ACT4088
Rev PrA, 24-Jul-07
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.
in setting the ACT4088's transient response and
ensuring stability. For most applications, choosing
R
FB1
= 49.9kΩ
provides good results. For applica-
tions with output voltages of 1.8V or lower, use a
larger R
FB1
value such as 80.6kΩ. Once R
FB1
is
chosen, use the following equation to choose R
FB2
:
R
FB 2
�½
R
FB1
V
OUT
1
0.81V
(1)
Setting the Output Voltage
An external voltage divider is used to set the output
voltage, as well as provide a known impedance
from V
OUT
to FB for compensation purposes. Con-
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
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.
The feedback resistor (R
FB1
) interacts with the inter-
nal compensation network, and plays an important
Innovative Products.
Active Solutions.
-6-
Copyright © 2007 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
