ACT8740
Rev PrB, 25-Feb-08
STEP-DOWN DC/DC CONVERTER
FUNCTIONAL DESCRIPTION
Programmable Operating Mode
General Description
By default, REG1 operates in fixed-frequency PWM
mode at medium to heavy loads, then transitions to
a proprietary power-saving mode at light loads in
order to save power. In applications where low
noise is critical, force fixed-frequency PWM opera-
tion across the entire load current range, at the ex-
pense of light-load efficiency, by setting the
REG1/MODE[ ] bit to [1].
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, and may be software-controlled via the I2C
interface by systems that require advanced power
management functions.
Power-OK
REG1 features a power-OK status bit that can be
read by the system microprocessor. If the output
voltage is lower than the power-OK threshold, typi-
cally 6% below the programmed regulation voltage,
REG1/OK[ ] will clear to 0.
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, pro-
viding a direct connection from the input to the out-
put (through the inductor), ensuring the lowest pos-
sible dropout voltage in battery-powered applica-
tions.
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.
Synchronous Rectification
Compensation
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.
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.
Enabling and Disabling REG1
Enable/disable functionality is typically implemented
as part of a controlled enable/disable scheme utiliz-
ing nMSTR and other system control features of the
ACT8740. REG1 is automatically enabled when-
ever either of the following conditions are met:
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.
1) nMSTR is driven low, or
2) ON1 is asserted high.
When none of these conditions are true, REG1 is
disabled, and its quiescent supply current drops to
less than 1µA.
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.
Programming the Output Voltage
By default, REG1 powers up and regulates to its
default output voltage. Once the system is enabled,
REG1’s output voltage may be programmed to a
different value, typically in order to reduce the
power consumption of a microprocessor in standby
mode. Program the output voltage via the I2C serial
interface by writing to the REG1/VSET1[ ] register.
Inductor Selection
REG1 utilizes current-mode control and a proprietary
internal compensation scheme to simultaneously
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ActivePMUTM is a trademark of Active-Semi.
Copyright © 2008 Active-Semi, Inc.
I2CTM is a trademark of Philips Electronics.
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