AN4444 datasheet(56/68 Pages) STMICROELECTRONICS

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Design guidelines

AN4444

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DocID025925 Rev 2

An example of communication with realistic values can be used to understand the point.

Let's consider the following conditions:



Modulation: B-PSK uncoded



PHY payload: 256 bytes



Packet transmit time = 55 ms



Duty cycle = 30%

With these parameters, sending 100 messages will take less than 20 seconds. According to

the model of Figure 46, in these conditions the ZthJA will reach 15 °C/W, allowing maximum

dissipation PD(MAX) with an ambient temperature up to 50 °C.

If the ambient temperature is 70 °C, PD(MAX) would be manageable for 6 consecutive

transmissions.

10.3

Oscillator section

The ST7590 internal oscillator circuitry requires a crystal having a maximum load

capacitance of 20 pF and a maximum ESR of 100

. It is recommended to choose a quartz

crystal with overall tolerance not greater than ± 50 ppm (including aging and temperature) 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 protections



Mains coupling circuits



Any PCB track or via carrying a 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 could be a good practice to reduce the effect of radiated

signals on the oscillator.

10.4

Power supply

The power supply requirements for the ST7590 reference design are listed in Table 2 on

page 8.

Anyway, 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, thus minimizing the overall disturbance

amplitude.

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