SA9904B Datasheet, PDF (9/12 Pages) List of Unclassifed Manufacturers – Three Phase Power / Energy IC with SPI Interface

SA9904B
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TYPICAL APPLICATION
In figure 11, the components required for the three phase
power/energy metering section of a meter, is shown. The
application uses current transformers for current sensing. The 4-
wire meter section is capable of measuring 3x230V/80A with
precision better than Class 1.
The most important external components for the SA9904B
integrated circuit are the current sense resistors, the voltage
sense resistors as well as the bias setting resistor.
BIAS RESISTOR
R7 defines all on-chip and reference currents. With R7=47kW,
optimum conditions are set.
CT TERMINATION RESISTOR
The voltage drop across the CT termination resistor at rated
current should be at least 20mV. The CT’s used have low phase
shift and a ratio of 1:2500.The CT is terminated with a 2.7W
resistor giving a voltage drop across the termination resistor
86.4mV at rated conditions (Imax for the meter).
CURRENT SENSE RESISTORS
The resistors R1 and R2 define the current level into the current
sense inputs of phase one of the device. The resistor values are
selected for an input current of 16µA on the current inputs at
rated conditions.
According to equation described in the Current Sense inputs
section:
R1 = R2 = (IL / 16µA ) x RSH / 2
= 80A /2500 / 16µA x 2.7W / 2
= 2.7kW
IL = Line current / CT Ratio
The three current channels are identical so R1 = R2 = R3 = R4
= R5 = R6.
VOLTAGE DIVIDER
The voltage divider is calculated for a voltage drop of 14V.
Equations for the voltage divider in figure 5 are:
RA = R16 + R19 + R22
RB = R8 || R13
Combining the two equations gives:
( RA + RB ) / 230V = RB / 14V
A 24k resistor is chosen for R13 and a 1M resistor is used for
R8.
Substituting the values result in:
RB = 23.44k
RA = RB x (230V / 14V - 1)
RA = 361.6k.
Resistor values of R16, R19 and R22 is chosen to be 120k
each.
The three voltage channels are identical so R14= R15= R16 =
R17 = R18 = R19 and R20 = R21= R22.
CRYSTAL OSCILLATOR
A color burst TV crystal with f = 3.5795MHz is used for the
oscillator. The oscillator frequency is divided down to
1.7897MHz on-chip, to supply the A/D converters as well as
the digital circuitry.
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