STPM32 datasheet(81/121 Pages) STMICROELECTRONICS

81 / 121 page

DS10272 Rev 8

81/121

STPM32, STPM33, STPM34

Application design and calibration

121

To set the desired LED pulse output, division factor LED_PWM can be set through

LPWx[3:0] bits in DSP_CR1 and DSP_CR2 configuration registers.

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 power) 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.

Table 36. LPWx bits, Cp, LED frequency relationships

LPWx

LED_PWM

CP [imp/kWh]

LED at PNom [Hz] Pulse value [Ws]

0000

0.0625

1024000

327.11

3.52

0001

0.125

512000

163.56

7.03

0010

0.25

256000

81.78

14.06

0011

0.5

128000

40.89

28.13

0100

64000

20.44

56.25

0101

32000

10.22

112.50

0110

16000

5.11

225

0111

8000

2.56

450

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

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