Diodes AP1507 - Manual (Page 9)

AP1507
150KHz, 3A PWM BUCK DC/DC CONVERTER
AP1507
Document number: DS31067 Rev. 3 - 2
9 of 12
www.diodes.com
May 2012
© Diodes Incorporated
Application Information
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 at 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 TO-252 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 0.8 in
2 , and ideally should have
2 or more square inches of 2 oz. additional copper area which
improves the thermal characteristics. With copper areas greater
than approximately 6in
2
, 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 AP1507 (TO-252 package) junction temperature rises
above ambient temperature with a 3A 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 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. (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, multi-
layer 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.