L6258EP_07 STMICROELECTRONICS [STMicroelectronics], L6258EP_07 Datasheet - Page 23

• Manufacturer Part Number: L6258EP_07
• Description: PWM controlled high current DMOS universal motor driver
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

L6258EP
Figure 9.
The schematic now shows the equivalent circuit of the stepper motor including a sine wave
voltage generator of the Bemf. The Bemf voltage of the motor is not constant, its value
changes depending on the speed of the motor.
Increasing the motor speed the Bemf voltage increases:
Bemf = Kt · ω
where:
Kt is the motor constant
ω is the motor speed in radiant per second
The formula defining the gain of the load considering the Bemf of the stepper motor
becomes:
we can see that the Bemf influences only the gain of the load block and does not introduce
any other additional pole or zero, so from the stability point of view the effect of the Bemf of
the motor is not critical because the phase margin remains the same.
Practically the only effect of the Bemf is to limit the gain of the total Aloop with a consequent
variation of the bandwidth of the system.
OUT+
OUT-
Electrical model of the load
ACload dB
ACload
Acload
=
=
20
Vsense
-------------------- -
Vout
⋅
log
=
V S Bemf
---------------------------- -
⎛
⎜
⎝
V S Bemf
---------------------------- -
=
–
V S
(
--------------------------------------------------------------- -
–
V S Bemf
V S
R
L
R
Bemf
–
L
L
S
⋅
⋅
----------------------
R L
V S
----------------------
R L
R S
)
+
⋅
R S
+
R S
----------------------
R L
R S
R
+
⎞
⎟
⎠
PWM current control loop
S
R S
to Sense
Amplifier
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