Functional description L6258EA
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
In the case of V
DAC
equal to zero, the transconductance loop is balanced at the value of Vr,
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
simultaneously driven from V
s
to ground; and the differential voltage across the load in this
case is zero and no current flows in the motor winding.
Tri_0
Tri_180
Error Ampl.
Output
First Comp.
Output
Second Comp.
Output
