Active-Semi
ACT8325
STEP-DOWN DC/DC CONVERTER
Rev0, 13-May-08
FUNCTIONAL DESCRIPTION
General Description
REG1 and REG2 are fixed-frequency, current-
mode, synchronous PWM step-down converters
that achieve peak efficiencies of up to 97%. REG1
is capable of supplying up to 350mA of output cur-
rent, while REG2 supports up to 550mA. These
regulators operate with a fixed frequency of
1.6MHz, minimizing noise in sensitive applications
and allowing the use of small external components.
REG1 and REG2 are available with a variety of
standard and custom output voltages, as described
in the Ordering Information section of this data-
sheet.
Compensation
REG1 and REG2 utilize current-mode control and a
proprietary internal compensation scheme to simul-
taneously simplify external component selection
and optimize transient performance over their full
operating range. No compensation design is re-
quired; simply follow a few simple guidelines de-
scribed below when choosing external components.
Thermal Shutdown
The ACT8325 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 ACT8325 die temperature exceeds
160°C, and prevents the regulators from being en-
abled until the IC temperature drops by 20°C (typ).
100% Duty Cycle Operation
Both REG1 and REG2 are 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 applications.
Input Capacitor Selection
The input capacitor reduces peak currents and
noise induced upon the voltage source. A 2.2µF
ceramic capacitor for each of REG1 and REG2 is
recommended for most applications.
Synchronous Rectification
REG1 and REG2 both feature integrated n-channel
synchronous rectifiers, maximizing efficiency and
minimizing the total solution size and cost by elimi-
nating the need for external rectifiers.
Output Capacitor Selection
For most applications, 10µF ceramic output capaci-
tors are recommended for both REG1 and REG2.
Although the these regulators were designed to
take advantage of the benefits of ceramic capaci-
tors, namely small size and very-low ESR, low-ESR
tantalum capacitors can provide acceptable results
as well.
Enabling and Disabling REG1 and REG2
Each of the ACT8325's regulators features inde-
pendent pin-controlled enable control. Drive ON1 to
a logic-high to enable REG1, drive ON1 to a logic-
low to disable REG1. Similarly, drive ON2 to a
logic-high to enable REG2, drive ON2 to a logic-low
to disable REG2. When disabled, each regulator's
supply current drops to less than 1µA.
Output Voltage Programming
Figure 4:
Output Voltage Programming
OUTx
ACT8325
FBx
R
FB2
C
FF
R
FB1
Soft-Start
REG1 and REG2 each include matched soft-start
circuitry. When enabled, the output voltages track
the internal 80µs soft-start ramp and both power up
in a monotonic manner that is independent of load-
ing on either output. This circuitry ensures that both
outputs power up in a controlled manner, greatly
simplifying power sequencing design considera-
tions.
Figure 4 shows the feedback network necessary to
set the output voltage when using the adjustable
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