NJM3776

s FUNCTIONAL DESCRIPTION

Each channel of the NJM3776 consists of the following sections:

• An output section with four output transistors, two sourcing and two sinking, intended to drive the four transistors

in an external H-bridge. Each transistor is capable of driving up to 200 mA continuous current.

• 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 sourcing output transistor in the circuit. Consequently the correspond-ing lower external

transistor, in the H-bridge, is turned off. 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 NJM3776 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. Due to the high output drive capability, this transient might exceed the max.

allowed voltage on the C inputs and damage the circuit. A resistor is placed in the feedback loop in order to prevent

this transient from damaging the circuit.

The current paths during turn-on, turn-off and phase shift are shown in figure 3.

Figure 3. Output stage with current paths

during turn-on, turn-off and phase shift

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Vmm

Rs

Fast Current Decay

Slow Current Decay

Motor Current

Time

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3