NJM3777
s
FUNCTIONAL DESCRIPTION
Each channel of the NJM3777 consists of the following sections: an output H-bridge with four transistors and four
recirculation diodes, capable of driving up to 800 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
S
, effectively connected in series with the motor
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
R
, the comparator resets the flip-
flop, which turns off the upper output transistor. The turn-off of one channel is independent of the other channel.
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 NJM3777 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 turn-on, turn-off and phase shift are shown in figure 3.
V
MM
1
2
3
R
S
Motor Current
1
2
3
Fast Current Decay
Slow Current Decay
Time
Figure 3. Output stage with current paths
during turn-on, turn-off and phase shift.
NJRC [ NEW JAPAN RADIO ]
