AN731: Replacing Traditional Opto-Drivers with the Si826x Driver Family

AN731
R EPLACING T RADITIONAL O P T O -D RIVERS WITH THE
Si8 26 X D RIVER F AMILY
1. Introduction
Optocoupler-based gate drivers (“opto-drivers”) have been around for decades and favored over other gate drive
solutions because they provide safety isolation and level shifting in a single package. Typical opto-driver
applications include motor and motion control, switch-mode power delivery, and test and medical systems.
Opto-drivers are, however, not without their problems; they exhibit weaknesses in critical areas making them lessthan-optimal for some applications and forcing designers to compromise circuit performance and/or add external
components. Until recently, replacing a legacy opto-driver with a modern CMOS isolated gate driver required
significant design and PCB changes that increased costs and design risks. The Si826x family changes this
dynamic by offering pin- and package-compatible opto-driver replacements that retrofit opto-driver sockets without
PCB modification and provide significant gains in performance and reliability. This application note examines these
twenty-first century opto-driver upgrades.
2. Si826x Operation
The input-side diode emulator mimics the behavior of an opto-driver LED to ensure compatibility with existing optodriver input circuits. The diode emulator enables the high-frequency transmitter when input current IF is at or above
its threshold value, sending a high-frequency carrier across the isolation barrier to the receiver. This high-selectivity
receiver forces the output driver high when sufficient in-band energy is detected. Input current below the IF
threshold disables the transmitter, causing the receiver to force the output driver low.
Si826x Isolated Gate Driver
NC
VDDI
IF
e
EN
XMIT
LED Emulator
Vo
ISOLATION
RF
CATHODE
VF
VDD
ISOLATION
ANODE
GNDI
RECV
GND
Si826x Isolated Gate Driver
ANODE
VDDI
e
EN
XMIT
LED Emulator
GNDI
ISOLATION
NC
VDD
ISOLATION
RF
VF
Vo
GND
NC
IF
30VDC(max)
VDD
RECV
30VDC(max)
VDD
Vo
GND
GND
CATHODE
Figure 1. Si826x 4 and 6-Pin Block Diagrams
Rev. 0.2 4/13
Copyright © 2013 by Silicon Laboratories
AN731
AN731
This architecture provides numerous advantages over opto-drivers:
Pin
and Package Drop-In Opto-Coupler Upgrade
Faster propagation time and better parametric stability over voltage and temperature compared to optodrivers.
Standard CMOS Process Technology
CMOS is a well-understood process technology with 40+ years of development and offers 5.5 times lower
FIT rate (failures-in-time) than legacy opto-drivers as well as an expanded operating temperature range of
–40 to +125 °C (compared to –40 to +85 °C for opto-drivers). The Si826x isolation barrier lifetime is 10
times higher than that of opto-drivers. Part-to-part matching is 14 times tighter than the Gallium Arsenide
(GaAs) process technologies used by opto-drivers.
Precise Current Thresholds with Hysteresis
The Si826x output is either low or high, with none of the ambiguous output states found in opto-drivers,
and excellent threshold stability over voltage and temperature.
Improved Performance
Faster and more precise timing, lower-power operation, higher reliability, reduced internal parasitic
coupling for better CMTI, and greater parametric stability over temperature than opto-drivers.
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 for the mold compound and polyimide tape used in opto-drivers.
Superior Surge Tolerance
Withstands 10 kV surge per IEC 60065.
Wide Product Range
The Si826x digital isolators are available in SOIC-8, DIP8, LGA8, and SDIP6 packages. (For more
information, see the Si826x isolated driver data sheet.)
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Rev. 0.2
AN731
3. Applications
3.1. Replacing an Existing Opto-Driver with the Si826x
Use
the Si826x data sheet or online guide to select the correct pin- and package-compatible Si826x
version for the end application.
Desolder and discard the existing opto-driver from the board.
Solder the selected Si826x device into the site previously occupied by the opto-driver.
If necessary, adjust the value of RF to achieve a maximum anode current of 6 mA.
Connect
power and verify proper system operation.
Note that the Si826x is compatible with typical opto-driver circuits commonly used to improve opto-driver CMTI
performance (typical components include shorting switches and reverse diode clamps). Components of this type
can be left in place or removed at the user's option.
4. Evaluation Board
The Si826x package and pin-compatibility make upgrading existing opto-driver circuits quick and easy. Simply
look-up the closest replacement to the incumbent opto-driver and replace it with the Si826x. The EVB shown in
Figure 2 exercises the Si826xA or B isolated gate driver and competing opto-driver simultaneously. An external
digital input signal is buffered by an onboard driver 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 25 °C, while Figure 4 shows the
outputs at 85 °C. Note the faster propagation delay rise times provided by the Si826x.
Figure 2. Si826x Isolated Gate Driver vs. Opto-Driver Comparison Board
Rev. 0.2
3
AN731
Figure 4 uses the same setup as Figure 2 but, operating at an elevated temperature of 85 °C. The Channel 1
waveform is the Si826x output, and the Channel 2 waveform is the HCPL-4506 output. 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 Si826x output performance is essentially the same, as shown in Figure 3.
Figure 3. Propagation Delays at 25 °C
Figure 4. Propagation Delays at 85 °C
4
Rev. 0.2
AN731
5. Summary
The Si826x isolated gate driver upgrade is the first and only enhanced opto-driver replacement technology. This
device family offers higher performance and greater reliability and ease-of-use than legacy opto-drivers. The
Si826x easily retrofits into existing opto-driver sites and requires no PCB changes. These devices are ideal for
retrofit or new designs.
Rev. 0.2
5
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DOCUMENT CHANGE LIST
Revision 0.1 to Revision 0.2

6
Extensive rewrite of document.
Rev. 0.2
AN731
NOTES:
Rev. 0.2
7
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