TIDA-00830
24V Stepper Motor Design with AutoTune™
TI Reference Design
2
2. Current Limiting
In order to properly commutate a stepper motor, the controller device must continuously regulate the
current through the motor coils at predetermined current values. An overview of the current regulation
scheme found in the DRV8880 is shown in
Figure 1
below.
Sine DAC
VREF
TRQ[0:1]
Device
Logic
1/Av
Internal
H-Bridge
Sense
Resistor
PWM Logic
+
-
1
2
3
4
TRQ[1:0] = 00
– 100%
TRQ[1:0] = 01
– 75%
TRQ[1:0] = 10
– 50%
TRQ[1:0] = 11
– 25%
TIME
V
O
L
T
A
G
E
Figure 1.
Single Phase Current Regulation Control Scheme
Figure 2.
1/4 Step Current Regulation Scaled by TRQ Percentage
The DRV8880 adjusts the current flowing through each phase of a stepper motor by internally
comparing a reference voltage generated by the internal sine DAC to the voltage measured across an
external sense resistor. This internal sine DAC is scaled based on the value of V
REF
and has a preset
table of voltage levels for each micro-stepping state. Current magnitude can be scaled using the
percent torque inputs TRQ[1:0] as shown in
Figure 2
above. Adjusting the current magnitude using
just the TRQ[1:0] bits simplifies the control process by not requiring a change in V
M
or a change in
V
REF
. If the voltage across the sense resistor is larger than the internal comparison voltage, the device
enters a different decay mode depending on the state of the decay mode setting pins and ATE pin.
Depending on if the sine DAC table is increasing or decreasing steps will also affect the decay mode
the device uses. All of these features included in the DRV8880s current regulation system allow the
driver to have constant control over how much current is being supplied to the stepper motor.
3. Current Ripple
While the device is enforcing a specific current level during each step, there is a tradeoff between using
slow or fast decay modes for current regulation. If the device uses too much fast decay then the level of
current ripple in the power supply will increase and can result in unwanted noise and vibration in the
stepper motor. However, if the device does not use enough fast decay then the current in the motor
will gradually increase and result in the device not being able to effectively regulate current. This can
damage the motor and create an uneven driving pattern. Some stepper motor drivers utilize a decay
mode called mixed decay to operate while the device is lowering current through the motor windings.
Mixed decay works by including a percentage of fast decay during t
OFF
and then the remaining amount
of t
OFF
is held at slow decay. This allows the current to drop quickly during the fast decay interval and
then slowly dissipate during the slow decay period, thus keeping the current ripple to a minimum. The
downside to this form of current decay is that each t
OFF
interval is often enforced regardless of how
