ACT8810
Active- Semi
Rev 4, 01-Oct-09
STEP-DOWN DC/DC CONVERTERS
FUNCTIONAL DESCRIPTION
General Description
1) ONx is asserted high to enable REGx,
2) REGx/ONx[ ] is set to 1 when ONx is high
In addition REG1, REG2, or REG3 may be enabled
when nPBIN is pushed low via 100kΩ resistance. It
depends on sequence is set. See the Control
Sequence section for more information.
When none of these conditions are true, REG1,
REG2 and REG3 are disabled, and each regulator’s
quiescent supply current drops to less than 1μA.
REG1, REG2, and REG3 are fixed-frequency,
current-mode, synchronous PWM step-down
converters that are capable of supplying up to 1.3A,
1.0A, and 0.55A of output current, respectively.
These regulators operate with a fixed frequency of
1.6MHz, minimizing noise in sensitive applications
and allowing the use of small external components,
and achieve peak efficiencies of up to 97%.
Each step-down DC/DC is available with a variety of
standard and custom output voltages, which may be
software-controlled by systems requiring advanced
power management functions, via the I2C interface.
Power-OK
REG1, REG2 and REG3 each feature a variety of
status bits that can be read by the system
microprocessor. If any output falls below its power-
OK threshold, typically 6% below the programmed
regulation voltage, REGx/OK[ ] is cleared to 0.
Buck Regulator PFM/PWM Operating
Modes
The buck converters offer PFM/PWM operating
modes to maximize efficiency under both light and
full load conditions. The device will automatically
transition from fixed frequency PWM mode to PFM
mode when the output current is approximately
100mA. In PFM mode, the device maintains output
voltage regulation by adjusting the switching
frequency. The device transitions into fixed
frequency PWM mode when the output current
reaches approximately 100mA.
Soft-Start
REG1, REG2 and REG3 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 loading on either output. This
circuitry ensures that each output powers up in a
controlled manner, greatly simplifying power
sequencing design considerations.
Compensation
100% Duty Cycle Operation
REG1, REG2 and REG3 utilize current-mode control
and a proprietary internal compensation scheme to
simultaneously simplify external component selection
and optimize transient performance over their full
operating range. No compensation design is required;
simply follow a few simple guide lines described
below when choosing external components.
REG1, REG2 and REG3 are each 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
Synchronous Rectification
The input capacitor reduces peak currents and
noise induced upon the voltage source. A 4.7μF
ceramic capacitor for each of REG1, REG2 and
REG3 is recommended for most applications.
REG1, REG2 and REG3 each feature integrated
channel synchronous rectifiers, maximizing
efficiency and minimizing the total solution size and
cost by eliminating the need for external rectifiers.
Output Capacitor Selection
Enabling and Disabling REG1, REG2
and REG3
REG1, REG2, and REG3 are typically enabled and
disabled using the ACT8810's closed-loop
enable/disable control scheme, including the nPBIN
input. Refer to the System Startup and Shutdown
section for more information about this function.
For most applications, 22μF ceramic output
capacitors are recommended for REG1 and 10μF
ceramic output capacitors are recommended for
REG2, REG3. Although the these regulators were
designed to take advantage of the benefits of
ceramic capacitors, namely small size and very-low
ESR, low-ESR tantalum capacitors can provide
acceptable results as well.
Each regulator is enabled when the following
conditions are met:
Innovative PowerTM
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ActivePMUTM and ActivePathTM are trademarks of Active-Semi.
I2CTM is a trademark of Philips Electronics.
Copyright © 2009 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
