AN4068 datasheet(47/63 Pages) STMICROELECTRONICS

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AN4068

Design guidelines

Doc ID 022923 Rev 2

47/63

An example of communication with realistic values can be used in order to understand the

point:

●

B-PSK: considering 64 bytes of payload per message, the time to transmit one

message is nearly 65 ms. Transmitting five messages per second, the duty cycle is

32.5%; at this rate, sending 100 messages takes 20 seconds. According to the model

in Figure 38, in these conditions the ZthJA reaches 16.0 °C/W, allowing maximum

dissipation PD(LIMIT) with an ambient temperature up to 66 °C.

9.3

Oscillator section

The ST7580 internal oscillator circuitry requires a crystal having a maximum load

capacitance of 20 pF and a maximum ESR of 100

Ω. It is recommended to choose a quartz

crystal with overall tolerance not greater than 150 ppm to ensure stability of carrier

frequency and digital timing.

It is very important to keep the crystal oscillator and the load capacitors as close as possible

to the device.

The resonant circuit must be far away from noise sources such as:

●

Power supply circuitry

●

Burst and surge protection

●

Mains coupling circuits

●

Any PCB track or via carrying an RF switching signal.

To properly shield and separate the oscillator section from the rest of the board, it is

recommended to use a ground plane on both sides of the PCB, filling all the area below the

crystal oscillator. No tracks or vias, except for the crystal connections, should cross the

ground plane.

Connecting the case to ground may be good practice in order to reduce the effect of

radiated signals on the oscillator.

9.4

Power supply

The power supply requirements for the ST7580 reference design are listed in Table 1.

However, the power supply circuit design is not only relevant in terms of available power.

Two points are particularly sensitive for a power line communication application:

●

The noise injected on the line

●

The input impedance of the power supply unit.

For the first point, a quasi-resonant switching mode power supply based on the ALTAIR04-

900 device has been chosen. This kind of switching controller spreads the switching

disturbances over a wide frequency range, therefore minimizing the overall disturbance

amplitude.

The second point involves the EMI input filter design. The suggested circuit in Figure 39 has

been designed to have minimum influence on the ST7580 line coupling circuit, in terms of

load impedance and linearity.

The 220 nF X2 capacitor has been put close to the bridge to avoid capacitive loading on the

ST7580 transmitted signal.

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