DRV8840_15 Datasheet, PDF (19/28 Pages) Texas Instruments – DC Motor Driver IC

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DRV8840
SLVSAB7D – MAY 2010 – REVISED DECEMBER 2015
10.3 Thermal Considerations
The DRV8840 has thermal shutdown (TSD) as described above. If the die temperature exceeds approximately
150°C, the device will be disabled until the temperature drops to a safe level.
Any tendency of the device to enter TSD is an indication of either excessive power dissipation, insufficient
heatsinking, or too high an ambient temperature.
10.3.1 Power Dissipation
Average power dissipation in the DRV8840 when running a DC motor can be roughly estimated by: Equation 3.
P 2 u R_DS(ON) u I_OUT ²
where
• P is the power dissipation of one H-bridge
• R_DS(ON) is the resistance of each FET
• I_OUT is the RMS output current being applied to each winding.
(3)
I_OUT is equal to the average current drawn by the DC motor. Note that at start-up and fault conditions this current
is much higher than normal running current; these peak currents and their duration also need to be taken into
consideration. The factor of 2 comes from the fact that at any instant two FETs are conducting winding current
(one high-side and one low-side).
The maximum amount of power that can be dissipated in the device is dependent on ambient temperature and
heatsinking.
Note that R_DS(ON) increases with temperature, so as the device heats, the power dissipation increases. This must
be taken into consideration when sizing the heatsink.
10.3.2 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 multilayer PCB with a ground plane,
this can be accomplished by adding a number of vias to connect the thermal pad to the ground plane. On PCBs
without internal planes, 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, see 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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