Datasheet

STPM10 Theory of operation
Doc ID 17728 Rev 4 39/53
In case of a precharge 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 a
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 - 5]
the resulting signal, together with the pre-processed and digitalized current signal:
Equation 8
i(t) = I
⋅ sin(ωt + ϕ); [see Figure 26 - 6]
are then available for the calculation process. These digital signals are also provided to two
additional stages, which then integrate of themselves, obtaining: