10.3.5.2 Example From Previous Calculations

P_Dissipation = 475 mW

T_A at Shutdown = 139°C

T_Case-Top at Shutdown = 155°C

Equation 61.

R_θJA WSON = 55°C/W

R_ψJC WSON = 21°C/W

WSON typical application produces R_θJA numbers in the range of 50°C/W to 65°C/W, and R_ψJC varies from 18°C/W to 28°C/W. These values are for PCBs with two and four layer boards with 0.5-oz copper, and 4 to 6 thermal vias to bottom side ground plane under the DAP.

For 5-pin SOT-23 package typical applications, R_θJA numbers range from 80°C/W to 110°C/W, and R_ψJC varies from 50°C/W to 65°C/W. These values are for PCBs with 2- and 4-layer boards with 0.5-oz copper, with 2 to 4 thermal vias from GND pin to bottom layer.

For typical thermal impedances and an ambient temperature maximum of 75°C: if the design requires more than 400 mW internal to the LM2735-Q1 be dissipated, or there is 750 mW of total power loss in the application, TI recommends using the 6-pin WSON package.

Figure 48. R_θJA vs Internal Dissipation for the WSON