ACT8332
Rev1, 24-May-10
STEP-DOWN DC/DC CONVERTER
FUNCTIONAL DESCRIPTION
Input Capacitor Selection
General Description
The input capacitor reduces peak currents and
noise induced upon the voltage source. A 2.2µF
ceramic input capacitor is recommended for most
applications.
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.
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
Thermal Shutdown
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
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 RFB2 = 51Kꢀ, then calculate RFB1 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.
⎛
⎞
VOUT1
VFB1
⎜
⎜
⎟
⎟
RFB1 = RFB2
−1
(1)
⎝
⎠
Where VFB1 is 0.625V
Innovative PowerTM
ActivePMUTM is a trademark of Active-Semi.
- 8 -
www.active-semi.com
Copyright © 2010 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
