STPM01 datasheet(22/60 Pages) STMICROELECTRONICS

22 / 60 page

Theory of operation

STPM01

22/60

Doc ID 10853 Rev 8

for tampering purposes or in case the line voltage is very stable, it is possible to use a

predefined value for computing the energy without sensing it.

In order to enable the SWM mode, the STPM01 must be configured with PST values of 4 or

5, (tamper enabled-Rogowsky coils). In this way, if the BFR error is detected, STPM01

enters in SWM. If BFR is cleared the energy calculation is performed normally, when BFR is

set (no voltage information is available) the energy computation is carried out using a

nominal voltage value according to the NOM configuration bits.

Since there is no more information on the phase shift between voltage and current, the

apparent rather than active power is used for tamper and energy computation. The

calculated apparent energy is the product between IRMS (effectively measured) and an

equivalent VRMS that can be calculated as follows:

VRMS = VPK*KNOM,

where VPK represents the maximum line voltage reading of the STPM01 and KNOM is a

coefficient that changes according to the following table:

For example, if a R1 = 783 kΩ and R2 = 475 Ω are used as resistor divider when the line

voltage is present, the positive voltage present at the input of the voltage channel of

STPM01 is:

since the maximum voltage value applicable to the voltage channel input of STPM01 is +0.3

V, the equivalent maximum line voltage applicable is:

VPK = R1+R2/R2 • 0.3 = 494.82

considering the case of NOM=2, the correspondent RMS values used for energy

computation are:

VRMS = VPK • 0.4219 = 208.76 [V]

Usually the supply voltage for the electronic meter is taken from the line voltage, in SWM,

since the line voltage is not present any more, some other power source must be used in

order to provide the necessary supply to STPM01 and the other electronic components of

the meter.

8.7

Power supply

The main STPM01 supply pin is the VCC pin. From the VCC pin two linear regulators provide

the necessary voltage for the analog part VDDA (3 V) and for the digital part VDDD (1.5 V).

The VSS pin represents the reference point for all the internal signals. 100 nF low ESR

Table 10.

Nominal voltage values

NOM

KNOM

00.3594

10.3906

20.4219

30.4531

RMS

⋅

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