MAXQ3181-RAN+ datasheet(54/84 Pages) MAXIM

54 / 84 page

Low-Power, Active Energy, Polyphase AFE

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Regardless of the internal units used, VOLT_CC and

AMP_CC can be tailored so that the LSB of the virtual

to have a 32-bit value representing milliamps, one

could multiply by a value that scaled the register such

that the LSB was 2-16mA. Then, discard the low-order

16 bits. The result is milliamps with 32 bits of precision;

thus, the maximum current that could be represented

would be 4,294,967,296mA, or just over 4MA.

The VOLT_CC and AMP_CC values can be calculated

from the full-scale voltage or full-scale current and the

desired value of one LSB in the display register:

Example: Assume the full-scale current is 102.4A, and

that we desire a 1nA LSB. The calculation would pro-

vide an AMP_CC value of:

102.4/(224 x 10-9) = 6104 = 0x17D8

Power

The MAXQ3181 measures energy. But power is just

energy per unit time, and the MAXQ3181 keeps track of

the time unit over which energy is accumulated. This is

simply the NS value, the fractional number of samples

that comprises one DSP cycle. So converting energy to

power is as simple as dividing the accumulated energy

over one DSP cycle by NS. Multiplying by a conversion

constant (PWR_CC) gives power in user-established

units.

The power registers (PWRP.A, PWRP.B, PWRP.C,

PWRS.A, PWRS.B, PWRS.C) are calculated by multiply-

ing the accumulated energy (A.ACT, A.APP, B.ACT,

B.APP, C.ACT, C.APP) by the conversion coefficient

(PWR_CC) and then dividing by NS. The result is the

48-bit average power over the most recent DSP cycle,

in units established by the conversion coefficient.

The PWR_CC value can be calculated from the full-

scale voltage, the full-scale current, and the desired

value of one LSB in the display register:

Example: For this example, assume the full-scale cur-

rent is 102.4A, the full-scale voltage is 558.1V, and that

the desired LSB is milliwatts after discarding the 16

LSB; that is, the desired LSB is 2-16 milliwatts. Perform

the following calculation:

102.4 x 558.1/(232 x 2-16 x 10-3) = 872 = 0x0368

Power Factor

Power factor is calculated as real power divided by

apparent power. Apparent power is computed as the

product of the RMS voltage and current measurement.

The power factor is multiplied by 214 before it is report-

ed; thus, unity power factor is given by 16,384 decimal

(0x4000).

Line Frequency

The line frequency is derived directly from the mean NS

values over the three phases. It is reported as millihertz;

thus, a 50Hz line frequency is reported as decimal

50,000 (0xC350).

Energy

Energy is read as the net energy directly scaled from

the appropriate registers. For example, the energy read

from the ENRP.A register (real energy, phase A) is

composed of the difference between the A.EAPOS (real

energy, positive direction, phase A) and A.EANEG (real

energy, negative direction, phase A) registers scaled

by the ENR_CC register.

Note that the energy registers (ENRP.A, ENRP.B,

ENRP.C, ENRP.T, ENRS.A, ENRS.B, ENRS.C, ENRS.T)

represent the energy, in every case, since the last over-

flow event. For this reason, software must keep track of

overflow and make adjustments accordingly when using

this register set.

To calculate the ENR_CC register value, begin with the

full-scale voltage and full-scale current, the frame time,

and the desired LSB value for energy. Then perform the

following calculation:

Example: It is essential to ensure that the correct units

are maintained throughout the calculation. In this exam-

ple, assume that the full-scale voltage is 558.1V, the

full-scale current is 102.4A, the frame time is the default

of 320μs, and the desired LSB is 100 milliwatt-hours

after the 32 bits are discarded; that is, the LSB is 0.1 x

2-32 watt-hours. Notice, however, that the frame time is

given in microseconds

and must be converted to hours

before the calculation can be performed: 320μs is 88.9

x 10-9 hours. So the calculation proceeds as follows:

102.4 x 558.1 x 88.9 x 10-9/(216 x 0.1 x 2-32) = 3329 =

0x0D01

ENR CC

ENR LSB

××

216

PWR CC

PWR LSB

2 32

VOLT CC

VOLT LSB

2 24

AMP CC

AMP LSB

2 24

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