STPM10 datasheet(19/47 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

DocID17728 Rev 5

19/47

Figure 14: First-

order ΣΔ A/D converter

7.4

Zero-crossing detection

The STPM10 has a zero-crossing detector circuit on the voltage channel which can be

used by application for synchronization of some utility equipment in the event of zero-

crossing of the line voltage. This circuit produces the internal signal ZCR which has a rising

edge every time the line voltage crosses zero, and a negative edge every time the voltage

reaches its positive or negative peak. The ZCR signal is then at twice the line voltage

frequency. The ZCR signal is available on the ZCR pin.

Figure 15: ZCR signal

7.5

Period and line voltage measurement

The period module measures the period of the base frequency of the voltage channel and

checks if the voltage signal frequency is within the fCLK/217 to fCLK/215 band. To do this, the

LIN signal is produced, which is low when the line voltage rises, and high when the line

voltage falls. This means that the LIN signal is the sign of dv/dt. With further elaboration,

the ZCR signal is also produced. On the trailing edge of LIN (line frequency) the period

counter starts counting up pulses of the fCLK/4 reference signal. The LIN signal is available

on the status bit register (see Table 10: "Status bit description" ). If the counted number of

pulses between two trailing edges of LIN is higher than 215, or if the counting is never

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