Die Attach

Material

Mold Compound

Gold Wire

Exposed Die

Attach Pad

Die

Cu

Exposed

Contact

JA

165 C

144 C

R

111 C / W

189 mW

q

°

-

°

=

=

°

JA

INTERNAL

165 C

Ta

R

P

q

°

-

=

LM2831

SNVS422D – AUGUST 2006 – REVISED SEPTEMBER 2015

www.ti.com

The second method can give a very accurate silicon junction temperature.

The first step is to determine RθJA of the application. The LM2831 has overtemperature 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 RθJA for any application can be characterized during the early stages of the design one

may calculate the RθJA 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 RθJA can be

determined.

(37)

Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be

found.

An example of calculating RθJA 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 cm × 3 cm. 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:

PINTERNAL = 189 mW

(38)

(39)

If the junction temperature was to be kept below 125°C, then the ambient temperature could not go above 109°C

Tj - (RθJA × PLOSS) = TA

(40)

125°C - (111°C/W × 189 mW) = 104°C

(41)

10.1.3 WSON Package

Figure 32. Internal WSON Connection

For certain high power applications, the PCB land may be modified to a "dog bone" shape (see Figure 33). By

increasing the size of ground plane, and adding thermal vias, the RθJA for the application can be reduced.

28

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