Texas Instruments opto-emulators
combine the behavior of traditional optocouplers with TI’s SiO2-based
isolation technology. Figure 3-1 shows that opto-emulators are pin-to-pin compatible with the industry’s most
popular optocouplers, facilitating seamless integration into existing designs and
providing equivalent input and output signal behavior.
Figure 3-1 is an illustrative cross section of a TI opto-emulator, showing the three die
inside which contain input, isolation, and output circuitry.
Table 3-1 shows a comparison
table of the dielectric strengths of different insulative materials traditionally found
in optocouplers along with SiO
2, found in TI's opto-emulators. Opto-emulators
boast improved high-voltage capabilities, making them designed for applications
demanding robust isolation. TI Opto-emulators leverage SiO
2 for the
insulating barrier, which is significantly stronger than air and materials used in many
optocouplers on the market. To learn more about TI’s SiO
2 technology and
reliability, read
Addressing High-Voltage Design Challenges With Reliable
and Affordable Isolation Technologies.
Table 3-1 Dielectric Strength of Various
Insulating Materials
Insulator
Materials |
Technology |
Dielectric
Strength |
Air |
Optocouplers |
approximately 1
VRMS/µm |
Epoxies |
Optocouplers |
approximately 20
VRMS/µm |
Silica Filled Mold
Compounds |
Optocouplers |
approximately 100
VRMS/µm |
SiO2 |
Opto-emulators |
approximately 500
VRMS/µm |
TI's opto-emulators replicate
the behavior of an LED on the input pins, so the signal transmission and electrical
parameters of the input circuit are similar to that of optocouplers. However, there
is no actual LED inside of opto-emulators, which results in several benefits:
- Since there is no internal LED
for signal transmission or transparent insulation material that can cloud or
yellow with time, the additional power that optocouplers require to compensate
for this degradation over their entire lifetime is not required when using TI’s
opto-emulators. Since opto-emulators do not transmit signals using an LED, this
over design practice does not apply. The signal transmission, power consumption,
and other operating parameters for TI’s opto-emulators are specified for their
entire operating lifetime and already account for process, voltage, and
temperature variations.
- Building on TI's existing digital
isolator technology, TI's first high-speed opto-emulator devices, the ISOM871x
family, feature a minimum CMTI specification of 125 kV/µs, surpassing some
traditional optocouplers by >100 kV/µs! This allows opto-emulators to be used
in applications with very high common-mode switching noise or high ringing
amplitudes where traditional optocouplers cannot be used.
- In typical applications, like in
feedback control loops of isolated power supplies, CTR variation of optocouplers
impacts the power supply feedback loop response, complicating feedback loop
design and creating challenges for system designers when factoring in the
appropriate compensation. TI opto-emulators, like the ISOM8110,
come standard with a variety of narrow CTR ranges that have higher stability
over lifetime and temperature.
- Typical high-speed optocouplers
support data rates from 100kbps up to 1Mbps, while ISOM8710 and
ISOM8711 can transmit data rates up to 25Mbps across the isolation
barrier. This enables higher throughput and use in more high-speed
applications.
- Many optocouplers are limited to
operation in temperatures up to +85°C. TI’s ISOM871x devices are specified for
operation from −40°C to +125°C, meaning their data sheet parameters are
specified for conditions many high-speed optocouplers are not designed for. TI's
ISOM811x family of opto-emulators support a further extended temperature range
of –55°C to +125°C.
TI's opto-emulators achieve
signal isolation much like our digital isolator devices. The emulation in
opto-emulators refers to the recreation of input and output structures that operate
like optocouplers while isolating signals using TI's isolation technology.
Standard optocouplers use an LED as
the input stage. When the input is turns the diode ON, these LEDs brighten as input
forward current increases. The light from the LED shines through an air or epoxy gap
onto a photo transistor inside of the package, which in turn sinks current on the
output side. This is the core operation of optocouplers, where the isolation barrier
is the air or epoxy gap between the LED and photo transistor, and additional
circuitry can be designed around the input or output create AC inputs or logic,
triac, or gate-driver outputs.
In opto-emulators, the input signal is transmitted across the isolation barrier
using an on-off keying (OOK) modulation scheme. The transmitter sends a high
frequency carrier across the barrier to represent one digital state and sends no
signal to represent the other digital state. Signal transmission in analog
opto-emulators functions similarly, and a receiver demodulates the signal after
advanced signal conditioning and produces the signal through the output stage. The
concept of the OOK modulation scheme is shown by the waveforms in Figure 3-3.