ACT8946AQJ203-T
Rev 1.0, 08-Jul-16
STEP-DOWN DC/DC REGULATORS
exhibits different characteristics that can greatly
affect performance over their temperature and
voltage ranges.
General Description
The ACT8946A features three synchronous, fixed-
frequency, current-mode PWM step down
converters that achieve peak efficiencies of up to
97%. REG1 and REG2 are capable of supplying up
to 1100mA of output current, while REG3 supports
up to 1200mA. 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.
Configuration Options
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.
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 4.
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
Enable / Disable Control
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.
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, REG2 and
REG3.
REG1, REG2, REG3 Turn-on Delay
Despite the advantages of ceramic capacitors, care
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
Each of REG1, REG2 and REG3 features a
programmable Turn-on Delay which help ensure a
reliable qualification. This delay is programmed by
DELAY[2:0], as shown in Table 5.
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