SA2002D Datasheet, PDF (7/10 Pages) Sames – Single Phase Kilowatt-hour Metering IC for Mechanical Display Applications

SA2002D
sames
TYPICAL APPLICATION
In figure 7, the components required for stand alone power
metering application, are shown. The application uses a shunt
resistor for the mains current sensing. The meter is designed
for 220V/40A I_MAX operation.
The most important external components for the SA2002D
integrated circuit are the current sense resistors, the voltage
sense resistors as well as the bias setting resistor.
BIAS RESISTOR
R13 defines all on-chip and reference currents. With
R13=24kW, optimum conditions are set. Device calibration is
done on the voltage input of the device.
SHUNT RESISTOR
The voltage drop across the shunt resistor at rated current
should be at least 20mV. A shunt resistor with a value of 625µW
is chosen. The voltage drop across the shunt resistor is 25mV
at rated conditions (Imax). The power dissipation in the current
sensor is:
P=(40A)² x 625µW = 1W.
CURRENT SENSE RESISTORS
The resistors R6 and R7 define the current level into the
current sense inputs of the device. The resistor values are
selected for an input current of 16µA on the current inputs of
the SA2002D at rated conditions.
According to equation described in the Current Sense inputs
section:
R6 = R7 = ( I_L / 16µA ) x R_SH / 2
= 40A / 16µA x 625µW / 2
= 781.25W
A resistor with value of 820W is chosen, the 5% deviation from
the calculated value will be compensated for when calculating
resistor values for the voltage path.
VOLTAGE DIVIDER
The voltage divider is calculated for a voltage drop of 14V +
5%(14.7V). Equations for the voltage divider in figure 7 are:
R1+R2+R3=RA and R12 || (R11+P1)=RB. Combining the two
equations gives:
(RA + RB) / 220V = RB / 14.7V
A 5k trimpot will be used in the voltage channel for meter
calibration. The center position on the pot is used in the
calculations. P1 = 2.5kW and values for resistors R11 = 22kW
and R12 =1MW is chosen.
Substituting the values will result in:
RB = 23.91kW and RA = RB x (230V/14.7V - 1) resulting in RA =
333kW so the resistor values of R1, R2 and R3 are chosen to
be 110kW.
PROGRAMMING
The resistor values are calculated for a 40A rated meter. The
LED pulse rate must be set accordingly by programming pins
R0, R1 and R2. Using the table Rated Conditions Select (On
page 5) pins R0 and R2 is set to V_SS and R1 set to V_DD. These
settings will configure the SA2002D for 220V/40A operation
with a LED pulse rate of 1600 pulses/kWh. The FAST pin is set
to V_SSfor normal operation.
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