Electronic Design

Novel Digital Isolators Rely On Capacitors

A new brand of isolator pushes CMOS signaling rates to 150 Mbits/s and magnetic isolation to six orders of magnitude beyond the specs.

On the manufacturing floor, digital isolators permit high-speed data transmission in the presence of rotating machines and other sources of large magnetic fields. In high-impedance circuit paths, isolation causes noise to appear across the isolation barrier, rather than at the receiver or more sensitive components. It also provides patient protection in medical applications and breaks potential ground loops between distant nodes. Previous isolators have been based on LEDs and photodiodes, transformers, or giant-magnetic-resistor (GMR) devices.

In a new design approach unveiled by Texas Instruments, drop-in replacement isolators achieve higher levels of magnetic isolation with monolithic capacitors. The caps reside on a receiver chip inside a module containing separate CMOS driver and receiver dies bond-wired together.

The TI ISO721 and ISO721M isolators provide six orders of magnitude more magnetic immunity than existing inductive devices (while paying a power penalty). They also are faster and use 60% less power than high-performance optocouplers. The modules have high immunity against data corruption due to fast voltage transients, providing a minimum protection level of 25 kV/ms.

The ISO721 provides transistor-transistor-logic (TTL) signal levels and a noise filter. Meanwhile, the ISO721M uses CMOS levels and omits the 721's noise filter, nearly halving its propagation delay.

INSIDE THE MODULE
Figure 1 shows the bond-wire interface between the transmitter and receiver die. The capacitors on the receiver die are formed with a metal top plate and conductive-silicon bottom plate on each side of a silicondioxide (SiO2) dielectric.

The isolators use a high-signaling rate and a low-signaling rate channel to communicate (Fig. 2). After single-endedtodifferential conversion, the high-rate channel transmits unencoded data transitions-across the capacitive barrier. Differential signaling allows small signal levels and small coupling capacitance.

Since this channel appears as a high impedance to common-mode noise and the receiver provides high commonmode noise rejection, the design offers good transient immunity. That's an important quality to possess for capacitive signal coupling.

Meanwhile, the low-rate channel encodes the data in a pulse-width-modulation (PWM) format. It then transmits the data across the barrier differentially. (This handles any long strings of ones or zeros.)

PERFORMANCE PUSH
The maximum signaling rate of an optocoupler depends on how quickly the LED can turn on and off. Based on what is currently available, the fastest optocouplers can achieve signaling rates of 50 Mbits/s. Existing transformer-and GMR-based digital isolators support an operating range from dc to 100 Mbits/s. The ISO721 takes the maximum signal rate to 150 for CMOS levels. (TTL tops out at 100 Mbits/s.) Typical propagation delays run 10 and 17 ns (CMOS and TTL), and jitter specs are 2 and 1 ns.

Beyond throughput, the ISO721's claim to fame is its immunity to magnetic interference. It substantially exceeds the standards for isolators set by Underwriters Laboratory (UL 1577), the International Electrotechnical Commission (IEC 60747-5-2), and the Canadian Standards Association (CSA Component Acceptance Notice 5A).

Specifically, the unshielded ISO721 successfully passed the Class-5 magnetic field immunity requirements of IEC 61000-4-8 power-frequency fields up to 100 A/m (125.6 10-6 Wb/m2) and IEC 61000-4-9 pulsed fields to 1000 A/m (1.256 10-3 Wb/m2).

Class 5 is the most severe in the IEC spec. It applies to "severe industrial environments characterized by conductors, bus bars, medium-voltage lines, or high-voltage lines carrying tens of kA," as well as "ground conductors of the lightning protection system." So, these isolators have a demonstrated ability to stand up to the heaviest manufacturing environments, including switchyard areas and power stations.

Figure 3 shows the test results. In an application note (SLLA181: http://focus. ti.com/lit/an/slla181/slla181.pdf), TI develops a noise budget based on an analysis of the circuit and the physical characteristics of the isolators.

From the tests and analysis, " magnetic coupling in the differential circuit of the low-speed signal of the ISO72x exceeding the noise budget requires a magnetic flux density greater than 12.3 Wb/m2 (123 kgauss) at 1 MHz," TI concludes.

"This would be the field generated by over 10 million A in a 0.1-m conductor 0.1 m away from the device," the TI note continues. "It is unlikely that this will occur in nature or any manufactured equipment. If it did, it is more likely that surrounding circuitry would fail before the barrier circuit of the ISO72x."

Designers who use digital isolators also want to know what happens at the output if the input signal is lost. TI's isolators use a periodic pulse to determine if the input structure has power and is working. If the output side of the isolator doesn't receive a pulse after 4 ms, the output goes high.

The TI isolators consume 60 mW at 5 V and 21.5 mW at 3.3 V. Comparable magnetic data isolators draw 4.3 mW at 5 V, while fast optoisolators consume roughly 100 mW.

Accelerated reliability testing yields 90% confidence typical values of 504,408 hours mean time to failure and 1983 failures in 109 hours failures in time (FIT). Housed in an eight-pin small-outline IC, the isolators cost $1.65 in 1000-unit quantities.

Texas Instruments
www.ti.com

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