Advanced Power

Electronics Corp.

APE1702

FUNCTION DESCRIPTION

EN

THERMAL CONSIDERATIONS

6

Allows the switching regulator circuit to be shutdown using logic level signals thus

dropping the total input supple current to approximately 100uA. 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 VCC) shuts the regulator down. if this shutdown

feature is not needed, the EN pin can be wired to the ground pin.

The TO-263-5L 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 APE1702 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 APE1702 (TO-263-5L package)

operating as a buck-switching regulator in an ambient temperature of 25°C (still air).

These temperature increments are all approximate and are affected by many factors.

Higher ambient temperatures required more heat sinker.

For the best thermal performance, wide copper traces and generous amounts of

printed circuit board copper (need connect to the VSS pins) should be used in the

board layout, (One exception is the SW pin, which should not have large areas of

copper.) 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.