AP1509

150KHz, 2A PWM Buck DC/DC Converter

Anachip Corp.

www.anachip.com.tw

Rev.0.1 Nov 27, 2003

7/8

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

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 or it can be left open, in either case

the regulator will be in the ON condition.

Thermal Considerations

The SOP-8 package needs a heat sink under most

conditions. The size of the heatsink 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

are all approximate and there are many factors that

can affect these temperatures. Higher ambient

temperatures require more heat sinking.

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.