MAX71020 Datasheet, PDF (16/37 Pages) Maxim Integrated Products – Single-Chip Electricity Meter AFE

MAX71020
Single-Chip Electricity Meter AFE
500
400
300
200
100
0
-100
-200
CURRENT [A]
-300
VOLTAGE [V]
-400
ENERGY PER INTERVAL [Ws]
ACCUMULATED ENERGY [Ws]
-500
0
5
10
15
20
Figure 4. Voltage, Current, Momentary and Accumulated Energy
Figure 4 shows the shapes of V(t), I(t), the instantaneous
power and the accumulated energy resulting from 50
samples of the voltage and current signals over a period
of 20ms. The application of 240VAC and 100A results in
an accumulation of 480Ws (= 0.133Wh) over the 20ms
period, as indicated by the accumulated power curve. The
described sampling method works reliably, even in the
presence of dynamic phase shift and harmonic distortion.
Fault and Reset Behavior
Events at Power-Down
Power fault detection is performed by internal com-
parators that monitor the voltage at the V_3P3A pin and
also monitor the internally generated V_DD pin voltage
(1.8VDC). V_3P3SYS and V_3P3A must be connected
together at the PCB level, so that the comparators, which
are internally connected only to the V_3P3A, are able to
simultaneously monitor the common V_3P3SYS and V_3P3A
voltage. The following discussion assumes that V_3P3A
and V_3P3SYS are connected together at the PCB level.
During a power failure, as V_3P3A falls, two thresholds
are detected. The first threshold, at 3.0V, warns the host
microcontroller that the analog modules are no longer
accurate. The second threshold, at 2.8V, warns the host
microcontroller that a serious reduction in supply voltage
has occurred. OTP reads may be affected.
Reset Sequence
When the MAX71020 receives a reset signal, either from
the RESETZ pin or from the SPI, it asynchronously halts
what it was doing. It then clears RAM and initializes con-
figuration bits. An errant RESET can occur during an ESD
event. If this happens, the host must be notified. This is
accomplished by holding the INTZ output low until the
host clears it.
Applications Information
Sensor Connection
Figure 5 to Figure 8 show voltage-sensing resistive divid-
ers, current-sensing current transformers (CTs) and cur-
rent-sensing resistive shunts and how they are connect-
ed to the voltage and current inputs of the MAX71020. All
input signals to the MAX71020 sensor inputs are voltage
signals providing a scaled representation of either a
sensed voltage or current.
The analog input pins of the MAX71020 are
designed for sensors with low source imped-
ance. RC filters with resistance values higher
than those implemented in the demo boards
must not be used. Refer to the demo board schematics
for complete sensor input circuits and corresponding
component values.
Table 6. VSTAT[1:0]
VSTAT[1:0]
00
01
11
DESCRIPTION
System Power-OK. V_V3P3A > 3.0V. Analog modules are functional and accurate.
System Power is low. 2.8V < V_V3P3A < 3.0V. Analog modules not accurate.
System power below 2.8V. Ability to monitor power is about to fail.
  16