DRV8824_16 Datasheet, PDF (20/35 Pages) Texas Instruments – Stepper Motor Controller IC

DRV8824
SLVSA06J – OCTOBER 2009 – REVISED JULY 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 startup 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),
SPACE
¦_step —VWHSV  VHF RQG
v
§
¨©
rotations
minute
·
¸¹
u
360
§
¨©
q
rotation
·
¸¹
u
n_m
§
¨©
µsteps
step
·
¸¹
60
§
¨©
sec onds
minute
·
¸¹
u
T_step
§
¨©
q
step
·
¸¹
(3)
SPACE
¦_step —VWHSV  VHF RQG
120
§
¨©
rotations
minute
·
¸¹
u
360
§
¨©
q
rotation
·
¸¹
u
8
§
¨©
µsteps
step
·
¸¹
60
§
¨©
sec onds
minute
·
¸¹
u
1.8
§
¨©
q
step
·
¸¹
(4)
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 MODE 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 set full-scale current (I_FS) is the maximum current driven through either winding. This
quantity depends on the xVREF 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. The gain of DRV8825 is set for 5
V/V.
I_FS (A)
xVREF(V)
A_v u R_SENSE (:)
xVREF(V)
5 u R_SENSE (:)
(5)
To achieve I_FS = 1.25 A with R_SENSE of 0.4 Ω, xVREF should be 2.5 V.
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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