Datasheet

R
T
JA

=
165° - T
A
P
INTERNAL
R
)
JC

=
T
J
- T
C
P
INTERNAL
LM26420
www.ti.com SNVS579F –FEBRUARY 2009–REVISED MARCH 2013
The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can
greatly 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. Five to eight thermal vias should be placed under the exposed pad to the ground plane if the
WQFN package is used. Up to 12 thermal vias should be used in the TSSOP-20 package for optimum heat
transfer from the device to the ground plane.
Thermal impedance also depends on the thermal properties of the application's operating conditions (V
IN
, V
OUT
,
I
OUT
etc), and the surrounding circuitry.
Method 1: Silicon Junction Temperature Determination
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 20°C/Watt for the 16-pin WQFN 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:
(38)
Therefore:
T
j
= (R
ΦJC
x P
INTERNAL
) + T
C
(39)
From the previous example:
T
j
= 20°C/W x 0.304W + T
C
(40)
Method 2: Thermal Shutdown Temperature Determination
The second method, although more complicated, can give a very accurate silicon junction temperature.
The first step is to determine R
θJA
of the application. The LM26420 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 FETs stop 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.
(41)
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 LM26420 WQFN demonstration board is shown
below.
The four layer PCB is constructed using FR4 with 1 oz copper traces. The copper ground plane is on the bottom
layer. The ground plane is accessed by eight 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 152°C, and at that temperature, the device went
into thermal shutdown.
From the previous example:
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