STPM10 datasheet(41/47 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

DocID17728 Rev 5

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Calibrator value

KV = KI = 0.875

CI = CV = 128

In this way, KV and KI are tuned to obtain a precise measurement: for example CV = 0

generates a correction factor of -12.5% (KV = 0.75) and CV = 255 determines a correction

factor of +12.5% (KV = 1), and so on.

Based on the above, the following formulas relating to KV,I and CV,I are obtained:

KV,I = (CV,I / 128) · 0.125 + 0.75

CV,I = 1024 · KV,I - 768

The calibration procedure outputs CV and CI values, which allow the above power

sensitivity of the meter. This sensitivity is used to calculate target frequency at the LED pin

for nominal voltage and current values:

XF = f · 64

with:

f = PM · In · Vn / 3600000

From the values above and for both chosen amplification factor AI = 32 and initial

calibration data, the following target values can be calculated:

Target RMS reading for a given In:

XI = In · KS · AI · KI · GINT · GDF · GDIF · BI / (VBG · 1000) = 1573

Target RMS reading for a given Vn:

XV = f · BV · BI · DUD / (fM · XI) = 852

The output of the voltage divider is then:

VDIV = (XV · VBG)/ (2 · GDIF · AV · KV · GDF · GINT · BV) = 145.6 mV

Choosing R2

= 500 Ω (connected between VI and VSS), the R1 resistor (connected between

VLINE and VIP) value is obtained:

R1 = R2 · (Vn - VDIV) / VDIV

= 789.3 Ω

Indicating, with IA and VA, the real readings on the STPM10 RMS voltage and current

registers, and with XI and XV ideal values of RMS current and voltage readings already

calculated, the final values for calibrators can be calculated as:

XV = (Kv · VA) / 0.875

XI = (Ki · IA) / 0.875

If the computed final calibration data falls out of the calibration data range, the energy

meter should be recognized as bad, or the given presumptions and calculations above

should be checked. Otherwise, if the final data of the calibrators is written into the energy

meter, the RMS readings should be very close to the target values I and V, and the

frequency of the LED output should be very close to the target value f.

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