DRV8711_15 Datasheet, PDF (17/44 Pages) Texas Instruments – DRV8711 Stepper Motor Controller IC

www.ti.com
DRV8711
SLVSC40E – JUNE 2013 – REVISED MARCH 2015
Auto-mixed decay mode samples the current level at the end of the blanking time, and if the current is above the
Itrip threshold, immediately changes the H-bridge to fast decay. During fast decay, the (negative) current is
monitored, and when it falls below the Itrip threshold (and another blanking time has passed), the bridge is
switched to slow decay. Once the fixed off time expires, a new cycle is started.
If the bridge is turned on and at the end of TBLANK the current is below the Itrip threshold, the bridge remains on
until the current reaches Itrip. Then slow decay is entered for the fixed off time, and a new cycle begins.
See Figure 10 and Figure 11.
The upper waveform shows the behavior if I < Itrip at the end of tBLANK. At slow motor speeds, where back
EMF is not significant, the current increase during the ON phase is the same magnitude as the current decrease
in fast decay, because both times are controlled by tBLANK, and the rate of change is the same (full VM is
applied to the load inductance in both cases, but in opposite directions). In this case, the current will gradually be
driven down until the peak current is just hitting Itrip at the end of the blanking time, after which some cycles will
be slow decay, and some will be mixed decay.
I below Itrip
after tBLANK
Itrip
tON
tBLANK
tOFF
tON
tBLANK
tOFF
I above Itrip
after tBLANK
I < Itrip
tON
At Itrip and after
tBLANK, slow decay
tOFF
tBLANK tBLANK
Itrip
tON
tBLANK tBLANK
tOFF
I > Itrip, start
fast decay
When I < Itrip in fast decay
and tBLANK expires, change
to slow decay
Figure 10. I < Itrip at the End of tBLANK
On
Fast
Decay
Slow
Decay
If the Itrip level changes during a PWM cycle (in response to a step command to the indexer), the current cycle is
immediately terminated, and a new cycle is begun. Refer to the drawing below.
If the Itrip level has increased, the H-bridge will immediately turn on; if the Itrip level has decreased, fast decay
mode is begun immediately. The top waveform shows what happens when the Itrip threshold decreases during a
PWM cycle. The lower Itrip level results in the current being above the Itrip threshold at the end of tBLANK on
the following cycle. Fast decay is entered until the current is driven below the Itrip threshold.
Copyright © 2013–2015, Texas Instruments Incorporated
Product Folder Links: DRV8711
Submit Documentation Feedback
17