LM2738 Datasheet, PDF (14/28 Pages) National Semiconductor (TI) – 550kHz/1.6MHz 1.5A Step-Down DC-DC Switching Regulator

Thermal impedance is defined as:
Thermal impedance from the silicon junction to the ambient
air is defined as:
The PCB size, weight of copper used to route traces and
ground plane, and number of layers within the PCB can great-
ly effect R_θJA. The type and number of thermal vias can also
make a large difference in the thermal impedance. Thermal
vias are necessary in most applications. They conduct heat
from the surface of the PCB to the ground plane. Four to six
thermal vias should be placed under the exposed pad to the
ground plane if the LLP package is used.
Thermal impedance also depends on the thermal properties
due to the application's operating conditions (Vin, Vo, Io etc),
and the surrounding circuitry.
Silicon Junction Temperature Determination Method 1:
To accurately measure the silicon temperature for a given
application, two methods can be used. The first method re-
quires 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 8-pin LLP package
with the exposed pad. Knowing the internal dissipation from
the efficiency calculation given previously, and the case tem-
perature, which can be empirically measured on the bench
we have:
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 tempera-
ture, 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
National Semiconductor LM2738 LLP 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
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
125°C - (102°C/W x 207mW) = 104°C
LLP Package
Therefore:
T_j = (R_ΦJC x P_LOSS) + T_C
From the previous example:
T_j = (R_ΦJC x P_INTERNAL) + T_C
T_j = 30°C/W x 0.207W + 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
LM2738 has over-temperature protection circuitry. When the
silicon temperature reaches 165°C, the device stops switch-
ing. 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
FIGURE 8. Internal LLP Connection
30049174
For certain high power applications, the PCB land may be
modified to a "dog bone" shape (see Figure 9). By increasing
the size of ground plane, and adding thermal vias, the R_θJA
for the application can be reduced.
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