MAXQ3180 Maxim, MAXQ3180 Datasheet - Page 60

• Manufacturer Part Number: MAXQ3180
• Description: The MAXQ3180 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, power, energy, and many other metering and power-quality parameters of a polyphase load
• Manufacturer: Maxim

Low-Power, Multifunction, Polyphase AFE
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 MAXQ3180 accumulates raw sums and calculates
line-cycle integrals for each voltage-current pair sepa-
rately. The individual power accumulators are:
• PA = V
• PB = I
• PC = V
The PA and PC accumulators always operate in a sin-
gle mode: (V
the PC accumulator. Alternately, the operating mode of
the PB accumulator is defined by setting the
CONCFG[1:0] bits in the OPMODE1 register.
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 MAXQ3180 will decrement the ACC_TIMO
register every DSP cycle until it becomes 0. When
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.
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.
60
(V
A
______________________________________________________________________________________
- V
B
C
A
C
)
x V
x I
x I
A
Energy Accumulation Start Delay
A
C
B
x I
or -I
A
) for the PA accumulator, (V
B
x V
C
or -I
B
No-Load Feature
x (V
A
+ V
C
C
) or -I
x I
C
) for
B
x
So far in this discussion, the values being calculated
and managed in the MAXQ3180 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
MAXQ3180 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.
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.
Regardless of the internal units used, VOLT_CC and
AMP_CC can be tailored so that the LSB of the virtual
register can be any value. For example, if one wished
to have a 32-bit value representing milliamps, one
could multiply by a value that scaled the register such
that the LSB was 2
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.
-16
mA. Then, discard the low-order
On Demand Calculations
RMS Volts, RMS Amps