AP1509

Document number: DS31016 Rev. 6 - 2

9 of 12

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February 2020

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AP1509

Functions Description

Pin Functions

+VIN

This is the positive input supply for the IC switching regulator. A suitable input bypass capacitor must be presented at this pin to minimize voltage

transients and to supply the switching currents required by the regulator.

Ground

Circuit ground.

Output

Internal switch. The voltage at this pin switches between (+VIN - VSAT) and approximately -0.5V with a duty cycle of approximately VOUT/VIN. To

minimize coupling to sensitive circuitry, the PCB copper area connected to this pin should be minimized.

Feedback

This pin 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 150µA.

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 required, the SD pin can be wired to the ground pin.

Thermal Considerations

The SO-8 package requires 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 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 (SO-8 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

require more heat sinker.

For the best thermal performance, wide copper traces and generous amounts of PCB 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 PCB 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, add heat to the PCB, 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.