ACT4088

Rev0, 13-May-08

- 6 -

www.active-semi.com

Copyright © 2008 Active-Semi, Inc.

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

The ACT4088 is a current-mode step-down DC/DC

converter that provides excellent transient response

with no extra external compensation components.

This device contains an internal, low-resistance,

high-voltage power MOSFET, and operates at a

high 1.4MHz operating frequency to ensure a com-

pact, high-efficiency design with excellent AC and

DC performance.

Setting the Output Voltage

An external voltage divider is used to set the output

voltage, as well as provide a known impedance

nect a 50kΩ resistor from the output to FB to ensure

stable compensation, and select the bottom resistor

to provide the desired regulation voltage.

Figure 1:

Output Voltage Setting

nal compensation network, and plays an important

in setting the ACT4088's transient response and

ensuring stability. For most applications, choosing

tions with output voltages of 1.8V or lower, use a

Selecting the Inductor

The ACT4088 was optimized for use with a 4.7µH

inductor. When choosing an inductor, choose one

with a DC resistance of less than 250mΩ and a DC

current rating that is typically 30% higher than the

maximum load current.

During typical operation, the inductor maintains a

continuous current to output load. The inductor

current has a ripple that is dependent on the

inductance value.

Higher inductance reduces the peak-to-peak ripple

current. The trade off for high inductance value is

the increase in inductor core size and series

resistance, and a reduction in current handling

capability.

If efficiency at light loads (such as less than 100mA)

is critical in the application, a larger inductor is

recommended.

ACT4088

FB

IN

EN

FB

G

REFERENCE

& THERMAL

SHUTDOWN

CONTROL

SOFT-START

PWM Comparator

ILIM Comparator

HICCUP

REGULATOR

& UVLO

OSCILLATOR

COMPENSATION

DRIVER

CSA

Q1

BST

SW

EA

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

V

81

.

0

V

R

R

OUT

1

FB

2

FB

(1)