NJM3774

Figure 3. Output stage with current paths

during turn-on, turn-off and phase shift.

s FUNCTIONAL DESCRIPTION

Each channel of the NJM3774 consists of the following sections: an output H-bridge with four transistors, capable

of driving up to 1000mA continuous current to the motor winding; a logic section that controls the output transistors;

an S-R flip-flop; and a comparator. The clock-oscillator is common to both channels.

Constant current control is achieved by switching the output current to the windings. This is done by sensing the

peak current through the winding via a resistor, R

turn-on period. As the current increases, a voltage develops across the resistor, and is fed back to the comparator.

At the predetermined level defined by the voltage at the reference input V

turning off the output transistors. The current decreases until the clock oscillator triggers the flip-flop, turning on the

output transistors, and the cycle is repeated.

The current paths during turn-on, turn-off and phase shift are shown in figure 3. Note that the upper recirculation

diodes are connected to the circuit externally.

Fast Current Decay

Slow Current Decay

Motor Current

Time

1

2

3

3

1

2

R

S

External recirculation diodes