AN-3012 Datasheet, PDF (5/8 Pages) Fairchild Semiconductor – High Speed Logic Compatible

APPLICATION NOTE
AN-3012
Output Power Consumption versus
Propagation Delay
The passive resistive pull-up resistor does dissipate power
when the output of the optocoupler is switched into the logic
low state. Increasing the collector load resistor does reduce
power consumption at the price of longer turn-off delay and
larger pulse width distortion. Figure 12 shows the expected
increase propagation delay from the logic low to logic high.
160
tA = 23°C
VCC = 3.3V
140 ^IF ^{= 10mA}
Period = 1ms
PW = 100µs
120
100
tpLH (ns)
80
tpHL (ns)
60
40
0
2
4
6
8
10
12
RLOAD – COLLECTOR LOAD RESISTOR (kΩ)
Figure 12. Propagation vs Collector Load Resistor
Logic Interfacing
The FODM8061 is a versatile logic-to-logic interface
optocoupler. The low forward voltage of the LED permits
operation among logic families with supply ranges from
2.3V to 15V. Figure 13 shows an inverting interface between
a 3V 74LCX07 open drain buffer to output logic families
from 3.3V to 5V. The current to the LED is set by R1 (82Ω)
to approximately 10mA. R2 (430Ω) provides the pull-up
needed by the open drain ’07 gate.
VDD1 = 3.0V–3.6V
C1
100nF
1
R2
430Ω
VDD2 = 3.3V–5V
6
R3
350Ω
Output
5
Input
R1
82Ω
3
74LCX07
4
FODM8061
C2
100nF
Figure 13. Inverting Series LED drive
When a non-inverting interface is needed, the LED shunt
drive is used. This circuit schematic is shown in Figure 14.
V_DD1 = 3.0V–3.6V
C1
0.1µF
R1
220Ω
Input
1
74LCX07
V_DD2 = 3.3V–5V
6
R2
350Ω
Output
5
3
4
FODM8061
C2
0.1µF
Figure 14. Non-inverting Logic interface
The output transistor of the 74LCX07 shunts the drive
current away from the LED when its output is forced low.
This type of interface offers the lowest LED off-state
impedance resulting in improved LED noise immunity.
Interface Circuit for V_CC Greater
than 5.5V
Industrial systems today still use the 15V CD4xxx logic
gates, which offer exceptional logic noise immunity, mainly
due to their high switching thresholds and slow operating
speed. When a low voltage high speed optocoupler is needed
to interface into logic families with supply voltages greater
than 5.5V, a buffer transistor is needed. Figure 15 shows the
use of a cascode amplifier interface, which uses an
inexpensive bipolar 2N2222 NPN transistor with a Schottky
“Baker Clamp”. This circuit switches a 25mA load from a
25V supply. The cascode circuit maintains the same logic
sense as the original optocoupler interface and as Figure 20
shows the rise and fall times, and the propagation delays well
under 100ns. The 5V supply voltage of the optocoupler can
be supplied by a Zener diode, from a regulated supply.
REV. 1.0 3/9/10
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