Si87xx

14

Preliminary Rev. 0.5

4. Applications

The following sections detail the input and output circuits necessary for proper operation of the Si87xx family.

4.1. Input Circuit Design

Opto coupler manufacturers typically recommend the circuits shown in Figures 7 and 8. These circuits are

specifically designed to improve opto-coupler input common-mode rejection and increase noise immunity.

Figure 7. Si87xx Input Circuit

Figure 8. High CMR Si87xx Input Circuit

The optically-coupled circuit of Figure 7 turns the LED on when the control input is high. However, internal

capacitive coupling from the LED to the power and ground conductors can momentarily force the LED into its off

state when the anode and cathode inputs are subjected to a high common-mode transient. The circuit shown in

Figure 8 addresses this issue by using a value of R1 sufficiently low to overdrive the LED, ensuring it remains on

during an input common-mode transient. Q1 shorts the LED off in the low output state, again increasing common-

mode transient immunity.

Some opto coupler applications recommend reverse-biasing the LED when the control input is off to prevent

coupled noise from energizing the LED. The Si87xx input circuit requires less current and has twice the off-state

noise margin compared to opto couplers. However, high CMR opto coupler designs that overdrive the LED (see

Si87xx. In addition, there is no benefit in driving the Si87xx input diode into reverse bias when in the off state.

Consequently, opto coupler circuits using this technique should either leave the negative bias circuitry unpopulated

or modify the circuitry (e.g., add a clamp diode or current limiting resistor) to ensure that the anode pin of the

Si87xx is no more than –0.3 V with respect to the cathode when reverse-biased.

R1

1

2

3

4

Si87xx

Vdd

Open Drain or

Collector

Control

Input

ANODE

CATHODE

N/C

N/C

R1

1

2

3

4

Si87xx

Vdd

Control

Input

ANODE

CATHODE

N/C

N/C

Q1