RT8489

4

DS8489-00 November 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Operation

The internal gate driver circuit is powered from GBIAS pin

around 10V. The GBIAS voltage is generated by an internal

base driver to drive an external NPN emitter follower. The

OSC block generates a programmable frequency which

is set by an external resistor at RSET pin for RT8489

operation. At the beginning of the oscillator cycle, the

GATE turns high. The VOUT regulation unit automatically

detects the lowest sensed LEDx pin feedback voltage

among the 4 LED strings and compares to 0.7V. If the

lowest LEDx pin voltage is lower than 0.7V, the VC pin

(the output of the OP AMP in VOUT regulation unit) is

charged high. The ISENSE pin voltage is the triangular

feedback signal of the sensed switch current (which equals

inductor current ramp).The PWM comparator compares

ISENSE pin voltage to VC pin voltage. When ISENSE pin

voltage exceeds VC pin voltage, the PWM comparator

resets the latch and turns off GATE. If ISENSE pin voltage

exceeds VC pin voltage by the end of the switching cycle,

the GATE will be turned off for minimum off-time. The cycle

repeats when the GATE is turned on at the beginning of

the next switching cycle. By this PWM closed loop control,

the lowest sensed LEDx pin voltage among the 4 LED

strings is regulated to 0.7V.

As the system starts, the cap at the soft-start pin is slowly

charged up by an internal current source around 8

μA. The

slowly rising VC pin voltage allows the PWM duty to

increase gradually to achieve soft-start function.

The dimming can be set by varying ACTL pin analog or

PWM voltage signal. The internal sense threshold for the

4 current sources follows ACTL signal to achieve dimming

control.