Datasheet

R
T
JA

=
165°C - 140°C
213 mW
= 117°C/W
R
T
JA

=
165° - Ta
P
INTERNAL
LMR10515
www.ti.com SNVS728C –OCTOBER 2011–REVISED APRIL 2013
where T
C
is the temperature of the exposed pad and can be measured on the bottom side of the PCB.
Therefore:
T
j
= (R
θJC
x P
LOSS
) + T
C
From the previous example:
T
j
= (R
θJC
x P
INTERNAL
) + T
C
T
j
= 18°C/W x 0.213W + T
C
The second method can give a very accurate silicon junction temperature.
The first step is to determine R
θJA
of the application. The LMR10515 has over-temperature protection circuitry.
When the silicon temperature reaches 165°C, the device stops switching. The protection circuitry has a
hysteresis of about 15°C. Once the silicon temperature has decreased to approximately 150°C, the device will
start to switch again. Knowing this, the R
θJA
for any application can be characterized during the early stages of
the design one may calculate the R
θJA
by placing the PCB circuit into a thermal chamber. Raise the ambient
temperature in the given working application until the circuit enters thermal shutdown. If the SW-pin is monitored,
it will be obvious when the internal PFET stops switching, indicating a junction temperature of 165°C. Knowing
the internal power dissipation from the above methods, the junction temperature, and the ambient temperature
R
θJA
can be determined.
Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be
found.
An example of calculating R
θJA
for an application using the LMR10515 is shown below.
A sample PCB is placed in an oven with no forced airflow. The ambient temperature was raised to 140°C, and at
that temperature, the device went into thermal shutdown.
From the previous example:
P
INTERNAL
= 213 mW
Since the junction temperature must be kept below 125°C, then the maximum ambient temperature can be
calculated as:
T
j
- (R
θJA
x P
LOSS
) = T
A
125°C - (117°C/W x 213 mW) = 100°C
WSON Package
Internal WSON Connection
For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 24). By
increasing the size of ground plane, and adding thermal vias, the R
θJA
for the application can be reduced.
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