Datasheet

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Power Dissipation and Junction Temperature
P
D(max)
+
T
J
max *T
A
R
qJA
(2)
P
D
+
ǒ
V
I
*V
O
Ǔ
I
O
(3)
TPS70845, TPS70848
TPS70851, TPS70858
TPS70802
SLVS301D – JUNE 2000 – REVISED DECEMBER 2007
Specified regulator operation is assured to a junction temperature of +125 ° C; the maximum junction temperature
should be restricted to +125 ° C under normal operating conditions. This restriction limits the power dissipation the
regulator can handle in any given application. To ensure the junction temperature is within acceptable limits,
calculate the maximum allowable dissipation, P
D(max)
, and the actual dissipation, P
D
, which must be less than or
equal to P
D(max)
.
The maximum-power-dissipation limit is determined using the following equation:
where:
• T
Jmax
is the maximum allowable junction temperature
• R
θ JA
is the thermal resistance junction-to-ambient for the package, that is, 32.6 ° C/W for the 20-terminal PWP with no
airflow.
• T
A
is the ambient temperature
The regulator dissipation is calculated using:
Power dissipation resulting from quiescent current is negligible. Excessive power dissipation triggers the thermal
protection circuit.
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