DRV8821_16 Datasheet, PDF (22/32 Pages) Texas Instruments – Dual Stepper Motor Controller and Driver

DRV8821
SLVS912J – JANUARY 2009 – REVISED JANUARY 2016
www.ti.com
10.3 Thermal Considerations
The DRV8821 has thermal shutdown (TSD) as described Thermal Shutdown (TSD). 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.
10.3.1 Power Dissipation
Power dissipation in the DRV8821 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 Equation 5.
P = 4 R (I )²
TOT
· _DS(ON) · _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). Remember that the DRV8821 has
two stepper motor drivers, so the power dissipation of each must be added together to determine the total device
power dissipation.
The maximum amount of power that can be dissipated in the DRV8821 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.
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 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. Figure 19 shows
thermal resistance vs. copper plane area for both a single-sided PCB with 2-oz copper heatsink area, and a 4-
layer PCB with 1-oz copper and a solid ground plane. Both PCBs are 76 mm x 114 mm, and 1.6 mm thick. It can
be seen that the heatsink effectiveness increases rapidly to about 20 cm², then levels off somewhat for larger
areas.
Six pins on the center of each side of the package are also connected to the device ground. A copper area can
be used on the PCB that connects to the PowerPAD™ as well as to all the ground pins on each side of the
device. This is especially useful for single-layer PCB designs.
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