All semiconductor devices dissipate heat to some degree. There are three paths through which heat is
dissipated: package top to air, package bottom to board, and package leads to board. The dominant path for
heat dissipation in the MAX11500, MAX11501/MAX11502, MAX11504/MAX11505 video filters is through the
leads to the board.
Thermal resistance is a value for the thermal transfer properties of a material. It is referred to using the Greek
letter Q (theta), the first letter of the Greek word for heat, QerμoV (thermos).
The thermal resistance from the junction to case is, therefore, known as QJC. The thermal resistance from case
to ambient is known as QCA, and the thermal resistance from junction to ambient is known as QJA. Thermal
models are analogous to electrical models (Figure 1), and capacitance plays a part in both. Temperature is
analogous to voltage, and heat is analogous to current.
In the electrical world, if current flows through a resistance, a voltage differential will be generated. Similarly, if
heat flows through a thermal resistance, a temperature differential will be generated. In addition, if there is a
short transient heat flow, the thermal capacitances will shunt the heat and there will be only a small temperature
rise. Objects need time to heat up.