The amount of heat that an LDO linear regulator generates is directly proportional to the amount of power the regulator dissipates during operation. All integrated circuits have a maximum allowable junction temperature (T_Jmax) above which normal operation is not assured. A system designer must design the operating environment so that the operating junction temperature (T_J) does not exceed the maximum junction temperature (T_Jmax). The two main environmental variables that a designer can use to improve thermal performance are air flow and external heatsinks. The purpose of this information is to aid the designer in determining the proper operating environment for a linear regulator that is operating at a specific power level.

In general, the maximum expected power (P_Dmax) consumed by a linear regulator is computed as shown in Equation 6:

where:

• V_IN(avg) is the average input voltage
• V_OUT(avg) is the average output voltage
• I_OUT(avg) is the average output current
• I_Q is the quiescent current

For most TI LDO regulators, the quiescent current is insignificant compared to the average output current; therefore, the term V_IN(avg) x I_Q can be neglected. The operating junction temperature is computed by adding the ambient temperature (T_A) and the increase in temperature due to the regulator's power dissipation. The temperature rise is computed by multiplying the maximum expected power dissipation by the sum of the thermal resistances between the junction and the case (R_ΘJC), the case to heatsink (R_ΘCS), and the heatsink to ambient (R_ΘSA). Thermal resistances are measures of how effectively an object dissipates heat. Typically, the larger the device, the more surface area available for power dissipation and the lower the object's thermal resistance.

Figure 8-8 illustrates these thermal resistances for a SOT223 package mounted in a JEDEC low-K board.

Equation 7 summarizes the computation:

The R_ΘJC is specific to each regulator as determined by the package, lead frame, and die size provided in the data sheet of the regulator. The R_ΘSA is a function of the type and size of heatsink. For example, black body radiator type heatsinks can have R_ΘCS values ranging from 5°C/W for very large heatsinks to 50°C/W for very small heatsinks. The R_ΘCS is a function of how the package is attached to the heatsink. For example, if a thermal compound is used to attach a heatsink to a SOT223 package, R_ΘCS of 1°C/W is reasonable.

Even if no external black body radiator type heatsink is attached to the package, the board on which the regulator is mounted provides some heatsinking through the pin solder connections. Some packages, like the DDPAK and SOT223 packages, use a copper plane underneath the package or the circuit board ground plane for additional heatsinking to improve the thermal performance. Computer-aided thermal modeling can be used to compute very accurate approximations of the thermal performance of an integrated circuit in different operating environments (for example, different types of circuit boards, different types and sizes of heatsinks, different air flows, and more). Using these models, the three thermal resistances can be combined into one thermal resistance between junction and ambient (R_ΘJA). This R_ΘJA is valid only for the specific operating environment used in the computer model.

Equation 7 simplifies into Equation 8:

Rearranging Equation 8 gives Equation 9:

Using Equation 9 and the computer model generated curves shown in Figure 8-9, a designer can quickly compute the required heatsink thermal resistance/board area for a given ambient temperature, power dissipation, and operating environment.