10.3 Thermal Considerations

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 LM26420 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:

Equation 35.

Thermal impedance from the silicon junction to the ambient air is defined as:

Equation 36.

The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can greatly affect 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 must be placed under the exposed pad to the ground plane if the WQFN package is used. Up to 12 thermal vias must be used in the HTSSOP-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.