AP1509

150KHz, 2A PWM BUCK DC/DC CONVERTER

AP1509 Rev. 1

9 of 12

DECEMBER 2006

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Function Description

Pin Functions

This is the positive input supply for the IC switching regulator. A

suitable input bypass capacitor must be presented at this pin to

minimize voltage transients and to supply the switching currents

needed by the regulator.

Ground

Circuit ground.

Output

circuitry, the PC board copper area connected to this pin should

be minimized.

Feedback

Senses the regulated output voltage to complete the feedback

loop.

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 SD pin can be wired to

the ground pin.

Thermal Considerations

The SOP-8L 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

AP1509 junction temperature rises above ambient temperature

for a 2A load and different input and output voltages. The data for

these curves was taken with the AP1509 (SOP-8 package)

operating as a buck-switching regulator in an ambient

temperature of 25

all approximate and are affected by many factors. Higher ambient

temperatures require more heat sinker.

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 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 increments

are all approximate. The increments are affected by a lot of

factors. 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.