STPM34 datasheet(55/89 Pages) STMICROELECTRONICS

55 / 89 page

This example shows the correct dimensioning of a meter using a current transformer having the following

specification:

Table 33. Example 1 design data

Parameter

Value

VN nominal voltage

230 VRMS

IN nominal current

5 ARMS

IMax maximum current

40 ARMS

CP constant pulses

1000 imp/kWh

The dimension of the voltage channel considers the voltage divider resistor values as 770 kΩ and 470 Ω.

Setting CP= 64000 pulses/kWh (at LPWx = 1 - device default value) and according to calculation above the

following values are:

Table 34. Example 1 calculated data

Parameter

Value

Current sensor sensitivity

kS= Vref2∙CP∙ 1+R1R2

1800∙AV∙AI∙calV∙calI∙DClk=3.51mV/A

LED frequency at PN

LEDf=CP∙VN∙IN

3600000 =20.44Hz

VMAX

VMAX=12∙ VrefAV∙2∙R1+R2R2=347.8V

IMAX

IMAX=12∙ VrefAI∙2∙1kS=60.5A

IMIN

IMIN= Vref

AI∙calI∙kint∙kS∙215=5.97mA

LSBP

LSBP= Vref2∙ 1+R1R2

kint∙AV∙AI∙kS∙calV∙calI∙228=0.818mW/LSB

LSBE

LSBE= Vref2∙ 1+R1R2

3600∙DClk∙kint∙AV∙AI∙kS∙calV∙calI∙217=0.214mWs/LSB

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.

Table 35. 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

STPM32, STPM33, STPM34

Application design

DS10272 - Rev 13

page 55/89

STPM34 Datasheet(PDF) 55 Page - STMicroelectronics

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