lm2738xsdx National Semiconductor Corporation, lm2738xsdx Datasheet - Page 14

• Manufacturer Part Number: lm2738xsdx
• Description: Lm2738 550khz/1.6mhz 1.5a Step-down Dc-dc Switching Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
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
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
package to silicon junction.
R
junction.
In this data sheet we will use R
to measure top case temperature with a small thermocouple
attached to the top case.
R
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:
Therefore:
From the previous example:
The second method can give a very accurate silicon junction
temperature.
The first step is to determine R
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
stages of the design one may calculate the R
the PCB circuit into a thermal chamber. Raise the ambient
temperature in the given working application until the circuit
θJC
ΦJC
ΦJC
θJA
for any application can be characterized during the early
is the thermal impedance from all six sides of an IC
is approximately 30°C/Watt for the 8-pin LLP package
is the thermal impedance from top case to the silicon
θJA
. The type and number of thermal vias can also
T
T
j
j
T
= (R
= 30°C/W x 0.207W + T
j
= (R
ΦJC
ΦJC
x P
x P
INTERNAL
ΦJC
LOSS
θJA
so that it allows the user
) + T
of the application. The
) + T
C
C
C
θJA
by placing
14
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
Once this is determined, the maximum ambient temperature
allowed for a desired junction temperature can be found.
An example of calculating R
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:
If the junction temperature was to be kept below 125°C, then
the ambient temperature could not go above 109°C
LLP Package
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
for the application can be reduced.
125°C - (102°C/W x 207mW) = 104°C
FIGURE 8. Internal LLP Connection
T
j
P
- (R
INTERNAL
θJA
x P
θJA
= 207mW
LOSS
for an application using the
) = T
θJA
can be determined.
A
30049174
θJA