DRV8811_15 Datasheet, PDF (23/32 Pages) Texas Instruments – DRV8811 Stepper Motor Controller IC

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Thermal Information (continued)
DRV8811
SLVS865I – SEPTEMBER 2008 – REVISED JANUARY 2015
θ
JA
Temperature - °C
Figure 17. Typical R_DS(ON)
vs
Temperature
Figure 18. Thermal Resistance
vs
Copper Area
11.4 Power Dissipation
Power dissipation in the DRV8811 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)
(8)
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 DRV8811 is dependent on ambient temperature
and heatsinking. Figure 15 and Figure 16 show how the maximum allowable power dissipation varies according
to temperature and PCB construction. Figure 15 shows data for a JEDEC low-K board, 2-layers with 2-oz.
copper,
76 mm x 114 mm x 1.6 mm thick, with either no backside copper or a 24 cm² copper area on the backside.
Similarly, Figure 16 shows data for a JEDEC high-K board, 4 layers with 1-oz. copper, 76 mm x 114 mm x 1.6
mm thick, and a solid internal ground plane. In this case, the PowerPAD™ is tied to the ground plane using
thermal vias, and no additional outer layer copper.
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. Refer to Figure 17.
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