DRV8870: Power disipation

Hello Naoki,

Base on datasheet, the Rdson is 0.666ohm at 58° ambient temperature.

In P16, the junction temperature can reach to 80°, the Rdson is about 0.72ohm at 80 °.

Thanks.

Hello,

Thank you for your reply.
If I know the junction temperature reaches 80 degree in advance,
I don't calculate the power dissipation and the junction temperature, right?

Could you calculate the junction temperature at Ta=30 degree and IRMS=2A?

Best Regards,
Naoki Aoyama

For question 1, the answer is yes. you can calculate the power dissipation at the ambient temperature, not junction temperature.

For question 2, yes I can calculate the junction temperature.

Rdson = 0.565ohm*1.05=0.593ohm at Ta=30 °C

Power = I^2*Rdson=(2A)^2*0.593=2.373W

TJ=Ta+(P*Rja)=30°C+2.373W * 41.1 °/W=127.53°C

These value are calculated base on datasheet. In fact, the heatsinking should be take into consideration.

Below link is about thermal analysis, you can reference.

http://www.ti.com/adc/docs/midlevel.tsp?contentId=105986&DCMP=PCB_Thermal_Calc&HQS=NotApplicable+IL+thermal

Thanks.

Hello,
Thank you for your reply.
I calculate the junction temperature at Ta=58 degree and IRMS=0.8A , same way as you do.

Rdson= 0.565ohm*1.18 = 0.6667 ohm at Ta=58 degree
(At this point it is not already 0.72ohm)

Power= 0.8A*0.8A*0.6667 = 0.427W
TJ= 58+0.427W*41.1/W = 58+17.53= 75.53 degree
Even if the theta jc as 48/W on the datasheet, it is 78 degree.

I want to reconfirm that does the X axis of the Figure 1 show ambient temperature, not junction temperature?

Regards,
Naoki Aoyama