AP1512/A
50KHz, 2A/3A PWM BUCK DC/DC CONVERTER
AP1512/A Rev. 4 10 of 12 MAY 2008
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Functional Description
+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 SD 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 AP1512/A is available in a 5-pin surface mount TO-263 package.
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 AP1512/A (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. (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.
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.
