150KHZ， PWM BUCK DC/DC CONVERTER
2A
FSP3125
Function Description
Pin Description
+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 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
Sense 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
T
he SOP8L 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 FSP3125 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 FSP3125(SOP8L package) operating as a buck-switching regulator in an ambient temperature 25℃ (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.
4/9
2007-3-27
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
