SLVSCY9B August 2015 – July 2016 DRV8871
Power dissipation in the DRV8871 device is dominated by the power dissipated in the output FET resistance, RDS(on). Use the equation in the Drive Current section to calculate the estimated average power dissipation when driving a load.
Note that at startup, the current is much higher than normal running current; this peak current and its duration must be also be considered.
The maximum amount of power that can be dissipated in the device is dependent on ambient temperature and heatsinking.
RDS(on) increases with temperature, so as the device heats, the power dissipation increases. This fact must be taken into consideration when sizing the heatsink.
The power dissipation of the DRV8871 device is a function of RMS motor current and the FET resistance (RDS(ON)) of each output.
For this example, the ambient temperature is 58°C, and the junction temperature reaches 80°C. At 58°C, the sum of RDS(ON) is about 0.72 Ω. With an example motor current of 0.8 A, the dissipated power in the form of heat will be 0.8 A2 × 0.72 Ω = 0.46 W.
The temperature that the DRV8871 device reaches depends on the thermal resistance to the air and PCB. It is important to solder the device PowerPAD to the PCB ground plane, with vias to the top and bottom board layers, in order dissipate heat into the PCB and reduce the device temperature. In the example used here, the DRV8871 device had an effective thermal resistance RθJA of 48°C/W, and: