STPM10 datasheet(36/47 Pages) STMICROELECTRONICS

36 / 47 page

Theory of operation

STPM10

36/47

DocID17728 Rev 5

1. Disable the SPI module

2. Set the SDI pin, which is connected to SDA as an output

3. Activate SYN first, and then SCS

4. Apply a new bit value to SDI, and activate SCL

5. Deactivate SCL

6. Repeat the previous two steps seven times to complete a one byte transfer

7. Repeat the previous three steps for any remaining byte transfers

8. Set the SDI pin as an input

9. Deactivate SCS and SYN

10. Enable the SPI module

In case of a pre-charge command (0xFF), the emulation above is not necessary. Due to the

pull-up device on the SDA pin of the STPM10, the processor needs to perform the following

steps:

1. Activate SYN first in order to latch the results

2. After at least 1 μs, activate SCS

3. Write one byte to the transmitter of SPI (this produces 8 pulses on SCL with SDI=1)

4. Deactivate SYN

5. Optionally read the data records (the sequence of reading is altered)

6. Deactivate SCS

7.23

Energy calculation algorithm

Within the STPM10, the computing section of the measured active power uses a

completely new patented signal processing approach. This approach allows the device to

reach high level of performance in terms of accuracy. The signals, coming from the

sensors, for the instantaneous voltage are calculated as follows:

Equation 5:

v(t) = V·sin ωt

where V is the peak voltage and ω is related to the line frequency.

The instantaneous current is calculated using:

Equation 6:

i(t) = I· sin (ωt + ϕ) where I is the peak current, ω is related to the line frequency and ϕ is

the phase difference between voltage and current.

7.23.1

Active power

In the STPM10, after the pre-conditioning and the A/D conversion, the digital voltage signal

(which is dynamically more stable with respect to the current signal) is processed by a

differentiator stage which transforms:

Equation 7:

v(t) → v’(t) = dv/dt = V

⋅ ω ⋅ cos ωt

See Figure 26: "Active energy computation diagram" (5)

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