SI87XX Datasheet, PDF (14/36 Pages) Silicon Laboratories – 5 KV LED EMULATOR INPUT, OPEN COLLECTOR OUTPUT ISOLATORS

Si87xx
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.
Vdd
R1
Si87xx
1 N/C
2 ANODE
Control
Input
Open Drain or
Collector
3 CATHODE
4 N/C
Figure 7. Si87xx Input Circuit
Vdd
Si87xx
1 N/C
Control
Input
Q1
R1
2 _ANODE
3 CATHODE
4 N/C
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
Figure 8) may require increasing the value of R1 to limit input current I_F to its maximum rating when using the
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.
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Preliminary Rev. 0.5