ACT8712 View Datasheet(PDF) - Active-Semi, Inc
Part Name
Description
Manufacturer
ACT8712 Datasheet PDF : 36 Pages
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ACT8712
Rev0, 25-Feb-08
STEP-DOWN DC/DC CONVERTERS
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 550mA of output cur-
rent, while REG2 supports up to 750mA. 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. REG1 and REG2 also take advantage of the
I2C interface, and may be software-controlled in
systems that require advanced power management
functions.
100% Duty Cycle Operation
Both REG1 and REG2 are capable of operating at
up to 100% duty cycle. During 100% duty-cycle op-
eration, 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.
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.
Enabling and Disabling REG1 and REG2
Enable/disable functionality is typically implemented
as part of a controlled enable/disable scheme utiliz-
ing nMSTR and other system control features of the
ACT8712. REG1 and REG2 are automatically en-
abled whenever any of the following conditions are
met:
1) A valid input voltage is present at VIN,
2) nMSTR is driven low, or
3) ON1 is asserted high.
When none of these conditions are true, REG1 and
REG2 are disabled, and each regulator's quiescent
supply current drops to less than 1µA.
Programming the Output Voltage
By default, REG1 and REG2 each power up and
regulate to their default output voltage. Once the
system is enabled, each regulator’s output voltage
may be independently programmed to a different
value, typically in order to reduce the power con-
sumption of a microprocessor in standby mode.
Program the output voltages via the I2C serial inter-
face by writing to the REGx/VSETx[ ] and
REGx/VRANGE[ ] registers.
Programmable Operating Mode
By default, REG1 and REG2 operate in fixed-
frequency PWM mode at medium to heavy loads,
then transition 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 operation across the entire load current
range, at the expense of light-load efficiency, by
setting the REGx/MODE[ ] bit to [1].
Power-OK
REG1 and REG2 each feature a variety of status
bits that can be read by the system microprocessor.
If either output voltage is lower than the power-OK
threshold, typically 6% below the programmed
regulation voltage, REGx/OK[ ] will clear to 0.
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.
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.
Innovative Products. Active Solutions.
ActivePMUTM is a trademark of Active-Semi.
I2CTM is a trademark of Philips Electronics.
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www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
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