DRV8870_16 Datasheet, PDF (16/25 Pages) Texas Instruments – Brushed DC Motor Driver

DRV8870
SLVSCY8A – AUGUST 2015 – REVISED JANUARY 2016
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Power Dissipation (continued)
The maximum amount of power that can be dissipated in the device is dependent on ambient temperature and
heatsinking.
NOTE
R_DS(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 DRV8870 is a function of RMS motor current and the FET resistance (R_DS(ON)) of
each output.
Power | I_RMS2 u High-side R_DS(ON) Low-side R_DS(ON)
(3)
For this example, the ambient temperature is 58°C, and the junction temperature reaches 80°C. At 58°C, the
sum of R_DS(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 DRV8870 reaches will depend 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 DRV8870
had an effective thermal resistance R_θJA of 48°C/W, and:
T_J T_A (P_D u R_TJA ) 58qC (0.46 W u 48qC/W) 80qC
(4)
10.4.1 Heatsinking
The PowerPAD package uses an exposed pad to remove heat from the device. For proper operation, this pad
must be thermally connected to copper on the PCB to dissipate heat. On a multi-layer PCB with a ground plane,
this connection can be accomplished by adding a number of vias to connect the thermal pad to the ground plane.
On PCBs without internal planes, a copper area can be added on either side of the PCB to dissipate heat. If the
copper area is on the opposite side of the PCB from the device, thermal vias are used to transfer the heat
between top and bottom layers.
For details about how to design the PCB, refer to the TI application report, PowerPAD™ Thermally Enhanced
Package (SLMA002), and the TI application brief, PowerPAD Made Easy™ (SLMA004), available at www.ti.com.
In general, the more copper area that can be provided, the more power can be dissipated.
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