10.3 Thermal Definitions

T_J = Chip junction temperature

T_A = Ambient temperature

R_θJC = Thermal resistance from chip junction to device case

R_θJA = Thermal resistance from chip junction to ambient air

Heat in the LMR10515 due to internal power dissipation is removed through conduction and/or convection.

Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor).

Heat Transfer goes as:

Silicon → package → lead frame → PCB

Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural convection occurs when air currents rise from the hot device to cooler air.

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 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. Four to six thermal vias should be placed under the exposed pad to the ground plane if the WSON package is used.

Thermal impedance also depends on the thermal properties of the application 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 requires the user to know the thermal impedance of the silicon junction to case temperature.

R_θJC is approximately 18°C/Watt for the 6-pin WSON 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:

where T_C is the temperature of the exposed pad and can be measured on the bottom side of the PCB.

Therefore:

From the previous example:

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

The first step is to determine R_θJA of the application. The LMR10515 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 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 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 LMR10515 is shown below.

A sample PCB is placed in an oven with no forced airflow. The ambient temperature was raised to 140°C, and at that temperature, the device went into thermal shutdown.

From the previous example:

Since the junction temperature must be kept below 125°C, then the maximum ambient temperature can be calculated as: