APW7093 데이터 시트보기 (PDF) - Anpec Electronics
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APW7093 Datasheet PDF : 20 Pages
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APW7093
Function Descriptions
The APW7093 synchronous, current-mode, constant
off-time, PW M DC-DC converter steps down input
voltages of 3V to 5.5V to an adjustable output voltage
from 1.1V to VIN, as set by the voltage applied at
EXTREF. It sources and sinks up to 3A of output
current. Internal switches composed of a 90mΩPMOS
power switch and a 60m Ω NMOS synchronous-rectifier
switch improve efficiency, reduce component count,
and eliminate the need for an external Schottky diode
across the synchronous switch.
path for current to flow when the inductor is discharging.
Replacing the Schottky diode with a low-resistance
NMOS synchronous switch reduces conduction losses
and improves efficiency. The NMOS synchronous-rectifier
switch turns on following a short delay (typ. 20ns) af-
ter the PMOS power switch turns off, thus preventing
cross-conduction or “shoot-through.” In constant-off-
time mode, the synchronous-rectifier switch turns off
just prior to the PMOS power switch turning on. While
both switches are off, inductor current flows through
the internal body diode of the NMOS switch.
The APW7093 operates in a constant-off-time mode
under all loads. A single resistor-programmable
constant- tradeoffs in efficiency, switching noise,
component size, and cost. When power is drawn from
a regulated supply, constant-off-time PWM architecture
essentially provides constant-frequency operation. This
architecture has the inherent advantage of quick
response to line and load transients. The APW7093’s
current-mode, constant-off-time PWM architecture
regulates the output voltage by changing the PMOS
switch on-time relative to the constant off-time.
Constant-Off-Time Operation
In the constant-off-time architecture, the FB voltage
comparator turns the PMOS switch on at the end of
each off-time, keeping the device in continuous-
conduction mode. The PMOS switch remains on until
the feedback voltage exceeds the external reference
voltage (VEXTREF) or the positive current limit is reached.
When the PMOS switch turns off, it remains off for the
programmed off-time (TOFF). To control the current
under short-circuit conditions, the PMOS switch
remains off for approximately 4 x TOFF when VFB <
VEXTREF / 4.
Synchronous Rectification
In a step-down regulator without synchronous
rectification, an external Schottky diode provides a
Current Sourcing and Sinking
By operating in a constant-off-time, pseudo-fixed-
frequency mode, the APW7093 can both source and
sink current. Depending on the output current
requirement, the circuit operates in two modes. In the
first mode the output draws current and the APW7093
behaves as a regular buck controller, sourcing current
to the output from the input supply rail. However, when
the output is supplied by another source, the APW7093
operates in a second mode as a synchronous boost,
taking power from the output and returning it to the
input.
Thermal Resistance
Junction-to-ambient thermal resistance, θ JA, is highly
dependent on the amount of copper area immediately
surrounding the IC leads. The APW7093 QFN pack-
age has 1in square of copper area and a thermal
resistance of 50°C/W with no forced airflow. The
APW 7093 16-pin SSOP evaluation kit has 0.5 in
square of copper area and a thermal resistance of 80°C/
W with no forced airflow. Airflow over the board
significantly reduces the junction-to-ambient thermal
resistance. For heat sinking purposes, it is essential
to connect the exposed backside pad of the QFN
package to a large analog ground plane.
Copyright © ANPEC Electronics Corp.
10
Rev. A.5 - Jun., 2005
www.anpec.com.tw
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