DM385: How do get the DM385 CPU temperature ?

Raj,

See also the below link:

www.ti.com/.../spra953c.pdf

Conclusions

1) No Heat Sink - In most cases, the majority of thermal relief will occur through PCB conduction (vs convection to air). A reasonable way to estimate junction temp based on conduction is using equation 9 and PSIjb from the data sheet. It will estimate junction temp relative to measured PCB temp. So for example on DM8148 the PSIjb with no air flow is 3.44 deg/W so at 2 Watts the junction will be aprox 6.88 degrees above PCB temp. This applies when there is no heat sink

Tjunction = Tpcb + PSIjb x Power

2) With Heat Sink - A reasonable way to estimate junction temp when using a heat sink is using equation 6. It computes temp rise wrt to ambient and it does take into account conduction effects since it involves ThetaJA.

For example if published ThetaJA is 11.67 deg/W , ThetaJc is 0.39 deg/W and if heat sink Theta SA is 26.9 deg/W, then disregarding Theta cs, we find a temp rise of 8.17 deg/W above ambient

3) Note that If you disregard ThetaJcs, the calculation is just paralleling the thermal resistance of JA and the heat sink. That is, the heat sink won’t help much if ThetaSA is much bigger than IC ThetaJA because the parallel resistance is dominated by the smaller term. For example, in the example calculations listed, the heat sink resistance was 2.3X that of package ThetaJA. Therefore it resulted in about only 30% improvement in thermal relief.

4) Theta-cs is calculated based on the geometry of the thermal interface (area and thickness), and the material used. Thermal greases have typical thermal conductivities of ~0.8W/m-C. You could use your package geometry, the thickness assumed for the TIM, and Equation 7 to calculate Theta-cs.

Thanks,

Pavel