L6258EP

PWM current control loop

23/32

Figure 9.

Electrical model of the load

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

ωis 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+

Bemf

R

L

L

L

OUT-

R

S

to Sense

Amplifier

ACload

Vsense

Vout

---------------------

V

S

Bemf

–

()

R

S

R

L

R

S

+

----------------------

⋅

V

S

----------------------------------------------------------------

==

Acload

V

S

Bemf

–

V

S

-----------------------------

R

S

R

L

R

S

+

----------------------

⋅

=

ACload

dB

20

V

S

Bemf

–

V

S

-----------------------------

R

S

R

L

R

S

+

----------------------

⋅

⎝⎠

⎜⎟

⎛⎞

log

⋅

=