Datasheet

LM2738
SNVS556B –APRIL 2008–REVISED APRIL 2013 www.ti.com
The second method can give a very accurate silicon junction temperature.
The first step is to determine R
θJA
of the application. The LM2738 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 NFET 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.
(48)
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 Texas Instruments LM2738 WSON demonstration
board is shown below.
The four layer PCB is constructed using FR4 with ½ oz copper traces. The copper ground plane is on the bottom
layer. The ground plane is accessed by two vias. The board measures 3.0cm x 3.0cm. It was placed in an oven
with no forced airflow. The ambient temperature was raised to 144°C, and at that temperature, the device went
into thermal shutdown.
From the previous example:
P
INTERNAL
= 207mW (49)
(50)
If the junction temperature was to be kept below 125°C, then the ambient temperature could not go above
109°C.
T
j
- (R
θJA
x P
LOSS
) = T
A
(51)
125°C - (102°C/W x 207mW) = 104°C (52)
WSON Package
Figure 34. Internal WSON Connection
For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 35). By
increasing the size of ground plane, and adding thermal vias, the R
θJA
for the application can be reduced.
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