Datasheet

MAX71020
Single-Chip Electricity Meter AFE
28Maxim Integrated
Table 9. CE EQU Equations and Element Input Mapping
*Not supported by standard CE codes.
Table 10. CE Raw Data Access Locations
Constants
Constants used in the CE Data Memory tables are:
U f
0
is the fundamental frequency of the mains phases.
U I
MAX
is the external RMS current corresponding to the
maximum allowed voltage on the current inputs. For
the IB input, this is 250mV peak (176.8mV
RMS
). In the
MAX71020, IA normally has a preamplifier enabled
on the IA inputs, so I
MAX
needs to be adjusted to
27.78mV peak (19.64mV
RMS
) for the IAP-IAN inputs.
For a 250µΩ shunt resistor, I
MAX
becomes 78A
(19.64mV
RMS
/250µΩ = 78.57A) for IA, and 707A
(176.8mV
RMS
/250µΩ = 707.2A
RMS
) for IB.
U V
MAX
is the external RMS voltage corresponding to
250mV peak at the VA and VB inputs.
U N
ACC
, the accumulation count for energy measure-
ments (typically 2520).
U The duration of the accumulation interval for energy
measurements is N
ACC
/F
S
= 2520/2,520.6 ≈ 1s.
U X is a gain constant of the pulse generators. Its value
is determined by PULSE_FAST and PULSE_SLOW.
U Voltage LSB (for sag threshold) = V
MAX
x 7.879810 - 9V.
The system constants I
MAX
and V
MAX
are used by the
host controller to convert internal digital quantities (as
used by the CE) to external, real-world metering quantities.
Their values are determined by the scaling of the voltage
and current sensors used in an actual meter. The LSB
values used in this document relate digital quantities at the
CE or MPU interface to external meter input quantities. For
example, if a SAG threshold of 80V
RMS
is desired at the
meter input, the digital value that should be programmed
into SAG_THR (register 0x024) would be 80V
RMS
x
SQRT(2)/SAG_THR
LSB
, where SAG_THR
LSB
is the LSB
value in the description of SAG_THR (see Table 12).
Environment
Default settings are assumed to be V
MAX
= 600V, I
MAX
=
707A, and kH = 1.
CE Calculations
In Table 9, the Load Engine selects the desired equation
by writing to the METER_CFG register.
CE Front-End Data (Raw Data)
Access to the raw data provided by the AFE is possible
by reading registers 0x100–0x003 as shown in Table 10.
CE Status and Control
The CE Status Word, CESTATUS, is useful for generating
early warnings to the host controller (Table 7). It con-
tains sag warnings for phase A and B, as well as F0, the
derived clock operating at the line fre quency. The host
controller can read the CE status word at every CE_BUSY
interrupt.
CESTATUS provides information about the status of volt-
age and input AC signal frequency, which are useful for
generating an early power-fail warning to initiate neces-
sary data storage. CESTATUS represents the status flags
for the preceding CE code pass (CE_BUSY interrupt).
The significance of the bits in CESTATUS is shown in
Table 7. The CE is initialized by the host controller using
CECONFIG (Table 7). This register contains the SAG_CNT,
PULSE_SLOW, and PULSE_FAST fields. The CECONFIG
bit definitions are given in Table 7. When the SAG_INT
bit (register 0x020[20]) is set to 1, a sag event generates
EQU
WATT AND VAR FORMULA
(WSUM/VARSUM)
INPUTS USED FOR ENERGY/CURRENT CALCULATION
W0SUM/
VAR0SUM
W1SUM/
VAR1SUM
I0SQ
SUM
I1SQ
SUM
0
VA IA – 1 element, 2W 1
φ
VA x IA VA x IB IA —
1*
VA x (IA-IB)/2 – 1 element, 3W 1
φ
VA x (IA-IB)/2 — IA-IB IB
2*
VA x IA + VB x IB– 2 element, 3W 1
φ
VA x IA VB x IB IA IB
PIN REGISTER
IA 0x100
VA 0x101
IB 0x102
VB 0x103