RT9368 datasheet(8/10 Pages) RICHTEK

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RT9368

DS9368-07 April 2011

www.richtek.com

Applications Information

The RT9368 is a high efficiency charge pump white LED

driver. It provides 4 channels low dropout voltage current

source to regulated 4 white LEDs current. For high

efficiency, the RT9368 implements x1/x1.5/x2 mode

charge pump. An external RSET is used to set the current

of white LED. RT9368 has input current regulation to reduce

the input ripple.

Soft Start

The RT9368 includes a soft start circuit to limit the inrush

current at power on and mode switching. Soft start circuit

holds the input current level long enough for output

capacitor COUT reaching a desired voltage level. When the

soft start off, the RT9368 won

’ t sink spike current from

VIN.

Mode Decision

The RT9368 uses a smart mode decision method to select

the working mode for maximum efficiency. Mode decision

circuit senses the output and LED voltage for up/down

selection.

Dimming Control

When an external PWM signal is connected to the EN

pin, brightness of white LED is adjusted by the duty cycle.

LED Current Setting

The current of white LED connected to RT9368 can be

set by RSET. Every current flows through the white LED is

250 times greater than the current of RSET. The white LED

can be estimated by following equation :

Where VSET = 1.2V, and RSET is the resistor connected

from ISET to GND.

If the LED is not used, the LEDs pin should be connected

to GND. Figure 4 shows the connection for 3LEDs

application, LED4 pin is connection to GND directly.

LED4

LED2

LED1

LED3

Figure 4. Application for 3 LEDs

Layout Consideration

The RT9368 is a low dropout current source for white LED

driver. Careful PCB layout is necessary. For best

performance, place all peripheral components 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 RT9368.

All the traces of LED pins running from chip to LEDs

should be wide and short to reduce the parasitic

connection resistance.

Input capacitor (CIN) should be placed close to VIN (Pin

5) and connected to ground plane. The trace of VIN in

the PCB should be placed far away the sensitive devices

or shielded by the ground.

The GND should be connected to a strong ground plane

for heat sinking and noise protection.

The connection of RSET should be isolated from other

noisy traces. The short wire is recommended to prevent

EMI and noise coupling.

Output capacitor (COUT) should be placed close to VOUT

and connected to ground plane to reduce noise coupling

from charge pump to LEDs.

6. The traces running from pins to flying capacitor should

be short and wide to reduce parasitic resistance and

prevent noise radiation.

250

)

(

SET

LED

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