RT8489

11

DS8489-00 November 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Figure 3. To Prevent “hard-start”Sequence

Soft-Start

The soft-start of the RT8489 can be achieved by connecting

a capacitor from the SS pin to GND. The built in soft-start

circuit reduces the start-up current spike and output

voltage overshoot. The soft-start time is determined by

the external capacitor charged by an internal 8

μAconstant

charging current. The SS pin directly limits the rate of

voltage rise on the VC pin, which in turn limits the peak

switch current.

The soft-start interval is set by the soft-start capacitor

selection according to the following equation :

SS

SS

3.2V

t

= C

(s)

8A

μ

×

A typical value for the soft-start capacitor is 0.1

μF. The

soft-start pin reduces the oscillator frequency and the

maximum current in the switch. The soft-start capacitor

is discharged when EN/UVLO falls below its threshold,

during an over-temperature event, or during a GBIAS under

voltage event.

GBIAS Regulator Operation

The GBIAS pin requires a capacitor for stable operation

and also to store the charge for the large GATE switching

currents. Choose a 10V rated low ESR, X7R or X5R

ceramic capacitor for best performance. The value of the

once PWM pull low for over 10 ms

PWM

10ms

EN and PWM signal

EN 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