AN729: Replacing Traditional Optocouplers with Si87xx Digital Isolators

AN729
R EPLACING T RADITIONAL O PTOCOUPLERS WITH Si87 XX
D IGITA L I S O L A T O R S
1. Introduction
Opto-couplers are a decades-old technology widely used for signal isolation, typically providing safety isolation,
signal level shifting, and ground loop mitigation. They are commonly used in a wide range of end applications,
including data communication circuits, switch mode power systems, measurement and test systems, and isolated
data acquisition systems. Optocouplers have several weaknesses, including parametric instability with temperature
and device aging, significant internal parasitic couplings, long propagation delay times, narrow operating
temperature ranges, and relatively low reliability. Optocouplers have been the “go-to” isolation device of choice for
the past 30+ years because they were the only integrated solutions to the problem of signal isolation. Today's
advanced CMOS signal isolation products offer better timing performance, higher reliability, and lower power
consumption compared to optocouplers and are capturing sockets traditionally held by optocouplers. However,
converting to CMOS isolation devices has, most often, required circuit changes and PCB modifications that cost
money and create design risks, until now.
The Si87xx family can be used as a pin-compatible optocoupler upgrade in existing systems or as a design-in
component for new product designs. Package and pin compatibility allow the Si87xx to replace optocouplers
without PCB modifications, with substantial gains in device performance and reliability. Device operation is simple:
the Si87xx output is held low when anode current is above the turn-on current threshold and pulled high by an
internal or external pull-up resistor when anode current is below the turn-off current threshold. (For more
information about current thresholds and hysteresis values, see the Si87xx data sheet.)
2. Si87xx Operation
Figure 1 shows an Si87xx block diagram in which the input-side circuit contains a diode emulator, high-frequency
transmitter, and galvanic isolator, all of which are powered by the voltage present on the anode pin. The diode
emulator mimics the behavior of an optocoupler LED to ensure compatibility with existing optocoupler input circuits.
Device operation is straightforward: the diode emulator enables the transmitter when anode current is above its
turn-on threshold. This action causes the transmitter to propagate a high-frequency carrier across the isolation
barrier to the receiver, which, in turn, forces the output driver low. Conversely, an anode current below the turn-off
threshold disables the transmitter, causing the receiver to release the output pin to be pulled high by the pull-up
resistor.
VBIAS
VDD
NC
OUTPUT DIE
INPUT DIE
VDDI
RF
e
ENABLE
XMIT
CATHODE
GNDI
VDD2
ISOLATION
VF
ANODE
ISOLATION
IF
NC
RECV BUFFER
GND2
VO
GND
NC
Si8710 Digital Isolator Figure 1. Si8710 Digital Isolator Block Diagram
Rev. 0.1 8/12
Copyright © 2012 by Silicon Laboratories
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This simple architecture provides numerous advantages over optocouplers:
Pin
and Package Drop-In Opto-Coupler Upgrade
Faster propagation time, better parametric stability over voltage and temperature, 2x to 3x lower internal
parasitic coupling compared to optocouplers.
Standard CMOS Process Technology
CMOS is a well understood process technology with 40+ years of learning and offers 5.5 times lower
failures-in-time (FIT) rate than GaAs-based optocouplers, a time dependent dielectric breakdown (TDDB)
of 60 years, and a mean time-to-failure (MTTF) of 87 years. CMOS also provides an operating temperature
range of –40 to +125 °C compared to –40 to +85 °C for optocouplers, greater parametric stability over
voltage and temperature, and lower operating power versus optocouplers.
Precise Current Thresholds with Hysteresis
The Si826x output is either low or high, with no ambiguous output states and excellent threshold stability
over voltage and temperature. There are no current transfer ratio (CTR) issues to address.
Improved Performance
Shorter propagation delay time and PWD, wider operating temperature range, and greater parametric
operating stability than optocouplers.
Silicon Dioxide Based Capacitive Isolation Barrier
Silicon dioxide is an ultra-stable material enabling an unparalleled lifetime of over 60 years, compared to
less than 15 years in optocouplers.
Superior Surge Tolerance
Withstands 10 kV surge per IEC 60065
Wide Product Range
The Si87xx digital isolators are available in gull-wing PDIP8, SOIC8, LGA8, and SDIP6 packages and offer
optional internal pull-up resistor and external enable. (For more information, see the Si87xx Digital isolator
data sheet.)
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3. Applications
3.1. Replacing an Existing Optocoupler with the Si87xx
Use
the Si87xx data sheet or online guide to select the correct, pin-compatible Si87xx product for the
application at hand.
Desolder the existing optocoupler from the board and discard it.
Solder the selected Si87xx device into the site previously occupied by the optocoupler.
Adjust the value of RF to achieve a maximum current of 3 mA for the Si87xx A-grade and Si87xx C-grade
devices or 6 mA for the B-Grade device (see Equation 1 below).
Connect power and verify proper system operation.
Note that the Si87xx is also compatible with typical external devices that improve optocoupler CMTI performance,
such as shorting switches, reverse diode clamps, and others. Components of this type can be left in place or
removed at the user's option.
3.2. Using the Si87xx in New Designs
Typically, the only calculated value is that of the current limiting resistor, RF. The first step is deciding if the
application benefits more from low anode current (A-Grade or C-Grade) or from high common-mode transient
immunity (B-Grade). Once this decision is made and the anode current threshold and optimum ON current values
are known, values for calculating RF (see Equation 1) are straightforward.
V F – 2.0
R F = --------------------IF
Where:
R F is the value of the anode current limit resistor ( 
V F is the input-side forward voltage (V)
I F is the Anode forward input current (mA)
Equation 1.
Note that it is best to keep the values of both VF and RF low because this offers greater resistance against CMT
events. For example, a B-Grade Si87xx with VF = 5 V and RF = 833  exhibits higher CMTI than the same device
with VF = 10 V and RF = 1.6 k.
Rev. 0.1
3
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4. Evaluation Board
The Si87xx package and pin-compatibility makes upgrading existing optocoupler circuits quick and easy. Simply
look-up the closest replacement to the incumbent optocoupler and replace the optocoupler with the appropriate
Si87xx (it may also be necessary to adjust the value of Anode input resistor RF for optimum operation). The PCB of
Figure 2 exercises an Si87xx digital isolator and a competing optocoupler simultaneously. The external digital input
signal is buffered and fed into the inputs of both devices while the output signals are observed on an oscilloscope.
Figure 3 shows the outputs of both devices at room temperature, while Figure 4 shows the outputs at 80 °C. Note
the faster propagation delay rise times provided by the Si87xx device.
Figure 2. Opto Comparison EVB
Figure 3. Optocoupler Comparison EVB at Room Temperature
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Rev. 0.1
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Figure 4. Optocoupler Comparison EVB at 80 °C
Figure 4 uses the same setup as Figure 3, but, this time, operating at an elevated temperature. The blue waveform
is the output of the Si8710A, and the yellow waveform is the output of the HCPL-4506. As operating temperature
increases, the HCPL 4506 output falling edge is substantially slower, and the propagation delay worsens compared
to Figure 3. Note that the Si8710A output performance is essentially the same, as shown in Figure 3.
Rev. 0.1
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5. Summary
The Si87xx digital optocoupler upgrade is the first and only enhanced optocoupler replacement technology
available. This device family offers higher performance, greater reliability, increased ease-of-use, and more
intuitive design than traditional optocouplers. The Si87xx easily retrofits into existing optocoupler circuits and
requires no PCB changes. These devices are ideal for retrofit or new designs.
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NOTES:
Rev. 0.1
7
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