Industrial Converter and Protector IC
AM460
In 2-wire operation the IC ground is "virtual" (floating), as with a constant load resistance the supply voltage of
the device
V
CC
changes according to the current. As a rule, the following equation applies to 2-wire operation:
V
CC
=
V
S
−
I
OUT
(
V
IN
)
R
L
(4)
The reason for this is that in 2-wire operation the IC is connected in series to the actual load resistor
R
L
. This is
illustrated in Figure 3.
In 3-wire operation
V
CC
=
V
S
, as the IC ground is connected to the ground of the system.
Setting the voltage gain using the voltage output
Using amplifier stages OP1 and OP3 for signal conditioning the overall gain can be set by selecting suitable ex-
ternal resistors
R
1
and
R
2
. The transfer function for the output voltage is calculated by multiplying Equations 1
and 2 as follows:
V
OUT
=
V
INP
⋅
G
GAIN
⋅
G
OP
(5)
with
G
GAIN
= 1 +
R
1
/R
2
and
G
OP
= 2.2.
Setting the output current range using the current output
When using amplification stage OP1 together with the V/I converter for signal conditioning the offset of the
output current should first be compensated for by suitable selection of resistors
R
3
and
R
4
. To this end the OP1
input must be connected to ground (V
INP
= 0). With the short circuit at the input and by connecting up V/I con-
verter pin
VSET
as shown in Figure 2 the values of the output current according to Equation 3 are as follows:
I
OUT
(
V
INDAI
=
0
)
=
I
SET
=
V
REF
R
4
⋅
2
R
0
R
3
+
R
4
(6)
and thus for the ratio of the resistors
R
3
/R
4
:
R
3
V
REF
=
−
1
R
4
2
R
0
I
SET
(7)
The output current area is set in conjunction with the selection of external resistors
R
1
and
R
2
(or fine adjustment
with
R
0
). With Equations 1 and 3 the following is calculated for output current
I
OUT
:
I
OUT
=
V
INP
G
GAIN
R
+
I
SET
with
G
GAIN
=
1
+
1
8
R
0
R
2
(8)
Selecting the supply voltage
System supply voltage
V
S
needed to operate AM460 is dependent on the selected mode of operation.
•
When using voltage output pin
VOUT
the minimum
V
S
needed for operation is determined by the maximum
output voltage
V
OUTmax
required by the application. This is expressed as follows:
V
S
≥
V
OUT
max
+
5V
(9)
analog microelectronics
April 2003
9/18
AME [ ANALOG MICROELECTRONICS ]
