DRV8886AT Datasheet, PDF (17/40 Pages) Texas Instruments – 2-A Stepper Motor Driver With Integrated Current Sense and AutoTune Technology

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DRV8886AT
SLVSDO1 – JANUARY 2017
7.3.4 Current Regulation
The current through the motor windings is regulated by an adjustable, fixed-off-time PWM current-regulation
circuit. When an H-bridge is enabled, current rises through the winding at a rate dependent on the DC voltage,
inductance of the winding, and the magnitude of the back EMF present. When the current hits the current
regulation threshold, the bridge enters a decay mode for a fixed 20 μs, period of time to decrease the current.
After the off time expires, the bridge is re-enabled, starting another PWM cycle.
ITRIP
tBLANK
tDRIVE
tOFF
Figure 14. Current Chopping Waveform
The PWM regulation current is set by a comparator which monitors the voltage across the current sense
MOSFETs in parallel with the low-side power MOSFETs. The current sense MOSFETs are biased with a
reference current that is the output of a current-mode sine-weighted DAC whose full-scale reference current is
set by the current through the RREF pin. An external resistor is placed from the RREF pin to GND to set the
reference current. In addition, the TRQ pin can further scale the reference current.
Use Equation 1 to calculate the full-scale regulation current.
I_FS (A)
A_RREF (kA:) _{u TRQ (%)}
RREF (k :)
30 (kA:) _{u TRQ (%)}
RREF (k:)
(1)
For example, if a 30-kΩ resistor is connected to the RREF pin, the full-scale regulation current is 1 A (TRQ at
100%).
The TRQ pin is the input to a DAC used to scale the output current. Table 5 lists the current scalar value for
different inputs.
Table 5. Torque DAC Settings
TRQ
0
Z
1
CURRENT SCALAR (TRQ)
100%
75%
50%
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