STPM10 datasheet(43/51 Pages) STMICROELECTRONICS

43 / 51 page

STPM10

Theory of operation

Doc ID 17728 Rev 3

43/51

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 would fall 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 would be 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.

STPM10 数据表(PDF) 43 Page - STMicroelectronics