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ACT4088
(Rev.:2007)

28V, 1.5A, 1.4MHz Step-Down DC/DC Converter in SOT23-6

Active-Semi, Inc 

ACT4088

Rev PrA, 24-Jul-07

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

VIN

ON

OFF

Optional Connection:

IN if VIN < 5.5V

OUT if VOUT < 5.5V

2

IN

C1

10µF

6

BST

1

ACT4088 SW

3

EN

4

FB

G

5

RFB2

D2

1N4148

(Optional)

C3

22nF

L1

4.7µH

D1

B240A

RFB1

9.53kΩ

49.9kΩ

1%

1%

C2

22µF

VOUT

5V

Innovative Products. Active Solutions.

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Copyright © 2007 Active-Semi, Inc.

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