Functional description

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5

Functional description

The circuit is intended to drive a bipolar constant current through one motor winding. The

constant current is generated through switch mode regulation.

There is a choice of three different current levels with the two logic inputs lN0 and lN1. The

current can also be switched off completely.

5.1

Input logic

If any logic input is left open, the circuit treats it as a high-level input.

l

5.2

Phase input

The PHASE input pin determines the direction of current flow in the winding, depending on

the motor connections. The signal is fed through a Schmidt trigger for noise immunity, and

through a time delay in order to guarantee that no short-circuit occurs in the output stage

during phase-shift. A high level on the PHASE input causes the motor current flow from

OUTA through the winding to OUTB.

The lH0 and lH1 input pins select the current level in the motor winding. The values of the

different current levels are determined by the reference voltage VR together with the value of

5.3

Current sensor

comparators. Only one comparator is active at a time. It is activated by the input logic

according to the current level chosen with signals IN0 and IN1. The motor current flows

through the sensing resistor RS. When the current has increased so that the voltage across

RS becomes higher than the reference voltage on the other comparator input, the

comparator output goes high, which triggers the pulse generator and its output goes high

Table 8.

Truth table

IN0

IN1

Current level

H

H

No current

L

H

Low current

H

L

Medium current

L

L

Maximum current