DRV8846_15 Datasheet, PDF (24/35 Pages) Texas Instruments – DRV8846 Dual H-Bridge Stepper Motor Driver

DRV8846
SLLSEK2 – JUNE 2014
www.ti.com
9.2.2 Detailed Design Procedure
9.2.2.1 Stepper Motor Speed
The first step in configuring the DRV8846 requires the desired motor speed and stepping level. The DRV8846
can support from full step to 1/32 step mode.
If the target motor speed is too high, the motor will not spin. Make sure that the motor can support the target
speed.
For a desired motor speed (v), microstepping level (nm), and motor full step angle (θ_step),
¦_step
VWHSV
V

v(rpm) K n_m(steps)
6_step (G / step)
K
6
(2)
θ_step can be found in the stepper motor data sheet or often written on the motor itself.
For DRV8846, the microstepping levels are set by the M0/M1 pins and can be any of the settings in Table 2.
Higher microstepping means a smoother motor motion and less audible noise, but increases the switching losses
and requires a higher ƒ_step to achieve the same motor speed.
9.2.2.2 Current Regulation
The chopping current (I_CHOP) is the maximum current driven through either winding. This quantity will depend on
the sense resistor value (R_XISEN).
I_CHOP

VREF
6.6 K R_ISENSE
K TORQUE
(3)
I_CHOP is set by a comparator which compares the voltage across R_XISEN to a reference voltage. Note that I_CHOP
must follow Equation 4 to avoid saturating the motor.
I_CHOP (A)

R_L (:)

VM (V)
2 u R_DS(ON) (:)
 R_SENSE (:)
where
• VM is the motor supply voltage.
• R_L is the motor winding resistance.
(4)
9.2.2.3 Decay Modes
The DRV8846 supports four different decay modes: slow decay, fast decay, mixed decay, and adaptive decay.
The first selection to try is the adaptive decay mode, which adjusts the decay mode automatically to improve
current regulation. The current through the motor windings is regulated using a fixed-off-time PWM scheme. This
means that after any drive phase, when a motor has reached the current chopping threshold (I_CHOP), the
DRV8846 places the motor in one of the four decay modes until the PWM cycle has expired. Afterward, a new
drive phase starts.
The blanking time, t_BLANK, defines the minimum drive time for the current chopping. I_CHOP is ignored during t_BLANK,
so the winding current may overshoot the trip level during this blanking period.
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