TK111xxM
DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)
PACKAGE POWER DISSIPATION (P
D
)
This is the power dissipation level at which the thermal
sensor is activated. The IC contains an internal thermal
sensor which monitors the junction temperature. When the
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 (V
IN
x I
IN
)
and the output power (V
OUT
x I
OUT
) 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
the SOT-23L-6 device should be derated at 4.8 mW/°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 for the ambient temperature
of the PCB. The value obtained from P
D
/(150
°C
- T
A
) 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)
PD
3
DPD
6
4
5
25
50
75
TA ( C)
150
Procedure:
1)
2)
3)
4)
Find P
D
P
D1
is taken to be P
D
x (~ 0.8 - 0.9)
Plot P
D1
against 25
°C
Connect P
D1
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 P
D
against the point at which the
vertical line intersects the derating curve. This is taken
as the maximum power dissipation, D
PD
.
1.0
0.8
PD (mW)
0.6
FREE AIR
0.4
0.2
MOUNTED AS
SHOWN
T
j
= 0
jA
x P
D
+ T
A
For Toko ICs, the internal limit for junction temperature is
150
°C.
If the ambient temperature (T
A
) is 25
°C,
then:
150
°C
= 0
jA
x P
D
+ 25
°C
0
jA
= 125
°C/
P
D
P
D
is the value when the thermal sensor is activated. A
simple way to determine P
D
is to calculate V
IN
x I
IN
when
the output side is shorted. Input current gradually falls as
temperature rises. You should use the value when thermal
equilibrium is reached.
0
0
50
TA (°C)
100
150
SOT-23L-6 POWER DISSIPATION CURVE
March 1999 TOKO, Inc.
Page 11
TOKO [ TOKO, INC ]
