AP1506

150KHz, 3A PWM BUCK DC/DC CONVERTER

AP1506 Rev. 2

9 of 13

NOVEMBER 2006

www.diodes.com

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 kept to a minimum.

Feedback (FB)

Senses the regulated output voltage to complete the feedback

loop.

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

be wired to the ground pin.

Thermal Considerations

The AP1506 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 heat sink depends on the input voltage, the output

voltage, the load current and the ambient temperature. The

AP1506 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 AP1506 (TO-220 package)

operating as a buck-switching regulator in an ambient

temperature of 25

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 in

inches of 2 oz. Additional copper area improves the thermal

characteristics, but with copper areas greater than approximately

6 in

further thermal improvements are needed, double sided,

multi-layer PC board with large copper areas and/or airflow will

be recommended.

The AP1506 (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.

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.