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TK71550S Ver la hoja de datos (PDF) - Toko America Inc

Número de pieza
componentes Descripción
Fabricante
TK71550S
Toko
Toko America Inc  Toko
TK71550S Datasheet PDF : 13 Pages
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TK715xx
DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)
determine the power dissipation for shutdown when
mounted, attach the device on the actual PCB and
deliberately increase the output current (or raise the input
voltage) until the thermal protection circuit is activated.
Calculate the power dissipation of the device by subtracting
the output power from the input power. These
measurements should allow for the ambient temperature
of the PCB. The value obtained from PD /(150 °C - TA) is the
derating factor. The PCB mounting pad should provide
maximum thermal conductivity in order to maintain low
device temperatures. As a general rule, the lower the
temperature, the better the reliability of the device. The
thermal resistance when mounted is expressed as follows:
Tj = 0jA x PD + TA
For Toko ICs, the internal limit for junction temperature is
150 °C. If the ambient temperature (TA) is 25 °C, then:
150 °C = 0jA x PD + 25 °C
0jA = 125 °C/ PD
PD is the value when the thermal sensor is activated. A
simple way to determine PD is to calculate VIN x IIN when
the output side is shorted. Input current gradually falls as
temperature rises. You should use the value when thermal
equilibrium is reached.
3) Plot PD1 against 25 °C
4) Connect PD1 to the point corresponding to the 150 °C
with a straight line.
5) In design, take a vertical line from the maximum
operating temperature (e.g., 75 °C) to the derating
curve.
6) Read off the value of PD against the point at which the
vertical line intersects the derating curve. This is taken
as the maximum power dissipation, DPD.
The maximum operating current is:
IOUT = (DPD / (VIN(MAX) - VOUT)
500
400
300
200
100
0
0
MOUNTED AS
SHOWN
FREE AIR
50
100
150
TA (°C)
The range of usable currents can also be found from the
graph below.
SOT-23-3 POWER DISSIPATION CURVE
(mW)
3
PD
DPD
6
4
5
25
50
75
150
TA (°C)
Procedure:
1) Find PD
2) PD1 is taken to be PD x (~ 0.8 - 0.9)
January 1999 TOKO, Inc.
Page 9

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