L6223 datasheet(17/33 Pages) STMICROELECTRONICS

17 / 33 page

The behaviour of the oscillator at each phase

change allows the L6223 to drive high speed

unipolar stepper motors.

This is the main functional difference between the

L6223 and the L6223 (see fig. 21b).

In the latter, the phase change starts only when

the clock pulse sets the F-F (Fig. 22) that is when

the capacitor voltage reaches the discharge

threshold.

As a result, a variable delay between the leading

edge of the input signal and the beginning of the

current decay to zero can be expected.

Because of that, driving high speedy stepper mo-

tors is produced a noisy beating between chop-

ping frequency and phase change rate.

In the L6223 as soon as the phase change is

driven by the inputs, the oscillator voltage Jumps

to its top level, a new discharge period is gener-

ated and the chopping transistor is switched ON

(Q5 in fig. 22). The

advantages are a motor

phase change synchronous to the driving signal

and no beating for whatever rotation speed. By

setting pin 14 at a voltage equal or less than Vrs

= 0.9V, when the IC is normally supplied and the

oscillator is running, the 6 bit shift register is

quickly reset and the power outputs are disabled:

a delay of 700nsec max must be expected.

The use of this behaviour to reset the device at

the turn-on is not allowed; however the reset is

automatically provided by the Logic Supply Volt-

age crossing the threshold of 3.5V (typ.) both at

the turn-ON and at the turn-OFF.

Protection

The protection zeners on the outputs protect the

IC from overvoltage during chopping and phase

change. Actually, at the phase change, the out-

puts rise to a voltage equal to VO =2VS +Vm,

where VS is the power supply and Vm the product

of the motor resistance Rm with the peak phase

current Ip. Vs is doubled because in the unipolar

motor we have two coupled phases connected in

series(phase A and A, B and B) for each of the

two windings of the motor (MA, MB see Fig. 22).

The leakage inductance, seen from the outputs of

the L6223, can generate an overvoltage higher

than VO. To protect the IC, the zeners must be

able to sustain a power of 400W for 1 microsec

and must be able to conduct at a voltage Vz

higher than V0 =2Vs(max) +Vm. It’s important that

during the transition, at the max operating ambi-

ent temperature, the zener conduction is guaran-

teed for a voltage lower than 125V (see Absolute

Max Ratings). The ST-BZWO4-85 satisfies this

requirement. The diode connected to the com-

mon protects this input from the undergounds due

to the leakage inductances and/or the imbalanc-

ing between the phase currents.

Figure21a:Relationshipbetweencapacitordischarge

of the oscillator, window and sensing voltage

Figure 21b: Oscillator behaviour of the L6223 and

of the L6223 (simplified waveforms).

L6223

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