NJM3773

Figure 3. Output stage with current paths

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

s FUNCTIONAL DESCRIPTION

Each channel of the NJM3773 consists of the following sections: an output H-bridge with four transistors and four

recirculation diodes, capable of driving up to 750 mA 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 current-sensing resistor R

winding. As the current increases, a voltage develops across the sensing resistor, which is fed back to the com-

parator. At the predetermined level, defined by the voltage at the reference input V

flop, which turns off the upper output transistor. The turn-off function of the two channels works independently of

each other. The current decreases until the clock oscillator triggers the flip-flops of both channels simultaneously,

which turns on the output transistors again, and the cycle is repeated.

To prevent erroneous switching due to switching transients at turn-on, the

NJM3773 includes a digital filter. The clock oscillator provides a blanking pulse which is used for digital filtering of

the voltage transient across the current sensing resistor during turn-on.

The current paths during constant current switching and phase shift are shown in figure 3.

Fast Current Decay

Slow Current Decay

Motor Current

Time

1

2

3

3

2

1

R

S