AP1501
150KHz, 3A PWM BUCK DC/DC CONVERTER
AP1501 Rev. 11 9 of 12 JANUARY 2009
www.diodes.com © Diodes Incorporated
Functional 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
Senses the regulated output voltage to complete the feedback
loop.
ON/OFF
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 40V) shuts the regulator down.
If this shutdown feature is not needed, the ON/OFF pin can be
wired to the ground pin or it can be left open, in either case the
regulator will be in the ON condition.
Thermal Considerations
The AP1501 is available in 5-pin surface mount TO-263 and
TO-220.
The TO-220 package needs a heat sink under most conditions.
The size of the heat sink depends on the input voltage, the output
voltage, the load current and the ambient temperature. The
AP1501 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 AP1501 (TO-220 package)
operating as a buck switching regulator in an ambient
temperature of 25
o
C (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 is designed to be
soldered 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 0.4 in
2
, 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 in
2
, only small improvements in heat dissipation
are realized. If further thermal improvements are needed, double
sided, multilayer PC boards with large copper areas and/or
airflow are recommended.
The AP1501 (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 2A are
used, double sided or multilayer 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. (One 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.
Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors that will
affect these numbers. Some of these factors include board size,
shape, thickness, position, location, and even board temperature.
Other factors are, trace width, total printed circuit copper area,
copper thickness, single or double-sided, multilayer board and
the amount of solder on the board. The effectiveness of the
PC board to dissipate heat also depends on the size, quantity
and spacing of other components on the board, as well as
whether the surrounding air is still or moving.
Furthermore, some of these components such as the catch diode
will add heat to the PC board and the heat can vary as the input
voltage changes. For the inductor, depending on the physical
size, type of core material and the DC resistance, it could either
act as a heat sink taking heat away from the board, or it could
add heat to the board.
