RT9167/A
The value of junction to case thermal resistance
θ
JC
is
popular for users. This thermal parameter is convenient
for users to estimate the internal junction operated
temperature of packages while IC operating. It’ s
independent of PCB layout, the surroundings airflow effects
and temperature difference between junction to ambient.
The operated junction temperature can be calculated by
following formula :
T
J
= T
C
+ P
D
x
θ
JC
Where T
C
is the package case temperature measured by
thermal sensor, P
D
is the power dissipation defined by
user’ s function and the
θ
JC
is the junction to case thermal
resistance provided by IC manufacturer. Therefore it’ s easy
to estimate the junction temperature by any condition.
For example, how to calculate the junction temperature
of RT9167A-28CB SOT-23-5 package? If we use input
voltage V
IN
= 3.3V at an output current I
O
= 500mA and
the case temperature (pin 4 of SOT-23-5 package)
T
C
= 70°C measured by thermal couple while operating,
then our power dissipation is as follows:
P
D
= (3.3V
−
2.8V) x 500mA + 3.3V x 90μA
≅
250mW
And the junction temperature T
J
could be calculated as
following :
T
J
= T
C
+ P
D
x
θ
JC
T
J
= 70°C + 0.25W x 130°C/W
= 70°C + 32.5°C
= 102.5°C < T
J(MAX)
=125°C
For this operation application, T
J
is lower than absolute
maximum operation junction temperature 125°C and it’s
safe to use.
DS9167/A-26 March 2007
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11
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