AN469 APPLICATION NOTE

2/6

The oscillator provides pulses to set the two flipflops, which in turn cause the outputs to activate the power ac-

tuator. Once the outputs have been activated the current in the load starts to increase, limited by the inductive

characteristic of the load.

Figure 2. Application Circuit for Unipolar 4 Phase Stepper Motor.

When the current in the load winding reaches the programmed peak value, the voltage across the sense resistor

terrupts the drive and allows the current to decay through a recirculating path until the next oscillator pulse oc-

curs. The peak current in each winding is programmed by selecting the value of the sense resistor and Vref and

is given by the equation :

(2)

The minimum output pulse width is determined by the pulse width of the oscillator, or other signal applied to the

sync input. The internal oscillator is designed to provide narrow pulses to the sync input but the pulse width

should be considered carefully.

In some applications it is desirable to set the pulse width of this sync pulse to be just longer than the turn on

delay time of the actuator stage. This may be useful in systems where the switching noise or recovery current

of the catch diodes, which passes through the sense resistor, causes the comparator to sense a current above

the peak current. By making the sync pulse wide enough to hold the flip-flop set at the time the switching tran-

sient occurs will cause the device to ignore this false data. When the internal oscillator is used the pulse width

can be modified by changing the value of the capacitor on pin 1.

Increasing the capacitance will widen the pulse width. The L6506 may be used with either a bridge driver, as

shown in figure 1, for bipolar motors or a quad darlington array, as shown in figure 2, for 4 phase unipolar mo-

tors. For eigher configuration, half step may be implemented using the 4 phase inputs with the input waveforms

shown in figure 3.

The recirculation path for the motor current is through a catch diode for unipolar motors, or a catch diode and

I

p eak

V

ref

R

s ens e

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