SI8261AAC-C-IS Datasheet, PDF (17/40 Pages) Silicon Laboratories – 5 KV LED EMULATOR INPUT, 4.0 A ISOLATED GATE DRIVERS

Si826x
5. Applications
The following sections detail the input and output circuits necessary for proper operation. Power dissipation and
layout considerations are also discussed.
5.1. Input Circuit Design
Opto driver manufacturers typically recommend the circuits shown in Figures 14 and 15. These circuits are
specifically designed to improve opto-coupler input common-mode rejection and increase noise immunity.
Vext
R1
Si826x
1 N/C
2 ANODE
Control
Input
Open Drain or
Collector
3 CATHODE
4 N/C
Figure 14. Si826x Input Circuit
Vext
Si826x
1 N/C
Control
Input
Q1
R1
2 _ANODE
3 CATHODE
4 N/C
Figure 15. High CMR Si826x Input Circuit
The optically-coupled driver circuit of Figure 14 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 15 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 driver applications recommend reverse-biasing the LED when the control input is off to prevent coupled
noise from energizing the LED. The Si826x 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 15) may require increasing the value of R1 to limit input current I_F to its maximum rating when using the
Si826x. In addition, there is no benefit in driving the Si826x input diode into reverse bias when in the off state.
Rev. 1.3
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