ACT8935
Rev 4, 17-Sep-13
STEP-DOWN DC/DC REGULATORS
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
exhibits different characteristics that can greatly
affect performance over their temperature and
voltage ranges.
General Description
The ACT8935 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
Compensation
By default, each regulator powers up and regulates
to its 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 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
VSET[-] register as shown in Table 8.
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 and REG3,
while 15μF ceramic output capacitor is
recommended for REG2.
PWRSTAT Control Bit (REG3)
PWRSTAT[-] is a control bit which configures
REG3's behavior with respect to the PBIN input.
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