DRV8844_15 Datasheet, PDF (16/25 Pages) Texas Instruments – DRV8844 Quad ½-H-Bridge Driver IC

DRV8844
SLVSBA2C – JULY 2012 – REVISED MAY 2015
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Thermal Considerations (continued)
11.3.1 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 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, refer to TI application report SLMA002, PowerPAD™ Thermally
Enhanced Package and TI application brief SLMA004, PowerPAD™ Made Easy, available at www.ti.com.
In general, the more copper area that can be provided, the more power can be dissipated.
11.4 Power Dissipation
Power dissipation in the DRV8844 is dominated by the power dissipated in the output FET resistance, or R_DS(ON).
Average power dissipation of each H-bridge when running a DC motor can be roughly estimated by Equation 1.
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
(1)
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 total device dissipation will be the power dissipated in each of the two H-bridges added together.
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.
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