The thermal characteristics of the LM21215 are specified using the parameter θ_JA, which relates the junction temperature to the ambient temperature. Although the value of θ_JA is dependent on many variables, it still can be used to approximate the operating junction temperature of the device.

To obtain an estimate of the device junction temperature, one can use the following relationship:

Equation 22.

and

Equation 23.

where

• T_J is the junction temperature in °C.
• P_IN is the input power in Watts (P_IN = V_IN × I_IN).
• θ_JA is the junction to ambient thermal resistance for the LM21215.
• T_A is the ambient temperature in °C.
• I_OUT is the output load current in A.

It is important to always keep the operating junction temperature (T_J) below 125°C for reliable operation. If the junction temperature exceeds 165°C, the device cycles in and out of thermal shutdown. If thermal shutdown occurs, it is a sign of inadequate heat sinking or excessive power dissipation in the device.

Figure 9-2 provides a better approximation of the θ_JA for a given PCB copper area. The PCB used in this test consisted of four layers: 1-oz. copper was used for the internal layers while the external layers were plated to 2-oz. copper weight. To provide an optimal thermal connection, a 3 × 5 array of 8-mil. vias under the thermal pad were used, and an additional twelve 8-mil. vias under the rest of the device were used to connect the four layers.

Figure 9-2 Thermal Resistance vs PCB Area (4-Layer Board)

Figure 9-3 shows a plot of the maximum ambient temperature vs output current for the typical application circuit shown in Figure 8-1, assuming a θ_JA value of 24°C/W.

Figure 9-3 Maximum Ambient Temperature vs Output Current (0 LFM)