DRV8818_15 Datasheet, PDF (12/31 Pages) Texas Instruments – DRV8818 Stepper Motor Controller IC

DRV8818
SLVSAX9D – SEPTEMBER 2011 – REVISED JANUARY 2015
Feature Description (continued)
VM
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1
xOUT1
3
2
xOUT2
1 Drive current
2 Fast decay (reverse)
3 Slow decay (brake)
Figure 7. Decay Mode
The DRV8818 also supports a mixed decay mode. Mixed decay mode begins as fast decay, but after a period of
time switches to slow decay mode for the remainder of the fixed off time.
Fast and mixed decay modes are only active if the current through the winding is decreasing; if the current is
increasing, then slow decay is always used.
Which decay mode is used is selected by the voltage on the DECAY pin. If the voltage is greater than 0.6 x V_CC,
slow decay mode is always used. If DECAY is less than 0.21 x V_CC, the device operates in fast decay mode
when the current through the winding is decreasing. If the voltage is between these levels, mixed decay mode is
enabled.
In mixed decay mode, the voltage on the DECAY pin sets the point in the cycle that the change to slow decay
mode occurs. This time can be approximated by:
t
FD
=
R
·C
·
In
æ
çè
0.6
V
·
V
CC
ö
÷ø
DECAY
(4)
Mixed decay mode is only used while the current though the winding is decreasing; slow decay is used while the
current is increasing.
Operation of the blanking, fixed off time, and mixed decay mode is illustrated in Figure 8.
12
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