ACT8332
STEP-DOWN DC/DC CONVERTER
FUNCTIONAL DESCRIPTION
General Description
REG1 is a fixed-frequency, current-mode, synchro-
nous PWM step-down converters that achieves a
peak efficiency of up to 97%. REG1 is capable of
supplying up to 350mA of output current and oper-
ates with a fixed frequency of 1.6MHz, minimizing
noise in sensitive applications and allowing the use
of small external components. REG1 is available
with a variety of standard and custom output volt-
ages, as well as an adjustable output voltage option.
Rev0, 14-Mar-08
Input Capacitor Selection
The input capacitor reduces peak currents and
noise induced upon the voltage source. A 2.2µF
ceramic input capacitor is recommended for most
applications.
Output Capacitor Selection
For most applications, a 10µF ceramic output ca-
pacitor is recommended. Although REG1 was de-
signed to take advantage of the benefits of ceramic
capacitors, namely small size and very-low ESR,
low-ESR tantalum capacitors can provide accept-
able results as well.
100% Duty Cycle Operation
REG1 is capable of operating at up to 100% duty
cycle. 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 appli-
cations.
Inductor Selection
REG1 utilizes current-mode control and a proprie-
tary internal compensation scheme to simultane-
ously simplify external component selection and
optimize transient performance over its full operat-
ing range. REG1 was optimized for operation with a
3.3µH inductor, although inductors in the 2.2µH to
4.7µH range can be used. Choose an inductor with
a low DC-resistance, and avoid inductor saturation
by choosing inductors with DC ratings that exceed
the maximum output current of the application by at
least 30%.
Synchronous Rectification
REG1 features an integrated n-channel synchro-
nous rectifier, which maximizes efficiency and mini-
mizes the total solution size and cost by eliminating
the need for an external rectifier.
Enabling and Disabling REG1
REG1 is enabled or disabled using ON1. Drive ON1
to a logic-high to enable REG1. Drive ON1 to a
logic-low to disable REG1, reducing supply current
to less than 1µA
Thermal Shutdown
The ACT8332 integrates thermal shutdown protec-
tion circuitry to prevent damage resulting from ex-
cessive thermal stress, as may be encountered un-
der fault conditions. This circuitry disables all regu-
lators if the ACT8332 die temperature exceeds
160°C, and prevents the regulators from being en-
abled until the IC temperature drops by 20°C (typ).
Soft-Start
REG1 includes internal soft-start circuitry, and en-
abled its output voltage tracks an internal 80µs soft-
start ramp so that it powers up in a monotonic man-
ner that is independent of loading.
Output Voltage Programming
Figure 4 shows the feedback network necessary to
set the output voltage when using the adjustable
output voltage option. Select components as fol-
lows: Set R
FB2
= 51KΩ, then calculate R
FB1
using
the following equation:
Compensation
REG1 utilizes current-mode control and a proprie-
tary internal compensation scheme to simultane-
ously simplify external component selection and
optimize transient performance over its full operat-
ing range. No compensation design is required,
simply follow a few simple guidelines described be-
low when choosing external components.
⎛
V
⎞
R
FB1
=
R
FB2
⎜
OUT1
−
1
⎟
⎜
V
⎟
⎝
FB1
⎠
Where V
FB1
is 0.625V
(1)
Innovative Power
TM
ActivePMU
TM
-8-
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