AP1518 Datasheet, PDF (8/12 Pages) Anachip Corp – 300KHz, 3A PWM Buck DC/DC Converter

300KHz, 3A PWM Buck DC/DC Converter
AP1518
„ Function Description
Pin Functions
+V_IN
This is the positive input supply for the IC switching
regulator. A suitable input bypass capacitor must be
present at this pin to minimize voltage transients
and to supply the switching currents needed by the
regulator.
Ground
Circuit ground.
Output
Internal switch. The voltage at this pin switches
between (+V_IN – V_SAT) and approximately – 0.5V,
with a duty cycle of approximately V_OUT / V_IN. To
minimize coupling to sensitive circuitry, the PC
board copper area connected to this pin should be
kept a minimum.
Feedback (FB)
Senses the regulated output voltage to complete
the feedback loop.
ON /OFF (SD)
Allows the switching regulator circuit to be
shutdown using logic level signals thus dropping
the total input supply current to approximately
150uA. Pulling this pin below a threshold voltage of
approximately 1.3V turns the regulator on, and
pulling this pin above 1.3V (up to a maximum of
18V) shuts the regulator down. If this shutdown
feature is not needed, the ON /OFF pin can be
wired to the ground pin.
Thermal Considerations
The AP1518 is available in two packages: a 5-pin
TO-220 and a 5-pin surface mount TO-263.
The TO-220 package needs a heat sink under most
conditions. The size of the heatsink depends on the
input voltage, the output voltage, the load current
and the ambient temperature. The AP1518 junction
temperature rises above ambient temperature for a
3A load and different input and output voltages.
The data for these curves was taken with the
AP1518 (TO-220 package) operating as a
buck-switching regulator in an ambient temperature
of 25^oC (still air). These temperature rise numbers
are all approximate and there are many factors that
can affect these temperatures. Higher ambient
temperatures require more heat sinking.
The TO-263 surface mount package tab was
designed to be soldering to the copper on a printed
circuit board. The copper and the board are the
heat sink for this package and the other heat
producing components, such as the catch diode
and inductor. The PC board copper area that the
package is soldered to should be at least 0.8 in²,
and ideally should have 2 or more square inches of
2 oz. Additional copper area improves the thermal
characteristics, but with copper areas greater than
approximately 6 in², only small improvements in
heat dissipation are realized. If further thermal
improvements are needed, double sided,
multi-layer PC board with large copper areas and/or
airflow will be recommended.
The AP1518 (TO-263 package) junction
temperature rises above ambient temperature with
a 2A load for various input and output voltages.
This data was taken with the circuit operating as a
buck-switching regulator with all components
mounted on a PC board to simulate the junction
temperature under actual operating conditions.
This curve can be used for a quick check for the
approximate junction temperature for various
conditions, but be aware that there are many
factors that can affect the junction temperature.
When load currents higher than 3A are used,
double sided or multi-layer PC boards with large
copper areas and/or airflow might be needed,
especially for high ambient temperatures and high
output voltages.
For the best thermal performance, wide copper
traces and generous amounts of printed circuit
board copper should be used in the board layout.
(Once exception to this is the output (switch) pin,
which should not have large areas of copper.)
Large areas of copper provide the best transfer of
heat (lower thermal resistance) to the surrounding
air, and moving air lowers the thermal resistance
even further.
Anachip Corp.
www.anachip.com.tw
Rev.1.1 Oct 29, 2004
8/12