150KHZ 3A PWM BUCK DC/DC CONVERTER
FSP3123
FUNCTION DESCRIPTION
Pin Function
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 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 ON/OFF pin can be wired to the ground pin.
Thermal Considerations
The FSP3123 is available in three packages, a 5-pin TO-220, 5-pin TO263 and a 8-pin PDIP Package. The
TO-220 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 FSP3123 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 FSP3123 (TO-220 package) operating as a buck switching regulator in an ambient temperature of 25oC
(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 board with large
copper areas and/or airflow are recommended.
The FSP3123 (TO-263 package) junction temperature rise above ambient temperature with a 3A 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. (Once 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, single or double-sided, multilayer 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.
2007-6-12
4/11
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
