DRV8814_16 Datasheet, PDF (17/26 Pages) Texas Instruments – DC Motor Driver IC

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DRV8814
SLVSAB9E – MAY 2010 – REVISED NOVEMBER 2015
Thermal Considerations (continued)
10.3.2 Power Dissipation
Power dissipation in the DRV8814 is dominated by the power dissipated in the output FET resistance, or
RDS(ON). Average power dissipation when running a stepper motor can be roughly estimated by Equation 3.
2
P_TOT 4 u R_DS(ON) u I_OUT(RMS)
where
• P_TOT is the total power dissipation
• R_DS(ON) is the resistance of each FET
• I_OUT(RMS) is the RMS output current being applied to each winding
• I_OUT(RMS) is equal to approximately 0.7 × the full-scale output current setting
(3)
The factor of 4 comes from the fact that there are two motor windings, and at any instant two FETs are
conducting winding current for each winding (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.
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.
10.3.3 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 PowerPAD™ Thermally Enhanced
Package, SLMA002, and 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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