Figure 7-8 Power-Line Communication Driver

The OPA561 is an excellent choice to drive ac power lines for low-speed communication applications. The device provides an easily implemented, reliable option that is superior to discrete power transistor circuits. Advantages include:

1. Fully integrated device
2. Integrated shutdown circuitry for send-and-receive switching
3. Thermal shutdown
4. Adjustable current limit
5. Shutdown flag
6. Power savings
7. Small PowerPAD integrated circuit package

Typically such a system consists of a microcontroller, a modem IC and the power-line interface circuitry. See Figure 7-8 for the half-duplex power line communication system.

The system uses a synchronous FSK-modem, capable of 600-baud and 1200-baud data rates, and supports two different FSK channels in the 60‑kHz to 80‑kHz range. A microcontroller such as the MSP430™ MCU is used to control the modem IC.

The OPA561 analog interface circuitry drives the FSK modem signals on the ac power line. The circuitry filters the transmit signal (ATO) from the ST7536 to suppress the 2nd-harmonic distortion of the transmit signal. The circuitry also amplifies the ATO signal and provides the very low output impedance necessary to properly drive the line. The impedance of a typical power line at 70‑kHz ranges from 1 Ω to 100 Ω. The OPA561 is an excellent choice for this type of load. The transformer provides isolation and additional filtering. C9 prevents 50‑Hz to 60‑Hz current from flowing in the transformer. Choose this capacitor carefully for proper voltage rating and safety characteristics.

The receive input signal is amplified (G = 100) and applied to the modem IC. The OPA561 is disabled in receive mode to avoid loading the line.