SA9605A
The only changes which now have to be implemented to interface the device to different
rated systems are: change the current sense resistors; and select the required PGM0
and PGM1.
No change to the post divider or micro-controller software is required if the FOUT2 pin
is used as described.
TYPICAL APPLICATION
In the Application Circuit (Figure 1), the components required for a three phase power
metering application are shown.
Terminated current sensors (current transformers) are connected to the current sensor
inputs of the SA9605A through current setting resistors (R
8
..R
13
).
The resistor values for standard operation are selected for an input current of 16µA into
the SA9605A, at the rated line current.
The values of these resistors are calculated as follows:
Phase 1:
R
8
= R
9
= (I
L1
/16µA) * R
18
/2
Phase 2:
R
10
= R
11
= (I
L2
/16µA) * R
19
/2
Phase 3:
R
12
= R
13
= (I
L3
/16µA) * R
20
/2
Where I
LX
= Secondary CT current at rated conditions.
R
18
, R
19
and R
20
= Current transformer termination resistors for the three phases.
R
1
+ R
1A
, R
4
and R
15
set the current for the phase 1 voltage sense input. R
2
+ R
2A
, R
5
+
P
5
and R
16
set the current for phase 2 and R
3
+ R
3A
, R
6
+ P
6
and R
17
set the current for phase
3. The values should be selected so that the input currents into the voltage sense inputs
(virtual ground) are set to 14µA for rated line voltage. Capacitors C1, C2 and C3 are for
decoupling and phase compensation.
R
14
+ P
14
defines all on-chip bias and reference currents. With R
14
+ P
14
= 24kΩ, optimum
conditions are set. R
14
+ P
14
may be varied within ± 10% for calibration purposes. Any
changes to R
14
+ P
14
will affect the output quadratically (i.e:
∆R
= +5%,
∆f
= +10%).
The formula for calculating the Output Frequency (f) is given below:
FOSC
(I
I1
I
V1
) + (I
I2
I
V2
) + (I
I3
I
V3
)
f = 11.16 * FOUTX *
*
3.5795MHz
3 * I
R 2
Where FOUTX = Nominal rated frequency (1160Hz)
FOSC
= Oscillator frequency (2MHz ...... 4MHz)
I
I1
, I
I2
, I
I3
= Input currents for current inputs (16µA at rated)
I
V1
, I
V2
, I
V3
= Input currents for voltage inputs (14µA at rated)
I
R
= Reference current (typically 50µA)
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