ACT4088

Rev PrA, 24-Jul-07

- 8 -

www.active-semi.com

Copyright © 2007 Active-Semi, Inc.

Soft-Start

The ACT4088 provides an internal soft-start fea-

ture, which ramps the output voltage and output

current are from 0 to the full value over 0.5 millisec-

onds. This feature prevents output voltage over-

shoot at light loads as well as to prevent large in-

rush currents upon startup. The soft-start circuitry is

internally reset anytime the IC is disabled using the

EN pin, as well as if the IC reaches hiccup mode or

thermal shutdown. In all of these cases, soft-start

provides a smooth, controlled restart after the fault

is removed.

Frequency Foldback

The voltage at FB is monitored by a comparator to

detect an extreme output overload condition. If the

voltage at the FB pin falls to below 0.3V, the inter-

nal oscillator slows to a decreased frequency of

467kHz, 33% of the nominal value. This prevents

the inductor current from rising excessively during a

dead-short condition, potentially resulting in induc-

tor saturation.

Figure 2:

ACT4088 Typical 5V/1.5A Output Application

ACT4088

IN

EN

BST

SW

FB

G

C1

10µF

D1

B240A

C3

22nF

L1

4.7µH

C2

22µF

5V

49.9k

Ω

1%

9.53k

Ω

1%

6

5

4

3

2

1

OFF

ON

D2

1N4148

(Optional)

Optional Connection: