Functional description

L6258

14/32

2.6

Current control loop

The current control loop is a transconductance amplifier working in PWM mode.

The motor current is a function of the programmed DAC voltage.

To keep under control the output current, the current control modulates the duty cycle of the

two outputs OUT_A and OUT_B, and a sensing resistor Rs is connected in series with the

motor winding in order to produce a voltage feedback compared with the programmed

voltage of the DAC.

The duty cycle modulation of the two outputs is generated comparing the voltage at the

outputs of the error amplifier, with the two triangular wave references.

In order to drive the output bridge with the duty cycle modulation explained before, the

signals driving each output (OUTA & OUTB) are generated by the use of the two

comparators having as reference two triangular wave signals Tri_0 and Tri_180 of the same

amplitude, the same average value (in our case Vr), but with a 180° of phase shift each

other.

The two triangular wave references are respectively applied to the inverting input of the first

comparator and to the non inverting input of the second comparator.

The other two inputs of the comparators are connected together to the error amplifier output

voltage resulting by the difference between the programmed DAC. The reset of the

comparison between the mentioned signals is shown in Figure 6.

Figure 6.

Output comparator waveforms

so the outputs of the two comparators are signals having the same phase and 50% of duty

cycle.

As we have already mentioned, in this situation, the two outputs OUT_A and OUT_B are

case is zero and no current flows in the motor winding.

Tri_0

Tri_180

Error Ampl.

Output

First Comp.

Output

Second Comp.

Output