ACT4088
Rev 2, 15-Nov-12
FUNCTIONAL BLOCK DIAGRAM
IN
CSA
REGULATOR
& UVLO
EN
BST
CONTROL
SOFT-START
PWM Comparator
ILIM Comparator
HICCUP
Q1
DRIVER
OSCILLATOR
SW
REFERENCE
& THERMAL
SHUTDOWN
COMPENSATION
EA
FB
G
in setting the ACT4088's transient response and
ensuring stability. For most applications, choosing
RFB1 = 49.9kꢀ provides good results. For applica-
tions with output voltages of 1.8V or lower, use a
larger RFB1 value such as 80.6kꢀ. Once RFB1 is
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.
chosen, use the following equation to choose RFB2
:
RFB1
RFB2
=
(1)
V
⎛
⎜
⎞
⎟
OUT
−1
0.81V
⎝
⎠
Selecting the Inductor
Setting the Output Voltage
The ACT4088 was optimized for use with a 4.7µH
inductor. When choosing an inductor, choose one
with a DC resistance of less than 250mꢀ and a DC
current rating that is typically 30% higher than the
maximum load current.
An external voltage divider is used to set the output
voltage, as well as provide a known impedance
from VOUT to FB for compensation purposes. Con-
nect a 50kꢀ resistor from the output to FB to ensure
stable compensation, and select the bottom resistor
to provide the desired regulation voltage.
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.
Figure 1:
Output Voltage Setting
VOUT
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.
RFB1
ACT4088
FB
RFB2
If efficiency at light loads (such as less than 100mA)
is critical in the application, a larger inductor is
recommended.
The feedback resistor (RFB1) interacts with the inter-
nal compensation network, and plays an important
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