STPM32 datasheet(34/89 Pages) STMICROELECTRONICS

34 / 89 page

R= x2R

(7)

By assumption, the feedback signal R is DC type and therefore, it can be extracted from the average operation

and the above equation can be rearranged into:

Equation 8

R2= x2

(8)

By a square-root operation on both sides of previous equation we get:

Equation 9

R= x2

(9)

which is RMS value exact definition.

With an AC input signal:

Equation 10

x=xt =Asinωt

(10)

x2=A2sin2ωt =A21−cos2ωt2

The LP filter cuts the 2nd harmonic component of input signal multiplying it by a dumping factor α:

Equation 11

R=A 1−αcos2ωt2 A2 1−α2cosωt

(11)

R result is a DC signal plus the 2nd harmonic ripple with the amplitude of α/2. For dumping factor | α |<<1:

Equation 12

R A2

(12)

RMS updated values are available in DSP_REG14 and DSP_REG15 registers every 128 μs.

Raw ADC samples are also available for post-processing by MCU in registers from DSP_REG2 to DSP_REG9.

By taking into account the internal parameters in Table 12 and the analog front end components in Table 13, LSB

values of voltage and current registers are the following:

Table 14. STPM3x current voltage LSB values

Parameter

Value

Voltage RMS LSB value

LSBVRMS= Vref∙ 1+R1R2

calV∙AV∙215 V

Current RMS LSB value

LSBIRMS= Vref

calI∙AI∙kS∙kint∙217A

Instantaneous voltage normalized

vn Vnorm= −1 ∙223∙vn 23−vn 22:0

223

Instantaneous current normalized

in Inorm= −1∙223∙in 23−in 22:0

223

Instantaneous voltage LSB value

LSBVMOM= Vref∙ 1+R1R2

calV∙AV∙223 V

Instantaneous current LSB value

LSBIMOM= Vref

calI∙AI∙kS∙kint∙223A

8.4.8

Zero-crossing signal

Zero-crossing signals of voltage and current come from fundamental values of voltage and current and output

from LPF filter. Resolution of the zero-crossing signal is 8 μs given by FCLK clock = 125 kHz.

STPM32, STPM33, STPM34

Functional description of hardwired DSP

DS10272 - Rev 13

page 34/89

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