ACT8933
Rev 3, 15-Nov-12
STEP-DOWN DC/DC REGULATORS
must be taken during the design process to ensure
stable operation over the full operating voltage and
temperature range. Ceramic capacitors are available
in a variety of dielectrics, each of which exhibits
different characteristics that can greatly affect
performance over their temperature and voltage
ranges.
General Description
The ACT8933 features three synchronous, fixed-
frequency, current-mode PWM step down converters
that achieve peak efficiencies of up to 97%. REG1
and REG3 are capable of supplying up to 900mA of
output current, while REG2 supports up to 700mA.
These regulators operate with a fixed frequency of
2MHz, minimizing noise in sensitive applications and
allowing the use of small external components.
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.
100% Duty Cycle Operation
Each regulator is 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.
Inductor Selection
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. These devices were optimized for
operation with 2.2μH inductors, although inductors in
the 1.5μH to 3.3μH range can be used. 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, and REG3 each feature integrated n-
channel synchronous rectifiers, maximizing efficiency
and minimizing the total solution size and cost by
eliminating the need for external rectifiers.
Soft-Start
When enabled, each output voltages tracks an
internal 400μs soft-start ramp, minimizing input
current during startup and allowing each regulator to
power up in a smooth, monotonic manner that is
independent of output load conditions.
Configuration Options
Output Voltage Programming
By default, each regulator powers up and regulates to
its default output voltage. Output voltage is selectable
by setting VSEL pin that when VSEL is low, output
voltage is programmed by VSET1[-] bits, and when
VSEL is high, output voltage is programmed by
VSET2[-] bits. However, once the system is enabled,
each regulator's output voltage may be independently
programmed to a different value, typically in order to
minimize the power consumption of the
microprocessor during some operating modes.
Program the output voltages via the I2C serial
interface by writing to the regulator's VSET1[-]
register if VSEL is low or VSET2[-] register if VSEL is
high as shown in Table 8.
Compensation
Each buck regulator utilizes current-mode control and
a proprietary internal compensation scheme to
simultaneously simplify external component selection
and optimize transient performance over its full
operating range. No compensation design is
required; simply follow a few simple guidelines
described below when choosing external
components.
Input Capacitor Selection
The input capacitor reduces peak currents and noise
induced upon the voltage source. A 4.7μF ceramic
capacitor is recommended for each regulator in most
applications.
Enable / Disable Control
During normal operation, each buck may be enabled
or disabled via the I2C interface by writing to that
regulator's ON[ ] bit. The regulator accept rising or
falling edge of ON[ ] bit as on/off signal. To enable
the regulator, clear ON[ ] to 0 first then set to 1. To
disable the regulator, set ON[ ] to 1 first then clear it
to 0.
Output Capacitor Selection
For most applications, 22μF ceramic output
capacitors are recommended for REG1 and REG3,
while 15μF ceramic output capacitor is recommended
for REG2.
Despite the advantages of ceramic capacitors, care
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