STPM10 datasheet(20/51 Pages) STMICROELECTRONICS

20 / 51 page

Theory of operation

STPM10

20/51

Doc ID 17728 Rev 3

7.7

Load monitoring

The STPM10 includes a no-load condition detection circuit with adjustable threshold. This

circuit monitors the voltage and the current channels and, when the measured voltage is

below the set threshold, the internal signal BIL becomes high. Information about this signal

is also available in the status bit BIL.

The no-load condition occurs when the product of the VRMS and IRMS register values is

below a given value. This value can be set with the LTCH configuration bits. Four different

no-load threshold values can be chosen according to the two configuration bits LTCH (see

Table 8).

When a no-load condition occurs (BIL=1) the integration of power is suspended and the

tamper module is disabled. The BIL signal can be accessed only through the SPI interface.

7.8

Error detection

In addition to the no-load condition and the line frequency band, the integration of power can

also be suspended due to an error detected on the source signals.

There are two kinds of error-detection circuits involved. The first checks all the

Σ Δ signals

from the analog part if any is stacked at 1 or 0 within the 1/128 of fCLK period of observation.

In case of a detected error, the corresponding

Σ Δ signal is replaced with an idle Σ Δ signal,

which represents a constant value of 0. All error and other resolved flags are treated as bits

of a device status and can be read out by means of the SPI interface.

Another error condition occurs if LED pin output signals are different from the internal

signals that drive them. This can occur if some of these pins are forced to GND or to some

other imposed voltage value. In this case, the internal status bit PIN is activated, providing

the information that some hardware problem has been detected.

7.9

Tamper detection module

The STPM10 is able to measure the current in both live and neutral wire with a time domain

multiplexing approach on a unique sigma delta modulator. This mechanism is adopted to

implement anti-tamper function. If this function is selected (see Table 7), the live and neutral

wire currents are monitored; when the difference between the two measurements exceeds a

rated threshold the STPM10 enters the "tamper state", while in "normal state" the two

measurements are below the threshold.

In particular, both channels are not observed all the time, rather a time multiplex mechanism

is used. During the observation time of each channel, its active energy is calculated. A

tamper condition occurs when the absolute value of the difference between the two active

Table 8.

No-load detection thresholds

LTCH

KLTCH

0800

1600

3200

6400

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