RT9167/A
The value of junction to case thermal resistance θJC 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 :
TJ = TC + PD x θJC
Where TC is the package case temperature measured by
thermal sensor, PD is the power dissipation defined by
user’ s function and the θJC 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 VIN = 3.3V at an output current IO = 500mA and
the case temperature (pin 4 of SOT-23-5 package)
TC = 70°C measured by thermal couple while operating,
then our power dissipation is as follows:
PD = (3.3V − 2.8V) x 500mA + 3.3V x 90μA ≅ 250mW
And the junction temperature TJ could be calculated as
following :
TJ = TC + PD x θJC
TJ = 70°C + 0.25W x 130°C/W
= 70°C + 32.5°C
= 102.5°C < TJ(MAX) =125°C
For this operation application, TJ 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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