L6223A ST Microelectronics, Inc., L6223A Datasheet - Page 17

• Manufacturer Part Number: L6223A
• Description: Dmos Programmable High Speed Unipolar Stepper Motor Driver
• Manufacturer: ST Microelectronics, Inc.

Figure21a: Relationshipbetweencapacitordischarge
of the oscillator, window and sensing voltage
The behaviour of the oscillator at each phase
change allows the L6223A to drive high speed
unipolar stepper motors.
This is the main functional difference between the
L6223A 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 L6223A 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
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
advantages are a motor
Figure 21b: Oscillator behaviour of the L6223 and
of the L6223A (simplified waveforms).
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 V
where V
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 L6223A, can generate an overvoltage higher
than V
able to sustain a power of 400W for 1 microsec
and must be able to conduct at a voltage V
higher than V
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.
O
. To protect the IC, the zeners must be
S
is the power supply and V
0
= 2V
s(max)
+ V
m
. It’s important that
O
m
= 2V
the product
L6223A
S
+ V
17/33
m
z
,