Active-Semi
FUNCTIONAL DESCRIPTION
The patent-pending ACT8309/ACT8310/ACT8311
are current-mode, fixed-frequency PWM, synchro-
nous step-down DC/DC converters that are capable
of delivering up to 1.5A (ACT8311) with efficiency of
up to 95%. These devices feature very low-
resistance power MOSFETs and 100% duty cycle
operation, making them ideal choices for portable
applications requiring a 3.0V or 3.3V rail to be gen-
erated form a Li+ battery. These device operate
with a fixed frequency of 1.35MHz, minimizing noise
in sensitive equipment as well as optimizing both
efficiency and component size and cost.
ACT8309/8310/8311
Rev 0, 15-Jun-10
For most applications, a 22µF ceramic output ca-
pacitor is recommended. Although the these regula-
tors were designed to take advantage of the bene-
fits of ceramic capacitors, namely small size and
very-low ESR, low-ESR tantalum capacitors can
provide acceptable results as well.
Inductor Selection
These devices were optimized for operation with
3.3µH inductors, although inductors in the 1.5µH to
4.7µH range may be used. Choose an inductors
with a low DC-resistance, and avoid inductor satu-
ration by choosing inductors with DC ratings that
exceed the maximum output current of the applica-
tion by at least 30%.
100% Duty Cycle Operation
These devices are capable of operating at up to
100% duty cycle operation. During 100% duty cycle
operation, the high-side power MOSFET is held on
continuously, providing a direct connection from the
input to the output (through the inductor), ensuring
the lowest possible dropout voltage in battery-
powered applications.
Output Voltage Setting
Figure 3 shows the feedback network necessary to
set the output voltage when using adjustable output
voltage options. output voltage options. Select com-
ponents as follows: Set R
FB2
= 200kΩ, calculate
R
FB1
using the following equation:
Synchronous Rectification
The ACT8309/ACT8310/ACT8311 each feature
integrated n-channel synchronous rectifiers, maxi-
mizing efficiency and minimizing the total solution
size and cost by eliminating the need for external
rectifiers.
⎛
V
⎞
R
FB1
=
R
FB2
⎜
OUT
−
1
⎟
⎜
V
⎟
⎝
FB
⎠
Where V
FB
is 0.8V.
Figure 3:
Output Voltage Setting
V
OUT
ACT8309/
ACT8310/
ACT8311
R
FB1
FB
R
FB2
Soft-Start
The ACT8309/ACT8310/ACT8311 each include
integrated soft-start circuitry. When enabled, the
output voltage track an internal 200µs soft-start
ramp so that the output power up in a controlled,
monotonic manner that is independent of loading.
Compensation
The ACT8309/ACT8310/ACT8311 each utilize cur-
rent-mode control and a proprietary internal compen-
sation scheme to simultaneously simplify external
component selection and optimize transient perform-
ance over their full operating range. No compensation
design is required, simply follow a few simple guide-
lines described below when choosing external com-
ponents.
Enable/Disable Control
These devices may be enabled and disabled using
the EN input. Drive EN to a logic-high to enable the
regulator, drive EN to a logic-low to disable the
regulator. When disabled, each regulator’s quies-
cent supply current drops to less than 1µA.
Capacitor Selection
The input capacitor reduces peak currents and
noise induced upon the voltage source. A 10µF
ceramic capacitor is recommended for most appli-
cations.
Innovative Power
TM
-6-
Power-OK Output (POK)
These devices feature an open-drain “Power-OK”
indicator output. This output sinks current whenever
the output voltage is (typically) 5% below the regu-
lation voltage, and goes high-impedance when the
output voltage is above this threshold. To generate
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