11.1.1.1 Heatsink Requirements

The LM317-N regulators have internal thermal shutdown to protect the device from over-heating. Under all operating conditions, the junction temperature of the LM317-N must not exceed the rated maximum junction temperature (T_J) of 150°C for the LM117, or 125°C for the LM317-N. A heatsink may be required depending on the maximum device power dissipation and the maximum ambient temperature of the application. To determine if a heatsink is needed, the power dissipated by the regulator, P_D, must be calculate with Equation 3:

Figure 36 shows the voltage and currents which are present in the circuit.

The next parameter which must be calculated is the maximum allowable temperature rise, T_R(MAX) in Equation 4:

where T_J(MAX) is the maximum allowable junction temperature (150°C for the LM117, or 125°C for the LM317-N), and T_A(MAX) is the maximum ambient temperature that will be encountered in the application.

Using the calculated values for T_R(MAX) and P_D, the maximum allowable value for the junction-to-ambient thermal resistance (R_θJA) can be calculated with Equation 5:

Figure 36. Power Dissipation Diagram

If the calculated maximum allowable thermal resistance is higher than the actual package rating, then no additional work is needed. If the calculated maximum allowable thermal resistance is lower than the actual package rating either the power dissipation (P_D) needs to be reduced, the maximum ambient temperature T_A(MAX) needs to be reduced, the thermal resistance (R_θJA) must be lowered by adding a heatsink, or some combination of these.

If a heatsink is needed, the value can be calculated from Equation 6:

When a value for θ_HA is found using the equation shown, a heatsink must be selected that has a value that is less than, or equal to, this number.

The θ_HA rating is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots temperature rise vs power dissipation for the heatsink.