AP1507

150KHz, 3A PWM Buck DC/DC Converter

Anachip Corp.

www.anachip.com.tw

Rev.1.1 Oct 29, 2004

8/9

Function Description

Pin Functions

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

minimize coupling to sensitive circuitry, the PC

board copper area connected to this pin should be

kept 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 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

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

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 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.