NJM3774

Programming

Figure 8 shows the different input and output sequences for full-step, half-step and modified halfstep operations.

Full-step mode

Both windings are energized at all the time with the same current, I

current direction (and the magnetic field direction) in one phase is reversed. The next step is then taken when the

other phase current reverses. The current changes go through a sequence of four different states which equal four

full steps until the initial state is reached again.

Half-step mode

In the half-step mode, the current in one winding is brought to zero before a complete current reversal is made. The

motor will then have taken two half steps equalling one full step in rotary movement. The cycle is repeated, but on

the other phase. A total of eight states are sequenced until the initial state is reached again.

Half-step mode can overcome potential resonance problems. Resonances appear as a sudden loss of torque at

one or more distinct stepping rates and must be avoided so as not to loose control of the motor´s shaft position.

One disadvantage with the half-step mode is the reduced torque in the half step positions, in which current flows

through one winding only. The torque in this position is approximately 70 % of the full step position torque.

Modified half-step mode.

The torque variations in half step mode will be elimi-nated if the current is increased about 1.4 times in the halfstep

position. A constant torque will further reduce resonances and mechanical noise, resulting in better performance,

life expectancy and reliability of the mechanical system.

Modifying the current levels must be done by bringing the reference voltage up (or down) from its nominal value

correspondingly. This can be done by using DACs or simple resistor divider networks, as shown in figure 9.

The NJM3774 is designed to handle about 1.4 times higher current in one channel on mode, for example 700

mA per winding in the full-step position, and 1000 mA in the half-step position.

70 % current

level

GND

+5 V

Ref

V

1.2 k

Ω

10 k

Ω

2.2 k

Ω

1.2 k

Ω

10 nF

V

and

V

on

NJM3774

R1

R2

Figure 9. Reduction of reference

voltage at the V