STPM10 datasheet(26/47 Pages) STMICROELECTRONICS

26 / 47 page

Theory of operation

STPM10

26/47

DocID17728 Rev 5

case in which the LED pin is configured to be driven from internal signal AW (active

energy) whose frequency is proportional to the active energy. The signal AW is taken from

the 11th bit of the active energy register, and consequently a relationship between the LSB

value of the active energy register and the number of pulses provided per each kWh (P)

can be defined as:

Equation 2:

�� =

1000

211 ⋅ ��

[��ℎ]

Due to the innovative and proprietary power calculation algorithm, the frequency signal is

not affected by any ripple at twice the line frequency. This feature strongly reduces the

calibration time of the meter. In a practical example where the desired P is 64000

pulses/kWh (=17.7 Hz*kW), we have:

Equation 3:

kAW = 7.63*10-6 Wh

This means that the reading of 0x00001 in the active energy register represents 7.63 μWh,

while 0xFFFFF represents 8 Wh. The LED pin can be driven from AW (active energy wide

band), AF (active energy limited at fundamental), RW (reactive energy) or SW (apparent

energy) according to the value of the KMOT bit. In this case, since the LED pin is driven by

different signals from that of AW, some other relationship between the LSB of the register

and the number of pulses per kWh provided by the meter (P) must be defined as follows:

Equation 4:

kAF = 4*kAW [Wh]

kRW = 2*kAW [VARh]

kSW = kAW [VAh]

Table 9: LED pin configuration

KMOT (2 bits)

Signal available on LED pin

Number of pulses

AW Type0*

P [kWh]

AF Type1*

P [kWh]

P [kVARh]

P [kVAh]

7.15

Status bit

The STPM10 includes 8 status bits, which provide information about the current status of

the meter. The status bits are the following:

Table 10: Status bit description

Bit #

Name

Description

Conditions

BIL

No-load condition

BIL = 0: no-load condition not detected

BIL = 1: no-load condition detected

BCF

ΣΔ signals status

BCF = 0: ΣΔ signals active

BCF = 1: one or both ΣΔ signals are stacked

BFR

Line frequency

BFR = 0: line frequency within the 45 Hz - 65 Hz range

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