WT2595M
Typical Application Circuit
FB
12V
DC Input
3
3.3V/2A
Output Load
L1
33uH
Out
WT2595M
1
2
Vin
Inductor
-33
4
8
7 6 5
SD
Gnd
Cin
Capacitor
Co
Capacitor
D1
Schottky Diode
Function Description
Pin Functions
+VIN
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.
Out Internal switch and power output. The voltage at this pin switches between (+VIN – VSAT) and approximately –
0.5V, with a duty cycle of approximately VOUT / VIN. The PC board copper area connected to this pin should be
kept a minimum in order to reduce the coupling sensitivity to the circuitry
Ground
Circuit ground.
Feedback
Complete the feedback loop by sensing the regulated output voltage
ON/OFF
Allows the switching regulator circuit to be shutdown using logic level signals thus dropping the total input
supply current to approximately 100uA. 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 feature is not needed, the ON/OFF pin must be wired to the ground pin, in either case the
regulator will be in the ON condition.
Thermal Considerations
The SOP-8 package needs a heat sink under most condictions . The size of the heatsink depends on
the input voltage, the output voltage, the load current and the ambient temperature. The WT2595M
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 WT2595M operating as a buck switching regulator in an ambient
temperature of 25oC (still air). These temperature increments are all approximate and are affected by many
factors. 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 amount of solder on the board. 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. (One exception is the out(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.
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 heat can vary as the input
voltage changes. For the inductor, depending on the phical 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.
WEITRON
http://www.weitron.com.tw
28-Sep-06
3/9
