AP1501

Document number: DS31014 Rev. 14 - 2

9 of 13

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March 2021

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AP1501

Operation Description

Pin Functions

+VIN

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 between (+VIN – VSAT) and approximately – 0.5V, with a duty cycle of approximately VOUT / VIN.

To 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.

ON/OFF

Allows the switching regulator circuit to be shut down 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 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 AP1501 is available in the 5-pin surface mount TO-263 and TO-220.

Under most conditions, the TO-220 package requires a heat sink. The size of the heat sink depends on the input voltage, the output voltage, the

load current, and the ambient temperature. The AP1501 junction temperature rises above ambient temperature for a 3A load and different input

and output voltages. The data for these curves was taken with the AP1501 (TO-220 package) operating as a buck switching regulator in an

ambient temperature of 25°C (still air). These temperature rise 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 0.4 in2, and ideally should have 2 or more square inches of 2 oz. Additional copper area improves the

thermal characteristics, but with copper areas greater than approximately 6 in2, only small improvements in heat dissipation are realized. If further

thermal improvements are needed, double-sided multilayer PC boards with large copper areas and/or airflow are recommended.

The AP1501 (TO-263 package) junction temperature rises 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 2A 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.

(One 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, the board being single or double-sided, the board being multilayered, 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 add heat to the board or act

as a heat sink taking heat away from the board.