DRV8811 Datasheet, PDF (10/19 Pages) Texas Instruments – STEPPER MOTOR CONTROLLER IC

DRV8811
SLVS865D – SEPTEMBER 2008 – REVISED JULY 2009 ................................................................................................................................................. www.ti.com
VM
1
xOUT1
3
2
xOUT2
1 Drive current
2 Fast decay (reverse)
3 Slow decay (brake)
Figure 2. Decay Mode
The DRV8811 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 always operates in fast decay
mode (when the winding current 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)
Operation of the blanking, fixed off time, and mixed decay mode is illustrated in Figure 3.
10
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