TK717xxS
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 tempera-
ture rise is greatly affected by the mounting pad configura-
tion 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 recom-
mended mounting pad, the power dissipation of the SOT23-
5 is increased to 500 mW. For operation at ambient
temperatures over 25 °C, the power dissipation of the
SOT23-5 device should be derated at 4.0 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 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.
PD(mW)
3
PD
DPD
6
4
5
25
50
75
TA (°C)
150
Procedure:
1) Find P
D
2) P
D1
is taken to be P
D
x (~0.8 - 0.9)
3) Plot P
D1
against 25 °C
4) 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
.
The maximum operating current is:
I
OUT
= (D
PD
/ (V
IN(MAX)
- V
OUT
)
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
0
jA
= 125 °C / P
D
(°C / mW)
P
D
is the value when the thermal protection circuit 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.
Page 8
October 2001 TOKO, Inc.
TOKO [ TOKO, INC ]
