All-CMOS IC emulates opto-coupler for isolation without life issues
Silicon Labs has introduced an all-CMOS drop-in alternative to high-speed digital opto-couplers, claiming that it offers longer life than traditional LED/photodiode isolators plus lower input turn-on and output quiescent current.
“Opto-couplers use LED-based technology which exhibits significant output variation over input current, temperature and age,” said Silicon Labs. “Because of their aging effects, opto-couplers are often specified to last only 10 years, posing a serious problem for industrial system designers who want to develop products with 20+ year warranties.”
The firm has produced LED-less isolation chips before, but not any that are package and footprint compatible with existing opto-couplers, and include an LED emulator front-end that makes the input behave electrically like traditional products.
Although they will fit existing footprints, in some cases the input current limiting resistor will need changing as the maximum is 3mA (for A and C variants, 6mA for Bs).
Dubbed the Si87xx series, inside the isolators there are two CMOS die linked by capacitive coupling between an RF transmitter on one and an RF receiver on the other.
“Si87xx is also compatible with typical external devices that improve opto-coupler common-mode transient immunity, 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,” said SiLabs.
Incoming ‘anode’ current powers both the transmitter and the LED V/I characteristic simulator.
While similar at room temperature, tested at 80°C with a 500kHz square wave, the output of a traditional opto-coupler (HCPL-4506, yellow) is appreciably slower than that of a Si8710A (blue), said Silicon Labs.
Higher reliability than opto-couplers is claimed.
“CMOS is a well understood technology and offers 5.5x lower failures-in-time [FIT] rate than GaAs-based opto-couplers, a time dependent dielectric breakdown [TDDB] of 60 years, and a mean time-to-failure [MTTF[ of 87 years,” said SiLabs.
Si8710 has an open-collector output.
Si8711 adds an integrated output pull-up resistor.
Si8712 adds an asynchronous output enable pin to the 8710.
SOIC-8 and gull-wing DIP-8 packages for use up to 3.75kV are available now, SO-6 and LGA-8 parts certified to 5kV are planned for Q1 2013.
Devices are compliant with IEC 60747-5-2 including 10 kV surge protection and “the family meets the requirements for IEC 60950-1, 61010-1, 60601-1 reinforced insulation, and provides up to 1.2kV working voltage with a 60+ year lifetime,” said SiLabs.
New and replacement applications are expected in solar power inverters, switch-mode PSUs, uninterruptible PSUs, industrial drives, programmable logic controllers (PLCs) and medical equipment.
There is an evaluation kit: Si87xxDIP8-KIT.