DRV8824_14 Datasheet, PDF (20/36 Pages) Texas Instruments – DRV8824 Stepper Motor Controller IC

DRV8824
SLVSA06I – OCTOBER 2009 – REVISED JUNE 2014
www.ti.com
9.2.2 Detailed Design Procedure
9.2.2.1 Stepper Motor Speed
The first step in configuring the DRV8824 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 will not spin. Make sure that the motor can support the target
speed.
For a desired motor speed (v), microstepping level (n_m), and motor full step angle (θ_step),
f_step(steps/second)
=
v_¾ (rpm)· n_m(steps)· 6
q_step(°/step)
(3)
θ_step can be found in the stepper motor datasheet or written on the motor itself.
For the DRV8824, the microstepping level is set by the USM pins and can be any of the settings in Table 1.
Higher microstepping will mean a smoother motor motion and less audible noise, but will increase switching
losses and require a higher ƒ_step to achieve the same motor speed.
9.2.2.2 Current Regulation
In a stepper motor, the full-scale current (I_FS) is the maximum current driven through either winding. This quantity
will depend on the VREF analog voltage and the sense resistor value (R_SENSE). During stepping, I_FS defines the
current chopping threshold (I_TRIP) for the maximum current step.
I_FS(A) = _A¾ _v^V_·^RRE_SE^F_NS⁽_E^V(_W⁾ ₎ = ₅¾ ^V_· ^RRE_SEN^F_SE^((V_W⁾⁾
(4)
I_FS is set by a comparator which compares the voltage across R_SENSE to a reference voltage. There is a current
sense amplifier built in with programmable gain through ISGAIN. Note that I_FS must also follow Equation 5 in
order to avoid saturating the motor. VM is the motor supply voltage and R_L is the motor winding resistance.
I_FS(A) < _RL¾ _{(W) + 2 · RDS}^V_(O^M_N)((W^V)) _{+ RSENSE(W)}
(5)
9.2.2.3 Decay Modes
The DRV8824 supports three different decay modes: slow decay, fast decay, and mixed decay. The current
through the motor windings is regulated using a fixed-frequency PWM scheme. This means that after any drive
phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8824 will place the
winding in one of the three 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_TRIP is ignored during t_BLANK,
so the winding current may overshoot the trip level.
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