TK112xxB
DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)
the output side is shorted. Input current gradually falls as
temperature rises. You should use the value when thermal
equilibrium is reached.
PACKAGE POWER DISSIPATION (PD)
This is the power dissipation level at which the thermal
sensor is activated. The IC contains an internal thermal
sensorwhichmonitorsthejunctiontemperature. Whenthe
junction temperature exceeds the monitor threshold of
150 °C, the IC is shut down. The junction temperature
rises as the difference between the input power (VIN x IIN)
and the output power (VOUT x IOUT) increases. The rate of
temperature rise is greatly affected by the mounting pad
configuration on the PCB, the board material, and the
ambient temperature. When the IC mounting has good
thermal conductivity, the junction temperature will be low
even if the power dissipation is great. When mounted on
the recommended mounting pad, the power dissipation of
the SOT-23L is increased to 600 mW. For operation at
ambient temperatures over 25 °C, the power dissipation of
the SOT-23L device should be derated at 4.8 mW/°C. The
power dissipation of the SOT-89 package is 900 mW when
mounted as recommended. Derate the power dissipation
at 7.2 mW/°C for operation above 25 °C. To 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
fortheambienttemperatureofthePCB.Thevalueobtained
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:
The range of usable currents can also be found from the
graph below.
(mW)
3
P
D
6
D
PD
4
5
25
50
75
(°C)
150
T
A
Procedure:
1) Find PD
2) PD1 is taken to be PD x (~0.8 - 0.9)
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
verticallineintersectsthederatingcurve. Thisistaken
as the maximum power dissipation, DPD
.
The maximum operating current is:
Tj = 0jA x PD + TA
IOUT = (DPD / (VIN(MAX) - VOUT
)
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
Page 16
October 1999 TOKO, Inc.
TOKO [ TOKO, INC ]
