DRV8886AT Datasheet, PDF (30/40 Pages) Texas Instruments – 2-A Stepper Motor Driver With Integrated Current Sense and AutoTune Technology

DRV8886AT
SLVSDO1 – JANUARY 2017
www.ti.com
8.2.2 Detailed Design Procedure
8.2.2.1 Stepper Motor Speed
The first step in configuring the DRV8886AT device 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 speed is too high, the motor does not spin. Make sure that the motor can support the target
speed.
Use Equation 3 to calculate ƒ_step for a desired motor speed (v), microstepping level (n_m), and motor full step
angle (θ_step)
¦_step VWHSV V
v (rpm) u 360 (q / rot)
T_step (q / step) u n_m (steps / microstep) u 60 (s / min)
(3)
The value of θ_step can be found in the stepper motor data sheet, or written on the motor.
For the DRV8886AT device, the microstepping level is set by the Mx pins and can be any of the settings listed in
Table 11. Higher microstepping results in a smoother motor motion and less audible noise, but increases
switching losses and requires a higher ƒ_step to achieve the same motor speed.
Table 11. Microstepping Indexer Settings
M1
M0
STEP MODE
0
0
Full step (2-phase excitation) with 71% current
0
1
1/16 step
1
0
1/2 step
1
1
1/4 step
0
Z
1/8 step
1
Z
Non-circular 1/2 step
For example, the motor is 1.8°/step for a target of 120 rpm at 1/8 microstep mode.
¦_step VWHSV V
120 rpm u 360q / rot
1.8q / step u1/ 8 steps / microstep u 60 s / min
N+]
(4)
8.2.2.2 Current Regulation
In a stepper motor, the full-scale current (I_FS) is the maximum current driven through either winding. This quantity
depends on the RREF resistor and the TRQ setting. During stepping, I_FS defines the current chopping threshold
(I_TRIP) for the maximum current step.
I_FS (A)
A_RREF (kA:)
RREF (k:)
30 (kA:) u TRQ%
RREF (k:)
(5)
NOTE
The I_FS current must also follow Equation 6 to avoid saturating the motor. VM is the motor
supply voltage, and R_L is the motor winding resistance.
I_FS (A)
VM (V)
R_L (:) 2 u R_DS(ON) (:)
(6)
8.2.2.3 Decay Modes
The DRV8886AT device supports three different decay modes: slow decay, slow-mixed and all mixed decay. The
current through the motor windings is regulated using an adjustable fixed-time-off scheme which means that after
any drive phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8886AT
places the winding in one of the three decay modes for t_OFF. After t_OFF, a new drive phase starts.
The blanking time, t_BLANK, defines the minimum drive time for the PWM current chopping. I_TRIP is ignored during
t_BLANK, so the winding current may overshoot the trip level.
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