ACT8342
Rev 3, 11-Sep-09
STEP-DOWN DC/DC CONVERTER
FUNCTIONAL DESCRIPTION
Input Capacitor Selection
General Description
The input capacitor reduces peak currents and
noise induced upon the voltage source. A 2.2µF
ceramic input capacitor is recommended for most
applications.
REG1 is
a
fixed-frequency, current-mode,
synchronous PWM step-down converters that
achieves a peak efficiency of up to 97%. REG1 is
capable of supplying up to 350mA of output current
and operates with a fixed frequency of 1.6MHz,
minimizing noise in sensitive applications and
allowing the use of small external components.
REG1 is available with a variety of standard and
custom output voltages, as well as an adjustable
output voltage option.
Output Capacitor Selection
For most applications, a 10µF ceramic output
capacitor is recommended. Although REG1 was
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.
100% Duty Cycle Operation
REG1 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 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. REG1 was optimized for
operation with a 3.3µH inductor, although inductors
in the 2.2µH to 4.7µH 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 of
the application by at least 30%.
Synchronous Rectification
REG1 features an integrated n-channel
synchronous rectifier, which maximizes efficiency
and minimizes the total solution size and cost by
eliminating the need for an external rectifier.
Thermal Shutdown
Enabling and Disabling REG1
The ACT8342 integrates thermal shutdown
protection circuitry to prevent damage resulting
from excessive thermal stress, as may be
encountered under fault conditions. This circuitry
disables all regulators if the ACT8342 die
temperature exceeds 160°C, and prevents the
regulators from being enabled until the IC
temperature drops by 20°C (typ).
REG1 is enabled or disabled using ON1. Drive ON1
to a logic-high to enable REG1. Drive ON1 to a
logic-low to disable REG1, reducing supply current
to less than 1µA.
Soft-Start
REG1 includes internal soft-start circuitry, and
enabled its output voltage tracks an internal 80µs
soft-start ramp so that it powers up in a monotonic
manner that is independent of loading.
Output Voltage Programming
Figure 4 shows the feedback network necessary to
set the output voltage when using the adjustable
output voltage option. Select components as
follows: Set RFB2 = 51Kꢀ, then calculate RFB1 using
the following equation:
Compensation
REG1 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.
⎛
⎞
VOUT1
VFB
(1)
⎜
⎜
⎟
⎟
RFB1 = RFB2
−1
⎝
⎠
Where VFB is 0.625V
Innovative PowerTM
ActivePMUTM is a trademark of Active-Semi.
- 11 -
www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
