RT8489

10

DS8489-00 November 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT8489 is a 4-CH programmable current source

controller for LED backlight or lighting application.

By detecting the minimum voltage required to drive each

LED string and setting the Boost output accordingly, this

topology reduces power dissipation and increases overall

efficiency of the LED lighting system.

The individual current source channel regulates the current

flow to give accurate current sinking for each LED string.

The external N-MOSFET current source will accommodate

the power dissipation difference among channels resulting

from the forward voltage difference between the LED

strings.

Both digital PWM dimming signal and analog voltage

signal are used to control the LED current of each channel.

With high speed current source N-MOSFET drivers, the

RT8489 features highly accurate current matching of

±3

percent, while providing very fast turn-on and turn-off times.

This allows a very narrow minimum on or off pulse, which

increases dimming range and provides higher linearity.

The RT8489 integrates adjustable switching frequency and

soft-start, and provides over temperature, over voltage and

current limit protection features.

Input UVLO

The input operating voltage range of the RT8489 is 7V to

28V. An input capacitor at the VCC pin can reduce ripple

voltage. It is recommended to use a ceramic 10

μF or larger

capacitor as the input capacitor. This IC provides an Under

Voltage Lockout (UVLO) function to enhance the stability

when start-up. The UVLO rising input voltage threshold is

set at 5.5V typically with a 0.7V hysteresis.

Power Sequence

Refer to below Figure 1 and 2. The recommended power

on sequence states that the PWM signal should be ready

function will be disabled. As for power off sequence, EN/

start”as shown as Figure 3.

Figure 1. Power On Sequence Control by EN

EN must be turned on later than VIN

and PWM signal

EN must be turned off earlier than

VIN and PWM signal

Power On sequence

EN

PWM

Power Off sequence

UVLO

Soft-Start

No Soft-Start

Abnormal Power-on sequence

If PWM turns on late