Diodes AP1509 - Manual (Page 10)

AP1509
150KHz, 2A PWM BUCK DC/DC CONVERTER
AP1509
Document number: DS31016 Rev. 4 - 2
10 of 13
www.diodes.com
June 2010
© Diodes Incorporated
Functions Description
Pin Functions
+V
IN
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 needed by the
regulator.
Ground
Circuit ground.
Output
Internal switch. The voltage at this pin switches between (+V
IN
– V
SAT
) and approximately – 0.5V, with a duty
cycle of approximately V
OUT
/ V
IN
. To minimize coupling to sensitive circuitry, the PC board copper area
connected to this pin should be minimized.
Feedback
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 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 18V) shuts the regulator down. If this
shutdown feature is not needed, the SD pin can be wired to the ground pin.
Thermal Considerations
The SOP-8L 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 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 (SOP-8L package) operating as a buck-switching regulator in an ambient
temperature of 25
o
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 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.