Liteon Semiconductor Corporation
150KHZ 3A PWM Buck DC/DC Converter
This is the positive input supply for the IC switching regulator. A suitable input bypass capacitor must be present
at this pin to minimiz e voltage trans ients and to supply the switching current s needed by the regulator.
Interna l switch. The voltage a t th is p in switches between ( VIN – VSAT) and approxima tely – 0.5 V, with a du ty
cycle of app roxima tely V OUT / VIN. T o minimize coupling to sensitive circuitry, the PC board copper are a
connected to this pin should be kept a minimum.
Senses the regulated output voltage to complete the feedback loop.
Allows the s witching reg ulator circuit to be shutd own using lo gic leve l sig nals thus dr opping the total input
supply curre nt to approximately 1 50uA. Pulling this pin below a threshold voltage of approxima tely 1.3 V turns the
regulator on , and pu lling this pin ab ove 1.3 V (u p to a ma ximum o f 18 V) shuts the r egulator do wn. If th is
shutdown feature is not needed, the ON/O FF pin can be wired to the ground pin.
The TO-252 surface mou nt package tab is designed to be soldered to the copper on a printed circuit b oard. Th e
copper and the board are the heat sink for this package and the other heat p roducing components, such as th e
catch diode and induc tor. The PC board copper ar ea 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
characteristics, but with copper areas greater than approximately 6 in 2, only sma ll improve ments in hea t
dissipation are realized. If further thermal improvements a re n eeded, double sid ed, m ultilayer PC board w ith
large copper areas and/or airflow are recommended.
The LSP312 4 (TO-252 p ackage) junction te mperature rise abo ve a mbient temperature with a 3 A lo ad for
various input and output voltages. This data was taken with the circuit operatin g as a buck switching regulator
with all components mo unted on a PC board to simulate th e junction temperature u nder actual operating
conditions. T his curve ca n be used for a quick check for the a pproxima te junction temperature for various
conditions, b ut be aware tha t there a re many factors that can affect the ju nction tempe rature. When load
currents higher than 2 A are used, doub le 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 per formance, wide copper tr aces and ge nerous amou nts o f prin ted circuit boa rd copper
should be us ed in the bo ard la yout. (Once exception to this is the ou tput (s witch) pin , which should not ha ve
large areas o f copper .) Large areas of copper provide the best transfer o f heat (lower ther mal resistanc e) to the
surrounding air, and moving air lowers the thermal resistance even further.
Package the rmal resistan ce and junction temperature rise nu mbers are all approximate, and there a re man y
factors that will a ffect th ese numbers. Some of th ese factors include board size, shape, th ickness, p osition,
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 d issipate hea t a lso depends on the size, qu antity and s pacing o f o ther compon ents 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 a nd 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.