SI871X Datasheet, PDF (17/35 Pages) Silicon Laboratories – Industrial automation

Si871x/2x
4. Applications
The following sections detail the input and output circuits necessary for proper operation of the Si871x/2x family.
4.1. Input Circuit Design
Opto coupler manufacturers typically recommend the circuits shown in Figures 9 and 10. These circuits are
specifically designed to improve opto-coupler input common-mode rejection and increase noise immunity.
Vdd
R1
Si871x/2x
1 N/C
2 ANODE
Control
Input
Open Drain or
Collector
3 CATHODE
4 N/C
Figure 9. Si871x/2x Input Circuit
Vdd
Si871x/2x
1 N/C
Control
Input
Q1
R1
2 ANODE
3 CATHODE
4 N/C
Figure 10. High CMR Si871x/2x Input Circuit
The optically-coupled circuit of Figure 9 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 10 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 Si871x/2x 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
Figure 10) may require increasing the value of R1 to limit input current I_F to its maximum rating when using the
Si871x/2x. In addition, there is no benefit in driving the Si871x/2x 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
Si871x/2x is no more than –0.3 V with respect to the cathode when reverse-biased.
Rev. 1.0
17