STPM32 datasheet(56/89 Pages) STMICROELECTRONICS

56 / 89 page

LPWx

LED_PWM

CP [imp/kWh]

LED at PNom [Hz]

Pulse value [Ws]

1000

4000

1.28

900

1001

2000

0.64

1800

1010

1000

0.32

3600

1011

128

500

0.16

7200

1100

256

250

0.08

14400

1101

512

125

0.04

28800

1110

1024

62.5

0.02

57600

1111

2048

31.25

0.01

115200

The closer value to desired CP is given by setting LPWx divider to 1010.

Any tolerance producing small variation of CP from 1000 imp/kWh can be compensated by calibration: setting

CHV and CHC bits.

9.2

Application calibration

The meter has to be calibrated so to compensate external component tolerances and internal VREF possible drift.

After the calibration, a meter using the STPM3x can reach IEC class 0.2 accuracy, taking into account that the

component choice follows the rules explained above, and the layout and signal routing minimize the noise

capture.

9.2.1

Voltage and current calibration (CHVx, CHCx bits)

Thanks to the device internal architecture and linearity, all calculated values (RMS, energies and powers) can be

calibrated in a single point, just calibrating voltage and current streams.

For this purpose, a known nominal voltage VN and current IN must be applied to the meter under calibration.

Referring to Section 9.1 and Section 5, having R1 or kS calculated as stated in the previous section, the target

values of voltage and current RMS registers, XV and XI respectively are calculated as follows:

Table 36. Calibration target values

Parameter

Value

Voltage register value at VN

XV=VN∙AV∙calV∙215

Vref∙ 1+R1R2

Current register value at IN

XI=IN∙AI∙calI∙kint∙kS∙217

Vref

Note:

For the above calculation, the calculated value of the component kS or R1 (accordingto the chosen design

method) must be used; the difference of the real component is compensated by calibration as a tolerance.

To start calibration, the device has to be programmed with the proper gain and current sensor; moreover, to obtain

the greatest correction dynamic, calibrators are initially set in the middle of their range (0x800), thus obtaining a

calibration range of ± 12.5% per voltage or current channel.

After applying VN and current IN to the meter, a certain number of voltage and current RMS samples must be read

and averaged (please, refer to averaged register values as VAV and IAV) to calculate voltage and channel

calibrators as follows:

Table 37. Calibrator calculation

Parameter

Value

Calibrator value

CHV=14336∙ XVVAV−12288

CHC=14336∙ XIIAV−12288

STPM32, STPM33, STPM34

Application calibration

DS10272 - Rev 13

page 56/89

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