ACT6390/ACT6391
Rev PrA, 01-Sep-07
FUNCTIONAL BLOCK DIAGRAM
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.
t
SS
�½
2 . 2
10
5
C
SS
Frequency Selection
(1)
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.
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 (C
SS
) 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, C
SS
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:
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:
V
R1
�½
R 2
OUT
V
FB
1
(2)
Where V
FB
is 1.24V.
Innovative Products.
Active Solutions.
-5-
Copyright © 2007 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
