MAXQ3181-RAN+ datasheet(53/84 Pages) MAXIM

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Low-Power, Active Energy, Polyphase AFE

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again, this bit cleared. If a zero crossing on phase A is

then followed by a zero crossing on phase C, then

phase B, this bit set to 1.

Power Calculation (Active and Apparent)

The power, energy, and RMS calculation process con-

sists of two tasks: continuous accumulation and post-

processing triggered every CYCNT line cycles. The

accumulation task accumulates raw data obtained from

the AFE during CYCNT line cycles. This task is per-

formed continuously in the background by the

MAXQ3181. When a CYCNT line cycles accumulation

stage has completed, which is determined by a dedi-

cated frame counter exceeding the NS level, the raw

integral accumulator values are saved for postprocess-

ing and cleared, beginning the next cycle of accumula-

tion task. Then, the DSP postprocessing is triggered to

process saved integrals and calculate energy, power,

etc., values. Note that the background accumulation

task continues while foreground postprocessing is tak-

ing place, i.e., both tasks are executed simultaneously

sharing CPU time. It is essential that the DSP postpro-

cessing calculations be completed before the next DSP

trigger to avoid losing accumulated data. The master

should allow enough processing time by adjusting the

R_ADCRATE register. Default settings provide plenty of

CPU time for both tasks.

The MAXQ3181 accumulates raw sums and calculates

line-cycle integrals for each voltage-current pair sepa-

rately. The individual power accumulators are:

• PA = VA x IA

• PB = IB x VB or -IB x VC or -IB x (VA + VC) or -IB x

(VA - VC)

• PC = VC x IC

The PA and PC accumulators always operate in a sin-

gle mode: (VA x IA) for the PA accumulator, (VC x IC) for

the PC accumulator. Alternately, the operating mode of

the PB accumulator is defined by setting the

CONCFG[1:0] bits in the OPMODE1 register.

Energy Accumulation Start Delay

All filters have a certain settling time before accurate

energy readings can be accumulated. To avoid accu-

mulation of invalid data from filters that are still settling,

an energy accumulation timeout period can be set in

the ACC_TIMO register. When ACC_TIMO > 0, comput-

ed energy is not accumulated for ACC_TIMO of DSP

cycles. The MAXQ3181 will decrement the ACC_TIMO

ACC_TIMO reaches 0 value, energy accumulation

begins (or resumes, if ACC_TIMO was set to nonzero

value by the master). Pulse outputs are also disabled

when ACC_TIMO > 0. The default value of ACC_TIMO

is 0x05.

No-Load Feature

To avoid “meter creep,” no energy accumulation should

take place when measured current is less than a cer-

tain threshold. The NOLOAD register can be pro-

grammed to enable and configure this feature. If the

measured X.IRMS value for a phase (A, B, or C) falls

below the NOLOAD threshold, the energy accumulators

for this phase are not incremented. Setting NOLOAD =

0 disables this feature. Full scale is represented by

0x10000.

On Demand Calculations

So far in this discussion, the values being calculated

and managed in the MAXQ3181 have been based on

fundamental units meaningful to the device itself: volt-

age as a binary fraction of full-scale voltage; current as

a binary fraction of full-scale current, and time as a non-

integer multiple of the ADC frame time.

But a practical electricity meter must report its results in

standard units, such as volts, amperes, and watts. The

MAXQ3181 contains a mechanism to convert the inter-

nal units (“meter units”) to real world units (“display

units”). This conversion is performed in the conversion

constant (CC) registers.

For some of these values (voltage, current) the calcula-

tion is simple: multiply by the conversion constant. For

other values (power, energy) the calculation is more

complex. In any case, the value in the CC register

affects only the conversion from a meter unit to a dis-

play unit; calibration is handled separately in the gain

adjustment registers for each recorded value.

The results of all on-demand calculations are reported

as 8-byte (64-bit) values of which no more than 6 bytes

(48 bits) are significant. Eight bytes are used as a com-

mon length; however, fewer bytes can be requested for

those registers known to have smaller maximum values.

For example, the power factor virtual register has a

maximum value that is expressed in only 3 bytes; con-

sequently, the register can be requested with a length

of 4 bytes without loss of data.

RMS Volts, RMS Amps

These registers (V.A, V.B, V.C, I.A, I.B, I.C) are calculat-

ed by simply multiplying the calculated RMS value

(A.VRMS, B.VRMS, C.VRMS, A.IRMS, B.IRMS, C.IRMS)

by the contents of the VOLT_CC or AMP_CC register.

Since the RMS voltage and RMS current are given in 32-

bit registers and the conversion coefficients are given in

16-bit registers, the result of the product is 48 bits.

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