On the subject of methods for PWM driving of a brushed DC motor, in the previous article an example of constant-current driving was explained. This time, PWM driving in the form of BTL amplifier input is explained.
Again we begin with a circuit example. This is an example of a circuit that performs PWM driving in the form of BTL amplifier input. The operation of the BTL amp input circuit is explained here as well, and should be referenced.
If the initial-stage amplifier voltage gain is Gv, then VIN1-VIN2 = (Vph-Vref)/Gv, OUT1 is at 100% on-duty (with the H side always on), and OUT2 is at 0% on-duty (with the L side always on).
With VIN1-VIN2 = (Vpl-Vref)/Gv, the on-duty of OUT1 is 0% (with the L side always on), and the on-duty of OUT2 is 100% (with the H side always on).
The following should be noted regarding this duty control. This configuration also requires a simultaneous-on prevention circuit to ensure that a state in which transistors on the high and the low sides are turned on simultaneously does not occur. When a simultaneous-on prevention circuit is present, it may not be possible to preserve linearity of control at duties near 0% and near 100%, and so the characteristics must be examined. Also, because there is variation in the offsets of initial-stage amplifiers, triangular-wave oscillators, and comparators, it is important to note that there are shifts in duty values relative to the input voltage.
・In H-bridge PWM driving in the form of BTL amplifier input, the difference in VIN voltages is used to control the duty.
・When a simultaneous-on prevention circuit is present, it may not be possible to maintain linearity near duties of 0% and 100%.
・It should also be noted that, because there is variation in the offsets of the initial-stage amplifier, the triangular-wave oscillator, and the comparator, the duty shifts with respect to the input voltage.