Peak current control with PWM and constant off-time

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Peak current control with PWM and constant off-time

The peak current control with PWM and constant off-time is one of the most common

algorithms implemented by stepper motor controllers.

The phase current is measured through a shunt resistor and the resulting voltage (see

Equation 4 A) is sent to a comparator input. The other input is a reference voltage setting

the peak current forced into the motor phase (Equation 4 B).

Equation 4

At the beginning of driving, the motor phase currents are zero and the controller turns on

one of the diagonals of the power bridge according to the expected current direction. The

phase current is increased until the target reference voltage is reached, then the controller

switches off the forcing side of the bridge making the phase current recirculate in the bottom

side of the power stage. The same effect may be obtained recirculating the current on the

top side; in this case the forcing side of the bridge is kept on and the opposite side is turned

on.

Figure 8. Constant off-time control sequence (low-side recirculation)

A



=

B



=

V

Ref

I

Peak

V

BUS

V

Sense

I

Ph

R

SHUNT

R

SHUNT

V

BUS

V

Sense

I

Ph

R

SHUNT

V

BUS

I

Ph

t

OFF

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