User Manual

AND8399/D
http://onsemi.com
11
Maximum Operating Speed
As explained in Transparency Mode, one needs to be
careful when to measure the BEMF. After the start of the coil
current zero crossing it takes some time before the real
BEMF can be measured (see Figure 11). Leaving the coil
current zero crossing stage too early will result in an
incorrect BEMF sampling (= the measured voltage on the
SLA−pin will not represent the real BEMF). This will limit
the maximum operating speed.
The maximum operating speed will depend on next
parameters:
− Stepper motor
− Coil current
− Operating voltage
− Stepping mode
It’s best to choose a stepper motor with low series
resistance (few Ohms or less). For high speed applications
it will also be important to choose a stepper motor with low
inductance.
The coil current has to be defined in such a way that
enough torque is produced by the stepper motor to rotate the
load without loosing steps.
For silent operation, a high stepping mode is advised. But
as will be seen later, this will limit the maximum rotation
speed.
The operating voltage will depend on the used motor,
operating speed and coil current. It’s important to operate the
stepper motor within his operating range.
Finding the maximum operating speed can best be done
by measuring. For this the AMIS−3052x Evaluation Kit can
be used. This Evaluation Kit can be ordered from the
ON Semiconductor website (www.onsemi.com
). Below
steps explain how to determine the maximum operating
speed by using the AMIS−3052x EVK.
Step 1: Remove capacitor C3 from the AMIS−3052x
Motherboard (schematics and layout can be
found in the Graphical User Interface). This
capacitor will filter the SLA signal. Although
it’s advised to add this capacitor in the final
application, this capacitor will influence our
measurement and should be removed.
Step 2: Build the Evaluation Kit setup (see
documentation provided with the Evaluation
Kit). Make sure that the correct operating
voltage is used (connect your own power supply
if needed) and that the correct stepper motor is
used.
Step 3: Do not connect any signal to the NXT−pin!
Step 4: Press the CLR−pin one time to clear the digital.
Step 5: Set the Coil Current and Stepping Mode
(Control Register 0).
Step 6: Set SLA Transparency to Transparent and
enable the motor driver (Control Register 2).
Step 7: Connect the NXT signal. Set the NXT
frequency very low. Rotor should be rotating
very slowly resulting in no BEMF.
Step 8: Verify if there is no offset created by reading the
Microstepping Position (Status Register 3).
This can easily be done by enabling Check SLA
Output in the Graphical User Interface (GUI) of
the AMIS−3052x Evaluation Kit (see
Figure 13). When checked, the Microstepping
Position will color red if an offset is created
(when Status Register 3 is read out).
Figure 13. Check SLA Output
If the Microstepping Position colors red after
reading out SR3, go back to Step 1!
Step 9: Measure the coil current in one of the coils,
measure the voltage on the SLA−pin and
monitor the NXT−pin. Best is to trigger the
oscilloscope on the coil current zero crossing.
Figure 14 gives the results measured with a
Nidec Servo stepper motor operated at 12 V and
415 mA coil current. Stepping mode was set to
1/8 microstepping.
A zoom is taken on the coil current zero
crossing. At the rising edge of the NXT signal
(purple curve) the coil current will be regulated
to zero. Notice that the SLA voltage (yellow
curve) claps to 5 V. This is because the coil
voltage (not visible on the oscilloscope plot)
will clamp to Vbat + 0.6 V (see Figure 11).
After this you see the SLA voltage drop to zero.
This is the transient part (see Figure 11). Keep
in mind that the SLA voltage is only update at
the PWM frequency rate (this gives the steps in
the SLA voltage).