RT9302 datasheet(8/9 Pages) RICHTEK

8 / 9 page

RT9302

DS9302-02

March 2007

www.richtek.com

Application Information

Input UVLO

The input operating voltage range of the RT9302 is 2.8V

to 5.5V. An input capacitor at the VIN pin could reduce

ripple voltage. It is recommended to use a ceramic 1

μF

or larger capacitance as the input capacitor. This IC provides

an under voltage lockout (UVLO) function to prevent it

from unstable issue when startup. The UVLO threshold

of input falling voltage is set at 2V typically with a

hysteresis 0.2V.

Output Current

RT9302 provides a constant output current for white LED.

Figure 1 shows a typical application circuit for 3 white

LEDs. Each channel supports up to 20mA output current

and regulates a constant current for uniform intensity. For

two or one LED application, the non-used channel must

be connected to VIN as shown in Figure 2.

Brightness Dimming Control

The RT9302 provides a 16-step brightness dimming

control. The LED current can be easily configured from

1.25mA to 20mA. The dimming of white LEDs' current

can be achieved by applying a pulse signal to the EN pin.

Figure 3 shows the configuration of the 16-step pulse

dimming control operation. The output current is set at

100% of 20mA typical output current when EN pin pull

high initially from shutdown condition. The step change of

pulse dimming can be implemented by EN pin changing

with a logic Low pulse. Each step change 1/16 of full output

current setting. The output current change of pulse

dimming is designed in decreasing sequence in terms of

(16/16, 15/16, ....., 1/16) and then (16/16, 15/16, .....)

cycle by cycle. The duration of logic Low pulse is between

0.5us and 10

μs. The duration of ENlogic High pulse should

be longer than 0.5

μs.

The LED current can be turned off by EN pin with pulling

low for 20

μs or longer. The typical leakage current is 4μA

while shutdown condition.

Layout Consideration

For best performance, careful PCB layout is necessary.

All peripheral components should be placed as close to

the IC as possible. A short connection is highly

recommended. The following guidelines should be strictly

followed when designing a PCB layout for the RT9302.

1. All the traces of LED and VIN running from chip to LEDs

should be wide and short to reduce the parasitic

connection resistance.

2. Input capacitor (CIN) should be placed close to VIN and

connected to ground plane. The anodes of LEDs must

be connected to CIN, not battery directly.

3. The GND should be connected to a strong ground plane

for heat sinking and noise protection.

Battery

Ground Plane

GPIO

All the traces of LED and V

IN running from

chip to LEDs should be wide and short to

reduce the parasitic connection resistance.

Input capacitor (C

IN) should

be placed close to V

IN and

connected to ground plane.

The Anodes of LEDs must

connect to C

, not battery

directly.

The GND should be connected to a strong

ground plane for heat sinking and noise

protection.

VIN

GND

LED3

LED2

LED1

Figure 4. PCB Layout Guide

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