AP1509

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Thermal Co nsiderati ons

The SOP8 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 a2A load

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

AP1509

(SOP8 package) operating as a buck-switching regulator in an ambient temperature of 25oC

(still air). These temperature increments are all approximate and are affected by many

factors. Higher ambient temperatures requires more heat sinker.

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

circuit board copper should be used in the board layout. (One exception 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 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.