TK111xxM
DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)
The range of usable currents can also be found from the
graph below.
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-6 is increased to 600 mW. For operation at
ambient temperatures over 25 °C, the power dissipation of
theSOT-23L-6deviceshouldbederatedat4.8mW/°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.
Calculatethepowerdissipationofthedevicebysubtracting
the output power from the input power. These
measurements should allow for the ambient temperature
ofthePCB. ThevalueobtainedfromPD /(150 °C-TA)isthe
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
thermalresistancewhenmountedisexpressedasfollows:
(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
vertical line intersects the derating curve. This is taken
as the maximum power dissipation, DPD
.
1.0
MOUNTED AS
0.8
SHOWN
Tj = 0jA x PD + TA
0.6
FREE AIR
For Toko ICs, the internal limit for junction temperature is
150 °C. If the ambient temperature (TA ) is 25 °C, then:
0.4
0.2
0
150 °C = 0jA x PD + 25 °C
0jA = 125 °C/ PD
0
50
100
150
T
(°C)
A
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.
SOT-23L-6 POWER DISSIPATION CURVE
March 1999 TOKO, Inc.
Page 11
TOKO [ TOKO, INC ]
