STPM11_08 datasheet(19/45 Pages) STMICROELECTRONICS

19 / 45 page

STPM11/12/13/14

Theory of operation

19/45

For example, if R1 = 783k

Ω 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

STPM1x is:

since the maximum voltage value applicable to the voltage channel input of STPM1x is

+0.3V, 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 is:

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 no longer present, another power source must be used in order to

provide the necessary supply to STPM1x and the other electronic components of the meter.

7.6

Power supply

The main STPM1X 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. The 100nF 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 STPM1X contains a Power-On-Reset (POR) detection circuit. If the VCC supply is less

than 2.5 V, then the STPM1X goes into an inactive state, all the functions are blocked

asserting and a reset condition is set. This is useful to ensure that the 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

supply voltages.

A BandGap voltage reference (VBG) of 1.23 V ±1% is used as reference voltage level

source for the two linear regulators and for the A/D converters. Also, this module produces

several bias currents and voltages for all other analog modules and for the OTP module. The

bandgap voltage temperature behavior can be changed in order to better compensate the

variation of sensor sensitivity with temperature. This task is performed with the BGTC

configuration bits.

RMS

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