It is good design practice to estimate power dissipation and maximum expected junction temperature of the TPS20xxC and TPS20xxC-2. The system designer can control choices of package, proximity to other power dissipating devices, and printed-circuit board (PCB) design based on these calculations. These have a direct influence on maximum junction temperature. Other factors, such as airflow and maximum ambient temperature, are often determined by system considerations. It is important to remember that these calculations do not include the effects of adjacent heat sources, and enhanced or restricted air flow.
Addition of extra PCB copper area around these devices is recommended to reduce the thermal impedance and maintain the junction temperature as low as practical. The lower junction temperatures achieved by soldering the pad improve the efficiency and reliability of both TPS20xxC and TPS20xxC-2 parts and the system. The following examples were used to determine the θJACustom thermal impedances noted in Thermal Information: SOT-23 and Thermal Information: MSOP-PowerPAD. They were based on use of the JEDEC high-k circuit board construction (2 signal and 2 plane) with 4, 1-oz. copper weight, layers.
While TI recommends that the DGN package PAD be soldered to circuit board copper fill and vias for low thermal impedance, there may be cases where this is not desired. For example, use of routing area under the IC. Some devices are available in packages without the PowerPad (DGK) specifically for this purpose. The θJA for the DGN package with the pad not soldered and no extra copper, is approximately 141°C/W for 0.5-A and 1-A rated parts, and 139°C/W for the 1.5-A and 2-A rated parts. The θJA for the DGK mounted per Figure 45 is 110.3°C/W. These values may be used in Equation 1 to determine the maximum junction temperature.
As shown in Equation 1, the following procedure requires iteration because power loss is due to the internal MOSFET I2 × RDS(ON), and RDS(ON) is a function of the junction temperature. As an initial estimate, use the RDS(ON) at 125°C from the Typical Characteristics, and the preferred package thermal resistance for the preferred board construction from the Thermal Information: SOT-23 table.
If the calculated TJ is substantially different from the original assumption, estimate a new value of RDS(ON) using the typical characteristic plot and recalculate.
If the resulting TJ is not less than 125°C, try a PCB construction or a package with lower θJA.