DRV8711 Datasheet, PDF (15/34 Pages) Texas Instruments – STEPPER MOTOR CONTROLLER IC

DRV8711
www.ti.com
SLVSC40 – JUNE 2013
The upper waveform shows the behavior if I < Itrip at the end of tBLANK. Note that (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, since 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 4. 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.
Itrip decrease
tON
tBLANK
Itrip
tOFF
tON
tBLANK
tOFF
Decrease in Itrip terminates
cycle, fast decay begins
When I < Itrip in fast decay
change to slow decay
Itrip increase
tON
tBLANK
tON
tBLANK
tOFF
Itrip
Increase in Itrip terminates
cycle, bridge turns on
Figure 5. Itrip Level Changing During a PWM Cycle
On
Fast
Decay
Slow
Decay
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