AP1501A

150KHz, 5A PWM BUCK DC/DC CONVERTER

AP1501A Rev. 7

9 of 12

SEPTEMBER 2008

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Functional 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

and approximately – 0.5V, with a duty cycle of approximately

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 ON/OFF 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 AP1501A is available in two packages, a 5-pin surface mount

TO-263 and TO-220.

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 AP1501A junction

temperature rises above ambient temperature for a 5A load and different

input and output voltages. The data for these curves was taken with the

AP1501A (TO-220 package) operating as a buck switching regulator in

an ambient temperature of 25

numbers 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 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 0.8 in

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

realized. If further thermal improvements are needed, double sided,

multilayer PC boards with large copper areas and/or airflow are

recommended.

The AP1501A (TO-263 package) junction temperature rise 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 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.