10.3.2 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-Q1 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 junction temperature has decreased to approximately 150°C, the device starts 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 is 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.

Equation 40.

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-Q1 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 cm × 3 cm. 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:

Equation 42.

If the junction temperature was to be kept below 125°C, then the ambient temperature could not go above 112°C.