STPM01FTR datasheet(20/56 Pages) STMICROELECTRONICS

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Theory of operation

STPM01

20/56

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 will be 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 = 783kΩ and R2 = 475Ω are used as resistor divider when the line

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

STPM01 will be:

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

+0.3V, the equivalent maximum line voltage applicable will be:

VPK = R1+R2/R2 • 0.3 = 494.82

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

computation will be:

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 (3V) and for the digital part VDDD (1.5V). The

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

should be connected between VCC and VSS, VDDA and VSS, VDDD and VSS. All these

capacitors must be located very close to the device.

The STPM01 contains a Power On Reset (POR) detection circuit. If the VCC supply is less

than 2.5V then the STPM01 goes into an inactive state, all the functions are blocked

asserting a reset condition. This is useful to ensure correct device operation at power-up

and during power-down. The power supply monitor has built-in hysteresis and filtering,

which give a high degree of immunity to false triggering due to noisy supplies.

Table 10.

Nominal voltage values

NOM

KNOM

0.3594

0.3906

0.4219

0.4531

RMS

⋅

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