150kHz, 2A PWM Buck DC/DC Converter
FUNCTION DESCRIPTION
Pin Functions
+VIN
TJ1509A
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 (+VIN – VSAT) and approximately – 0.5V, with a duty
cycle of approximately VOUT / VIN. To minimize coupling to sensitive circuitry, the PC board copper area
connected to this pin should be kept a minimum.
Feedback(FB)
Senses the regulated output voltage to complete the feedback loop.
ON/OFF (SD)
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 SD 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 SOP-8 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 TJ1509A 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 TJ1509A (SOP-8 package) operating as a buck-switching regulator in an ambient temperature
of 25℃ (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.
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, 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.
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HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
