ACT8847
Rev 5, 15-Nov-12
STEP-DOWN DC/DC REGULATORS
Input Capacitor Selection
General Description
The input capacitor reduces peak currents and noise
induced upon the voltage source. A 10μF ceramic
capacitor is recommended for each regulator in most
applications.
REG1, REG2, REG3, and REG4 are fixed-frequency,
current-mode, synchronous PWM step-down
converters that achieves peak efficiencies of up to
97%. These regulators operate with a fixed frequency
of 2.25MHz, minimizing noise in sensitive
applications and allowing the use of small external
components. Additionally, REG1, REG2, REG3, and
REG4 are available with a variety of standard and
custom output voltages, and may be software-
controlled via the I2C interface for systems that
require advanced power management functions.
Output Capacitor Selection
REG1, REG2, REG3, and REG4 were designed to
take advantage of the benefits of ceramic capacitors,
namely small size and very-low ESR. REG1, REG2,
REG3 and REG4 are designed to operate with 33uF
or 44uF output capacitor over most of their output
voltage ranges, although more capacitance may be
desired depending on the duty cycle and load step
requirements.
100% Duty Cycle Operation
REG1, REG2, REG3, and REG4 are capable of
operating at up to 100% duty cycle. During 100%
duty cycle operation, the high-side power MOSFETs
Two of the most common dielectrics are Y5V and
X5R. Whereas Y5V dielectrics are inexpensive and
can provide high capacitance in small packages, their
capacitance varies greatly over their voltage and
temperature ranges and are not recommended for
DC/DC applications. X5R and X7R dielectrics are
more suitable for output capacitor applications, as
their characteristics are more stable over their
operating ranges, and are highly recommended.
are 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.
Operating Mode
By default, REG1, REG2, REG3, and REG4 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.
Inductor Selection
REG1, REG2, REG3, and REG4 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. These
devices were optimized for operation with 2.2μH or
1μH inductors. Choose an inductor with a low DC-
resistance, and avoid inductor saturation by choosing
inductors with DC ratings that exceed the maximum
output current by at least 30%.
Synchronous Rectification
REG1, REG2, REG3, and REG4 each feature
integrated synchronous rectifiers, maximizing
efficiency and minimizing the total solution size and
cost by eliminating the need for external rectifiers.
Soft-Start
REG1, REG2, REG3, and REG4 include internal 400
us soft-start ramps which limit the rate of change of
the output voltage, minimizing input inrush current
and ensuring that the output powers up in a
monotonic manner that is independent of loading on
the outputs. This circuitry is effective any time the
regulator is enabled, as well as after responding to a
short-circuit or other fault condition.
Configuration Options
Output Voltage Programming
By default, each regulator powers up and regulates to
its default output voltage. For REG2, REG3 and
REG4, the output voltage is selectable by setting
corresponding VSEL pin that when VSEL is low,
output voltage is programmed by VSET0[-] bits, and
when VSEL is high, output voltage is programmed by
VSET1[-] bits. Also, once the system is enabled,
each regulator's output voltage may be independently
programmed to a different value. Program the output
voltages via the I2C serial interface by writing to the
regulator's VSET0[-] register if VSEL is low or
VSET1[-] register if VSEL is high as shown in Table
5.
Compensation
REG1, REG2, REG3, and REG4 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.
Innovative PowerTM
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