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ACT6391

1.7A/2.5A PWM Step-Up DC/DC Converters In MSOP

Active-Semi, Inc 

ACT6390/ACT6391

Rev 1, 22-Aug-13

FUNCTIONAL BLOCK DIAGRAM

EN

COMP

FB

1.24V

ERROR

AMPLIFIER

-

+

ERROR

COMPARATOR

CLOCK

SOFT

START

CONTROL

AND

DRIVE

LOGIC

IN

4.5µA

SS

SW

FREQ

OSCILLATOR

SLOPE

COMPENSATION

+

5.5µA

CURRENT SENSE

G

AMPLIFIER

FUNCTIONAL DESCRIPTION

The ACT6390 and ACT6391 are highly efficient

step-up DC/DC converters that employ a current-

mode, fixed frequency pulse-width modulation

(PWM) architecture with excellent line and load

regulation.

The ACT6390 and ACT6391 operate at constant

switching frequency under medium to high load cur-

rent conditions. At light loads, these devices oper-

ate in a pulse-skipping mode in order to improve

light-load efficiency.

Soft-Start

The ACT6390 and ACT6391 both offer a program-

mable soft-start function which minimizes inrush

current during startup. The soft-start period is pro-

grammed by connecting a capacitor (CSS) between

SS and G. Operation of the soft-start function is as

follows: when the IC is disabled, SS is actively dis-

charged to G. Upon enabling the IC, CSS is charged

with a 4.5µA current so that the voltage at SS in-

creases in a controlled manner. The peak inductor

current is limited by the voltage at SS, so that the

input current is limited until the soft-start period ex-

pires, and the regulator can achieve its full output

current rating.

The soft-start period can be calculated as a simple

function of the soft-start capacitor using the equa-

tion:

t SS = 2.2 × 10 5 × C SS

(1)

Frequency Selection

The ACT6390 and ACT6391 include a pin-

selectable operating frequency drive FREQ to a

logic high for 1.3MHz operation, drive FREQ to a

logic low for 700kHz operation.

Selectable operating frequency, in combination with

the external compensation network, allows a wide

range of flexibility in optimizing total solution size

and cost.

FREQ is internally pulled down by 5.5µA, this pin

may be left unconnected to achieve a 700kHz oper-

ating frequency.

Setting the Output Voltage

The ACT6390 and ACT6391 both feature external

adjustable output voltages of up to 12V. To program

the output voltage, simply connect a resistive volt-

age divider between the output, FB, and G, with

resistors set according to the following equation:

R1

=

R

2

×

⎢⎣⎡⎜⎜⎝⎛

VOUT

VFB

⎟⎟⎠⎞

−

⎤

1⎥

⎦

(2)

Where VFB is 1.24V.

Innovative PowerTM

-5-

www.active-semi.com

Copyright © 2013 Active-Semi, Inc.

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