voltage is 0.25V. Connect cathode of lowest LED and resistor

4

4

EN

Chip Enable Pin. Connect to 1.4V or higher to enable device, 0.4V or less to disable

device.

−

5

OVP

Over Voltage Protection Pin. Voltage sensing input to trigger the function of over voltage

protection, the trip point is 15.5V. Leave it unconnected to disable this function.

5

6

VCC

Input Voltage Pin. Must be locally bypass with 1uF capacitor to GND.