LMZ31530: Thermal Performance

Hi Kevin,

Yes, case temp will be related to the power loss and thermal resistances.

The customer design appears to be set to fsw = 650kHz, but Figure 3 in the datasheet should give a good indication of expected power loss with the Vin=12V Vout=0.9V Fsw = 500kHz power dissipation curve. It looks like there will be about 4.9W of dissipation at 25A

With this power dissipation and the thermal resistances in the Thermal Information table and assuming we have an ambient temperature of, say, 30°C, and natural convection conditions, we can estimate that junction temperature will be about Tj = Ta + Pd*ThetaJA --> 30°C + 4.9W*8.6°C/W = 72.1°C.

Case temperature will be lower than this by Pd*PsiJT or 4.9W*1.6°C/W = 7.84°C. In other words, estimated case temperature will be around 64.3°C.

I hope this helps,
Kris