SM8303 Datasheet, PDF (8/11 Pages) SamHop Microelectronics Corp. – 150KHz, 3A PWM Buck Switching Regulator

SAMHOP Microelectronics Corp.
150KHz, 3A PWM Buck Switching Regulator
SM8303
Function Description
Pin Functions
+V_IN
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.
Output
Internal switch and power output. 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. The PC board copper
area connected to this pin should be kept a
minimum in order to reduce the coupling sensitivity
to the circuitry
Ground
Circuit ground.
Feedback
Complete the feedback loop by sensing the
regulated output voltage
ON /OFF
Allows the switching regulator circuit to be
shutdown using logic level signals thus dropping
the total input supply current to approximately
100uA. 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
25V) 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 SM8303 is available in two packages, a 5-pin
TO-220 and a 5-pin surface mount TO-263.
The TO-220 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 SM8303 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
SM8303 (TO-220 package) operating as a buck
switching regulator in an ambient temperature of
25^oC (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 0.8 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.
Function Description (Continued)
The SM8303 (TO-263 package) junction
temperature rise 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 3A 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
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V1.0 May 17 ,2007