SI8220 SILABS [Silicon Laboratories], SI8220 Datasheet - Page 16

• Manufacturer Part Number: SI8220
• Description: 0.5 AND 2.5 AMP ISODRIVERS WITH OPTO INPUT (2.5, 3.75, AND 5.0 KVRMS)
• Manufacturer: SILABS [Silicon Laboratories]

Si8220/21
The optically-coupled driver circuit of Figure 13 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 14 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 13 and 14; however, some applications will
require increasing the value of R1 in order to limit I
output drive do not change for values of I
circuit configurations of Figure 15, which are more efficient than those of Figures 13 and 14. 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 15C).
16
Control
Input
See Text
S1
R1
+5V
1
2
3
4
N/C
ANODE
CATHODE
N/C
Figure 15. Si8220/21 Other Input Circuit Configurations
Si8220/21
Figure 14. High CMR Opto Driver Input Circuit
A
Control
Control
Input
Input
See Text
F
between I
S1
R1
+5V
R1
F
Vdd
to a maximum of 10 mA. The Si8220/21 propagation delay and
Rev. 0.22
Q1
F(MIN)
1
2
3
4
and I
CATHODE
N/C
N/C
ANODE
Si8220/21
4
1
2
3
B
F(MAX)
N/C
N/C
ANODE
CATHODE
OPTO DRIVER
. New designs should consider the input
MCU I/O
Port pin
R1
1
2
3
4
N/C
CATHODE
N/C
ANODE
Si8220/21
C