SPMD250STP

RESET, ENABLE and HOME signals

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RESET, ENABLE and HOME signals

The RESET is an asynchronous reset input which restores the module to the home position

(state 1 : ABCD = 0101). Reset is active when low.

The HOME output signals this condition and it is intended to be ANDed with the output of a

mechanical home position sensor.

The ENABLE input is used to start up the module after the system initialization. ENABLE is

active when high or unconnected.

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Motor current regulation

The two bipolar winding currents are controlled by two internal choppers in a PWM mode to

obtain good speed and torque characteristics.

An internal oscillator supplies pulses at the chopper frequency to both choppers.

When the outputs are enabled, the current through the windings raises until a peak value set

by Ioset and Rsense (see the equivalent block diagram) is reached. At this moment the

outputs are disabled and the current decays until the next oscillator pulse arrives.

The decay time of the current can be selected by the CONTROL input (pin 10). If the

CONTROL input is kept high or open the decay is slow, as shown in Figure 12, where the

equivalent power stage and the voltages on A and B are shown as well as the current

waveform on winding AB.

When the CONTROL input is forced low, the decay is fast as shown in fig. 13.

If the modules must drive a large motor that does not store much energy in the windings, the

chopper frequency must be decreased: this is easily obtained by connecting an external

capacitor between OSC pin and GND1.

In these conditions a fast decay (CONTROL LOW) would impose a low average current and

the torque could be inadequate. By selecting CONTROL HIGH, the average current is

increased thanks to the slow decay.

When the m odule is used in the fast-decay mode it is recommended to connect external

fast recovery, low drop diodes between each phase output and the supply return (GND).

The slow-decay should be the preferred operating recirculation mode because of the lower

power dissipation and low noise operations.