150Khz, 3A PWM Buck Switching Regulator
Typical Application Circuit
12V
FB
4
SM8313
DC Input
Vin
1
5
SM8313
-33
3
SD
2
Output
L1
68uF
3.3V/3A
Output Load
C
in
680uF
Gnd
D1
1N5824
Co
470uF
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.
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
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
25V) shuts the regulator down. If this shutdown
Thermal Considerations
The SM8313 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 SM8313 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
SM8313 (TO-220 package) operating as a buck
switching regulator in an ambient temperature of
25
o
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 0.8 in
2
, 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 in
2
, 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.
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.
7/10
SAMHOP [ SAMHOP MICROELECTRONICS CORP. ]
