High-voltage tech supports Borscht phone features

High-voltage tech supports Borscht phone features
Legerity's dual-channel subscriber line interface controller (SLIC) device has enhanced and optimized features to directly address the requirements of voice-over-broadband applications. This feature tunes conventional analog phone lines for higher bandwidth and channel density. The goal of the design is to reduce system level costs, space and power through higher levels of integration, and to provide better quality-of-service.

The Le78D11/Le77D11 is a two-chip interface providing a software-configurable solution to Borscht telephone line functions. Those functions are battery feeding, overvoltage protection, ringing (20 Hz, 90-volt; 1 second on, 2 seconds off), supervision, codec (and channel filtering), a hybrid line and testing for two lines.

The resulting system offers reduced complexity and a smaller, denser footprint, all with minimal external component cost. The Le77D11 device itself requires only two power supplies: +3.3-V and a +12-V unit, but the supply rails can range from +8 V to +40 V depending on the application. A single TTL-level clock source drives the two switching regulators that generate the required line voltage dynamically on a per-line basis. Six programmable states are available: low-power standby, disconnect, normal active, reverse polarity, ringing and line test. Binary fault detection is provided upon application of fault conditions or thermal overload. Remember: For Borscht functions (at least, for the ringing function), the device must interface with 90 V.

For short loop applications, the dual-channel SLIC device uses a dielectrically isolated, fully complementary bipolar technology specially developed for high-voltage applications like Borscht. The HV7 is a 150-V dielectrically isolated complementary high-speed bipolar process.

Legerity's flagship bipolar technology, HV7, has a higher operating voltage at a greater bandwidth than most other technologies.

Traditional analog technologies-even so-called complementary bipolars-are optimized around an NPN transistor, with the PNP having inferior characteristics, typically lower output impedance and a much lower bandwidth (sometimes two to three orders of magnitude lower). Because the NPN and PNP devices in HV7 complement each other, truly matched amplifiers are possible, which improves power and area efficiency. True push-pull amplifier configurations are available with matched transistors, and, as we know, amplifiers are the building blocks of all analog circuitry.

Dielectric isolation uses silicon-on-insulator and deep trench isolation, where all the devices are electrically insulated from one another. This produces lower cross talk between devices. There are other effects as well: No appreciable dc leakage current to substrate and a dramatically lower parasitic capacitance. This produces faster transistors, because they do not have to drive elevated parasitics. The actual capacitance is not voltage-dependent capacitance, which provides a more linear response to the base current).

Dielectric isolation also produces a much smaller device area since out-diffusion, a form of horizontal spreading, is not present. Thus, the bias points for devices are not limited by a substrate-to-junction potential. Components can operate from multiple positive or negative high-voltage rails.

HV7 Electrical Features

The biggest advantage of dielectrically isolated processes like HV7 is the high bandwidth they achieve. Using the above points coupled with optimized poly-emitter and dual-base devices, HV7 achieves 1-GHz transition frequencies (f_ts) on both the NPN and PNP. Older junction-isolation processes achieve 200 MHz on the NPN and 2 MHz on the PNP. The GHz-f_ts — combined with the high breakdown voltage — enables applications requiring high-speed signal paths.

HV7 has component breakdowns in excess of 150 V, placing it in a unique position in the worldwide technology space. A typical industry dielectrically isolated, high-bandwidth technology has component breakdowns of 12 V or less. HV7 is one of the few high-bandwidth bipolar technologies with higher than 75-V operation. Therefore, few real-world power interface circuits are possible with these technologies.

Frank Thiel is senior Fellow at Legerity Inc. (Austin, Texas).

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