STPM10 datasheet(21/51 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

Doc ID 17728 Rev 3

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energy values is greater than a certain percentage of the averaged energy during the

activated tamper module (see Equation 1).

This percentage value can be selected between two different values (12.5 % and 6.25 %)

according to the value of the configuration bit CRIT.

The tamper condition is detected when the following formula is satisfied:

Equation 1

EnergyCH1 - EnergyCH2 > KCRIT (EnergyCH1 + EnergyCH2)/2;

where KCRIT can be 12.5 % or 6.25 %.

The detection threshold is much higher than the accuracy difference of the current channels,

which should be less than 0.2 %, but, some headroom should be left for possible transition

effect, due to accidental synchronism of actual load current change with the rhythm of taking

the energy samples.

The tamper circuit works if the energies associated with the two current channels are both

positive or negative, if the two energies have different sign, the tamper is on all the time

however, the channel with the associated higher power is selected for the final computation

of energy.

When internal signals are not good enough to perform the calculations, i.e. line period is out

or range or

ΔΣ signals from analog section are stacked at high or low logic level, or no load

condition is activated, the tamper module is disabled and its state is preset to normal.

7.9.1

Detailed operational description

The meter is initially set to normal state, i.e. tamper not detected.In this condition the

primary channel is selected for final integration of energy. In such state the values of both

load currents should not differ more than the accuracy difference of the channels does.

Sixty-four periods of line voltage is used as a tamper checking period.

After 24 periods of line voltage two internal signals MUX and INH are changed in order to

enable secondary current channel and to freeze the last power and RMS values of primary

current channel. The following 16 periods of line frequency are used for tamper detection

integration. During this gap, the final energy calculation does not use the signal from

selected channel but the frozen values.

Four line periods after the INH switch, the integration of power from secondary current

channel is started and lasts four periods. Additional four line periods later MUX signal is

switched back to primary current channel and the integration for tamper detection is started.

The timings of MUX and INH signals are shown in Figure 18 below.

Figure 18.

Timings of tamper module - Primary channel selected

MUX

Ch 1Ch 2Ch 1Ch 1

INH

Tamper power integrators

Cycles

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