DRV8811_15 Datasheet, PDF (18/32 Pages) Texas Instruments – DRV8811 Stepper Motor Controller IC

DRV8811
SLVS865I – SEPTEMBER 2008 – REVISED JANUARY 2015
Typical Application (continued)
9.2.1 Design Requirements
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DESIGN PARAMETER
Supply Voltage
Motor Winding Resistance
Motor Winding Inductance
Motor Full Step Angle
Target Microstepping Level
Target Motor Speed
Target Full-Scale Current
Table 3. Design Parameters
REFERENCE
VM
R_L
I_L
θ_step
n_m
V
I_FS
EXAMPLE VALUE
24 V
4.0 Ω
3.7 mH
1.8°/step
8 µsteps per step
120 rpm
1.25 A
9.2.2 Detailed Design Procedure
9.2.2.1 Stepper Motor Speed
The first step in configuring the DRV8811 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 (nm), and motor full step angle (θ_step),
f_step  µsteps / second 
v ^§¨©
rotations
minute
·
¸¹
u
360
§
¨©
q
rotation
·
¸¹
u
n_m
§
¨©
µsteps
step
·
¸¹
60
§
¨©
seconds
minute
¸·¹u^step
§
¨©
q
step
·
¸¹
(5)
f_step  µsteps / second 
120
§
¨©
rotations
minute
·
¸¹
u
360
§
¨©
q
rotation
·
¸¹
u
8
§
¨©
µsteps
step
·
¸¹
60
§
¨©
seconds
minute
·
¸¹
u
1.8
§
¨©
q
step
·
¸¹
(6)
θ_step can be found in the stepper motor datasheet or written on the motor itself.
For the DRV8811, the microstepping level is set by the USMx pins. Higher microstepping will mean a smother
motor motion and less audible noise, but will increase switching losses and require a higher f_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 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. The gain of DRV8811 is set for 8
V/V.
I_FS  A
VREFV 
VREFV 
A_v x R_SENSE ß 8 x R_SENSE ß
(7)
To achieve IFS = 1.25 A with RSENSE of 0.1 Ω, VREF should be 1.56 V.
9.2.2.3 Decay Modes
The DRV8811 supports three different decay modes: slow decay, fast decay, and mixed decay. The current
through the motor windings is regulated using a fixed off time scheme.
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