STPM32 datasheet(19/89 Pages) STMICROELECTRONICS

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Theory of operation

8.1

General operation description

The STPM3x product family measures up to two line voltages and two line currents to perform active, reactive

and apparent power and energy, RMS and instantaneous values, and line frequency information measurement of

a single, split or poly-phase metering system.

The STPM3x generates up to two independent train pulse output signals proportional to the active, reactive,

apparent or cumulative power. It also generates up to two programmable interrupt output signals.

The internal register map and the configuration registers can be accessed by SPI or UART interface.

The STPM3x converts analog signals, through four independent channels in parallel via sigma-delta analog-to-

digital converters, into a binary stream of sigma-delta signals with the appropriate not overlapped control signal

generator.

This technique fits to measure electrical line parameters (voltage and current) via analog signals from voltage

sensors and current sensors (inductive Rogowski coil, current transformer or shunt resistors). Current channel

inputs are connected, through external anti-aliasing RC filter, to a Rogowski coil or current transformer (CT) or

shunt current sensor which converts line current into the appropriate voltage signal. Each current channel

includes a low-noise voltage preamplifier with a programmable gain. Voltage channels are connected to a line

voltage modulator (ADC). All channels have quiescent zero signal point on GND, so the STPM3x samples

differential signals on both channels with their zero point around GND.

The converted sigma-delta signals feed an internal decimation filter stage that decimates 4 MHz bitstreams of a

factor 512 allowing a 3.6 kHz bandwidth at -3 dB. The 24-bit voltage and current data feed an internal

configurable filtering block and the hardwired DSP that performs the final computation of metrology quantities.

The STPM3x also includes two programmable temperature compensated bandgap reference voltage generators

and low drop supply voltage regulator. All reference voltages are designed to eliminate the channel crosstalk.

The mode of operation and configuration of the device can be selected by dedicated configuration registers.

8.2

Functional description of the analog part

The analog part of the STPM3x consists of the following sections:

•

Power management section:

–

Reference voltage generators with programmable independent temperature compensation

–

+3 V low drop supply voltage regulator

–

+1.2 V low drop supply voltage regulator

•

Analog front end section:

–

Preamplifiers in the two current channels

–

2nd order sigma-delta modulators

•

Clock generator

•

Power-on-reset (POR)

8.2.1

Power management section

Supply pins for the analog part are: VCC, VDDA, VDDD and GND. GND pins represent the reference point.

VCC pin is the power supply input namely +3.3 V to GND_REG, it has to be connected to GND_REG via a 1 μF

capacitor.

VDDA and VDDD are analog output pins of internal +3.0 V and +1.2 V low drop voltage regulators.

At least 1 μF capacitor should be connected between VDDA and GNDA. At least 1 μF (better 4.7 μF) capacitor

should be connected between VDDD and GNDD. The input of the mentioned regulators is VCC.

There are two voltage references embedded in the STPM33 and STPM34, while the STPM32 embeds a single

reference.

It is possible to switch off each reference voltage and each voltage or current channel independently for power

saving purpose.

EN_REF1 and EN_REF2 bits in DSP_CR1 and DSP_CR2 switch on/off the voltage reference.

STPM32, STPM33, STPM34

Theory of operation

DS10272 - Rev 13

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