DRV8711_15 Datasheet, PDF (31/44 Pages) Texas Instruments – DRV8711 Stepper Motor Controller IC

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DRV8711
SLVSC40E – JUNE 2013 – REVISED MARCH 2015
8.2.2 Detailed Design Procedure
8.2.2.1 Stepper Motor Speed
The first step in configuring the DRV8711 requires the desired motor speed and microstepping level. If the target
application requires a constant speed, then a square wave with frequency ƒ_step must be applied to the STEP pin.
If the target motor start-up speed is too high, the motor will not spin. Make sure that the motor can support the
target speed or implement an acceleration profile to bring the motor up to speed.
For a desired motor speed (V), microstepping level (n_m), and motor full step angle (θ_step),
f_step
(µsteps/second) =
v
æ
çè
rotations
minute
60
æ
çè
ö
÷ø
×
360
æ
çè
°
rotation
ö
÷ø
×
n_m
æ
çè
µsteps
step
seconds ö
minute ^÷ø
×
θ_step
æ
çè
°
step
ö
÷ø
ö
÷ø
(4)
f_step (µsteps/second)
=
120
æ
çè
rotations
minute
ö
÷ø
´
360
æ
çè
°
rotation
ö
÷ø
´
8
æ
çè
µsteps
step
ö
÷ø
60
æ
çè
seconds
minute
ö
÷ø
´
1.8
æ
çè
°
step
ö
÷ø
(5)
θ_step can be found in the stepper motor data sheet or written on the motor itself.
For the DRV8711, the microstepping level is set by the MODE bits in the CTRL register. Higher microstepping
will mean a smoother motor motion and less audible noise, but will increase switching losses and require a
higher f_step to achieve the same motor speed.
8.2.2.2 Current Regulation
In a stepper motor, the set full-scale current (I_FS) is the maximum current driven through either winding. For the
DRV8711, this quantity will depend on the analog voltage, the programmed torque and gain values, and the
sense resistor value (R_SENSE). During stepping, IFS defines the current chopping threshold (I_TRIP) for the
maximum current step. The gain of DRV8711 is set for 5 V/V.
I_FS(A ) =
2.75 (V)´ TORQUE
256 ´ ISGAIN ´ R_SENSE (W)
(6)
To achieve I_FS = 1.25 A with R_SENSE of 0.2 Ω with a gain of 5, TORQUE should be set to 116(dec).
8.2.2.3 Decay Modes
The DRV8711 supports three different decay modes: slow decay, fast decay, and mixed decay. The DRV8711
also supports automatic mixed decay mode, which minimizes current ripple. The current through the motor
windings is regulated using programmable settings for blanking, decay and off time. This means that after any
drive phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8711 will place
the winding in the programmed decay modes until the cycle has expired. Afterward, a new drive phase starts.
The blanking time T_BLANK defines the minimum drive time for the current chopping. I_TRIP is ignored during T_BLANK,
so the winding current may overshoot the trip level.
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