SI8220 Datasheet, PDF (18/31 Pages) Silicon Laboratories – 0.5 AND 2.5 AMP ISODRIVERS WITH OPTO INPUT (2.5, 3.75, AND 5.0 KVRMS)

Si8220/21
Vext
OPTO DRIVER
1 N/C
Control
Input
Q1
R1
2 _ANODE
3 CATHODE
4 N/C
Figure 15. High CMR Opto Driver 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 also recommend reverse-biasing the LED when the
control input is off to prevent coupled noise from energizing the LED.
The Si8220/21 can be used with the input circuits shown in Figures 14 and 15; however, some applications will
require increasing the value of R1 in order to limit I_F to a maximum of 20 mA. The Si8220/21 propagation delay and
output drive do not change for values of I_F between I_F(MIN) and I_F(MAX). New designs should consider the input
circuit configurations of Figure 16, which are more efficient than those of Figures 14 and 15. As shown, S1
represents any suitable switch, such as a BJT or MOSFET, analog transmission gate, processor I/O, etc. Also, note
that the Si8220/21 input can be driven from the I/O port of any MCU or FPGA capable of sourcing a minimum of
5 mA (see Figure 16C).
Vext
R1
Control
Input
S1
See Text
Si8220/21
1 N/C
2 ANODE
Vext
Control
Input
S1
See Text
3 ^CATHODE
4 N/C
R1
Si8220/21
1 ^N/C
2 ^ANODE
3 ^CATHODE
4 _N/C
MCU I/O
Port pin
R1
A
B
Figure 16. Si8220/21 Other Input Circuit Configurations
Si8220/21
1 N/C
2 ANODE
3 ^CATHODE
4 N/C
C
18
Rev. 1.5