SA2002P Datasheet, PDF (10/14 Pages) Sames – Programmable Single Phase Kilowatt-hour Energy IC for Mechanical Counter Application

SA2002P
sames
TYPICAL APPLICATION
The analog (metering) interface described in this section is
designed for measuring 230V/40A with precision better than
Class 1.
The most important external components for the SA2002P
integrated circuit are the current sense resistors, the voltage
sense resistors and the bias setting resistor. The resistors
used in the metering section should be of the same type so
temperature effects are minimized.
Reference Voltage Bias resistor
R6 defines all on chip and reference currents. With R13 = 24kW
optimum conditions are set.
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 SA2002P at rated conditions.
According to equation described in the current sense inputs
section:
R6 = R7 = ( IL/ 16µA ) x RSH / 2
= 40A / 16µA x625W / 2
= 781.25W
A resistor with a value of 820W is chosen, the 5% deviation
from the calculated value will be compensated for when
calculating the values in the voltage path.
Voltage Input IVP
The voltage input of the SA2002P (IVP) is driven with a current
of 14µA at nominal mains voltage. This voltage input saturates
at approximately 17µA. At a nominal voltage current of 14µA
allows for 20% overdriving. The mains voltage is divided with a
voltage divider to 14V that is fed to the voltage input pins via a
1MW resistor.
Voltage Divider
The voltage divider is calculated for a voltage drop of 14V + 5%
(14.7V). Equations for the voltage divider in figure 4 are:
RB = R1 + R2 + R3 + R8
RB = R11 || R12
Combining the two equations gives:
(RA + RB) / 230V = RB / 14.7V
Values for resistors R8 = 10W, R11 = 24kW and R12 = 1MW is
chosen.
Substituting the values result in:
RB = 23.437kW
RA = RB x (230V / 14.7V – 1 )
RA = 343.27kW.
Standard resistor values of R1, R2 and R3 are chosen to be
150kW, 150kW and 43kW.
A combination for R1, R2 and R3 with 110k, 110k and 110k
would also be sufficient.
Programming
In order for the meter to function correctly the EEPROM must
be programmed with the relevant parameters eg. LED pulse
rate, Motor pulse rate, Creep threshold, AC mode
measurement direction and rated current.
The calibration is preformed as described in the previous
section.
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