DRV8818_12 Datasheet, PDF (13/20 Pages) Texas Instruments – STEPPER MOTOR CONTROLLER IC

DRV8818
www.ti.com
SLVSAX9A – SEPTEMBER 2011 – REVISED FEBRUARY 2012
THERMAL INFORMATION
Thermal Protection
The DRV8818 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 thermal shutdown is an indication of either excessive power dissipation,
insufficient heatsinking, or too high an ambient temperature.
Power Dissipation
Power dissipation in the DRV8818 is dominated by the power dissipated in the output FET resistance, or R_DS(ON).
Average power dissipation when running a stepper motor can be roughly estimated by:
P = 4 · R · (I ) _TOT
DS(ON)
2
OUT(RMS)
(5)
where P_TOT is the total power dissipation, R_DS(ON) is the resistance of each FET, and I_OUT(RMS) is the RMS output
current being applied to each winding. I_OUT(RMS) is equal to the approximately 0.7x the full-scale output current
setting. 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 DRV8818 is dependent on ambient temperature
and heatsinking. The thermal dissipation ratings table in the datasheet can be used to estimate the temperature
rise for typical PCB constructions.
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.
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.
Copyright © 2011–2012, Texas Instruments Incorporated
Product Folder Link(s): DRV8818
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