Datasheet

R
T
JA

=
165
o
C - 144
o
C
189 mW
= 111
o
C/W
R
T
JA

=
165° - Ta
P
INTERNAL
R
)
JC

=
T
J
- T
C
Power
LM2831
SNVS422C –AUGUST 2006–REVISED APRIL 2013 www.ti.com
Silicon Junction Temperature Determination Method 1:
To accurately measure the silicon temperature for a given application, two methods can be used. The first
method requires the user to know the thermal impedance of the silicon junction to top case temperature.
Some clarification needs to be made before we go any further.
R
θJC
is the thermal impedance from all six sides of an IC package to silicon junction.
R
ΦJC
is the thermal impedance from top case to the silicon junction.
In this data sheet we will use R
ΦJC
so that it allows the user to measure top case temperature with a small
thermocouple attached to the top case.
R
ΦJC
is approximately 30°C/Watt for the 6-pin WSON package with the exposed pad. Knowing the internal
dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically
measured on the bench we have:
(32)
Therefore:
T
j
= (R
ΦJC
x P
LOSS
) + T
C
(33)
From the previous example:
T
j
= (R
ΦJC
x P
INTERNAL
) + T
C
(34)
T
j
= 30°C/W x 0.189W + T
C
(35)
The second method can give a very accurate silicon junction temperature.
The first step is to determine R
θJA
of the application. The LM2831 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.
(36)
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 LM2831 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
= 189mW (37)
(38)
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
(39)
125°C - (111°C/W x 189mW) = 104°C (40)
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