Liteon Semiconductor Corporation
LSP3124
150KHZ 3A PWM Buck DC/DC Converter
FEATURES
Output voltage: 3.3V, 5.0V, 12V, and adjustable
output version
Adjustable version output voltage range:1.23V to
18V±4%
150KHz±15% fixed switching frequency
Voltage mode non-synchronous PWM control
Thermal-shutdown and current-limit protection
ON/OFF shutdown control input
Operating voltage can be up to 22V
Output load current: 3A
Low power standby mode
Built-in switching transistor on chip
TO252-5L packages
GENERAL DESCRIPTION
The LSP3124 series are monolithic IC that design for
a step-down DC/DC converter, and own the ability of
driving a 3A load without additional transistor
component. Due to reducing the number of external
component, the board space can be saved easily.
The external shutdown function can be controlled by
logic level and then come into standby mode. The
internal compensation makes feedback control have
good line and load regulation without external design.
Regarding protected function, thermal shutdown is to
prevent over temperature operating from damage,
and current limit is against over current operating of
the output switch. If current limit function occurred and
V
FB
is down to 0.5V below, the switching frequency
will be reduced. The LSP3124 series operates at a
switching frequency of 150 KHz thus allowing smaller
sized filter components than what would be needed
with lower frequency switching regulators. Other
features include a guaranteed ±4% tolerance on
output voltage under specified input voltage and
output load conditions, and ±15% on the oscillator
frequency. The output version included fixed 3.3V,
5V, 12V, and an adjustable type. The package is
available in a standard 5-lead TO-252 package.
APPLICATIONS
Simple High-efficiency step-down regulator
On-card switching regulators
Positive to negative converter
PIN CONFIGURATION
(Top View)
5
4
3
2
1
Metal Tab GND
SD
FB
GND
OUTPUT
VIN
Symbol
1
2
3
4
5
Name
Vin
Output
Gnd
FB
SD
Descriptions
Operating Voltage Input
Switching Output
Ground
Output Voltage Feedback Control
ON/OFF Shutdown
1/9
Rev1.3
Liteon Semiconductor Corporation
LSP3124
150KHZ 3A PWM Buck DC/DC Converter
FUNCTION DESCRIPTION
Pin Function
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 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 TO-252 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 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.
The LSP3124 (TO-252 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.
4/9
Rev1.3
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