AN3213 datasheet(45/64 Pages) STMICROELECTRONICS

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AN3213

ST7570 demonstration board description

Doc ID 17429 Rev 1

45/64

The transmission output level VOUT rms of 2.5 V and the current limit IOUT rms(LIMIT) of 1 A,

fixed for the ST7570 demonstration board, correspond to a maximum output power POUT of

2.5 W over a 2.5

Ω load. In these conditions, the required dissipation results as follows:

Equation 14

Referring to the relationship between dissipated power and temperature, it can be proved

that in a continuous transmission, i.e. with ZthJA at its steady-state value of 50 °C/W, with an

ambient temperature of 25 °C, the maximum dissipation can be 2 W. However, controlling

the transmission duty cycle and total duration it becomes possible to get higher dissipation.

A real IEC61334-5-1 standard protocol condition can be used as an example (References

5):

●

Maximum length MAC frame with alarms: at 2400 bps and 50 Hz mains frequency, the

packet time tPKT is 150 ms, the duty cycle d is 100 % and the transmission duration tTX

is 1 second, corresponding to seven MAC sub-frames. According to Figure 39, in these

conditions, ZthJA would reach a maximum value of 16 °C/W only. This allows the power

dissipation PD to reach 2.5 W over all the ambient temperature range of the ST7570,

while PD(LIMIT) may be reached with an ambient temperature up to 66 °C.

7.5

Oscillator section

The ST7570 crystal oscillator circuitry requires a crystal having a maximum load

capacitance of 20 pF and a maximum ESR of 100

Ω.

Moreover, the following requirements impacting on the quartz crystal choice are typically set

by ST7570 metering applications:

●

Communication frequencies: f0 = 74 kHz, f1 = 63.3 kHz with ±0.5 % tolerance

●

Delay between mains zero-crossing and the beginning of the communication window:

120 µs, with tolerance equal to ± 20 µs.

Figure 41 shows the carrier frequency deviation vs. the quartz crystal tolerance, simulated

through a waveform generator applying a square wave with frequency values around 8 MHz:

AN3213 Datasheet(PDF) 45 Page - STMicroelectronics