Digital optoisolators: an overview – Avago Technologies ACPL-224-500E User Manual
Page 18
AV02-4387EN
15
Avago Technologies
1
2
8
7
6
5
LED
3
SHIELD
HCPL-2400
HCPL-2430
HCPL-2201
HCPL-2230
PHOTO
DIODE
(PIN)
2
R
1
V
OUT
V
CC
TOTEM POLE
OUTPUT STAGE
V
IN
ENABLE
CASCADED
AMPLIFIER
1
2
8
7
6
5
LED
3
SHIELD
HCPL-2601
PHOTO
DIODE
(PIN)
2
R
1
R
2
V
OUT
V
CC
HIGH SPEED
SCHOTTKY
TRANSISTOR
V
IN
ENABLE
CASCADED
AMPLIFIER
1
1
2
8
7
6
5
LOGIC
GATE INPUT
3
4
SHIELD
HCPL-7721
HCPL-0721
INTEGRATED
CURRENT
SOURCE
2
C
2
V
OUT
V
DD2
CMOS
PUSH-PULL
OUTPUT
STAGE
V
IN
V
DD1
C
1
NC*
*PIN 3 IS THE ANODE OF THE INTERNAL LED AND MUST BE LEFT UNCONNECTED FOR
GUARANTEED DATA SHEET PERFORMANCE.
C
1
, C
2
= 0.01 TO 0.1
µF
Figure 3. Cascaded Amplifier Optoisolator
Figure 4. Totem Pole Output Optoisolator
Figure 5. High Speed CMOS Optoisolators
Digital Optoisolators:
An Overview
Figure 3 shows a photodiode cas-
caded amplifier, coupled to a Schott-
ky transistor, open collector output
stage. In order for an open collector
output stage to function, a pull-up
resistor is required at the output of
the Optoisolator.
Figure 4 shows a photodiode and
cascaded amplifier, coupled to a
totem pole output stage. This type
of output stage eliminates the ne-
cessity of using a pull-up resistor
at the output. The outputs can sink
or source current allowing higher
output power drive capability.
Figure 5 shows a push-pull CMOS
output stage. These types of optoi-
solators are designed to achieve
highest speed performance (up to
25 M bit/s for HCPL-7721/7720). The
amplifier stage presents very low
input impedance to the photodiode,
and does not become saturated,
while allowing the signal to be am-
plified several folds through the am-
plifier stages, allowing operation at
relatively low LED drive currents (2
to 5 mA).