Datasheet

Theory of operation STPM10
22/53 Doc ID 17728 Rev 4
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
Tabl e 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 f
CLK
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 Ta b l e 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
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:
Table 8. No-load detection thresholds
LTCH K
LTCH
0800
11600
23200
36400