TD1509 Techcode Semiconductor, TD1509 Datasheet - Page 11

• Manufacturer Part Number: TD1509
• Description: 2A 150KHZ PWM Buck DC/DC Converter
• Manufacturer: Techcode Semiconductor

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Function Description
Pin Functions
+V
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
with a duty cycle of approximately V
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
50uA. 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
32V) 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 TD1509 is available in SOP8/DIP8 package.
The SOP8/DIP8 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 TD1509 junction temperature rises above
ambient temperature for a 2A load and different
input and output voltages. The data for these
curves was taken with the TD1509 (SOP8/DIP8
package) operating as a buck-switching regulator
in an ambient temperature of 25
These
December, 20, 2005.
2A 150KHZ PWM Buck DC/DC Converter
IN
temperature
IN
– V
SAT
) and approximately – 0.5V,
rise
numbers
o
OUT
C (still air).
/ V
are
IN
. To
all
11
approximate and there are many factors that can
affect
temperatures require more heat sinking.
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
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, multi-layer
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.
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output voltage to FB The
voltage divider divides the
output voltage down by the ratio:
VFB = VOUT * R1 / (R1 + R2)
Thus the output voltage is:
VOUT = 1.235 * (R1 + R2) / R1
R1 can be as high as 100KΩ, but a typical
value is 10KΩ. Using that value, R2 is
determined by:
R2 ~= 8.1 * (VOUT – 1.235) (KΩ)
Inductor
The inductor is required to supply constant
current to the output load while being driven by
these
thermal
temperatures.
resistance
Techcode Semiconductor Limited
Higher
Datasheet
and
TD1509
ambient
junction