Datasheet

11
LTC1655/LTC1655L
TYPICAL APPLICATIONS
U
This circuit shows how to use an LTC1655 to make an
optoisolated digitally controlled 4mA to 20mA process
controller. The controller circuitry, including the
optoisolation, is powered by the loop voltage that can have
a wide range of 6V to 30V. The 2.048V reference output of
the LTC1655 is used for the 4mA offset current and V
OUT
is used for the digitally controlled 0mA to 16mA current.
R
S
is a sense resistor and the op amp modulates the
transistor Q1 to provide the 4mA to 20mA current through
this resistor. The potentiometers allow for offset and full-
scale adjustment. The control circuitry dissipates well
under the 4mA budget at zero scale.
1655/55L TA03
3k
10k
1k
75k
1%
5k
150k
1%
20k
Q1
2N3440
R
S
10Ω
V
LOOP
6V TO 30V
I
OUT
OUTIN
CLK
D
IN
CS/LD
CLK
D
IN
CS/LD
CLK
D
IN
CS/LD
V
CC
V
OUT
1µF
LTC1655
4N28
OPTOISOLATORS
5V
500Ω
LT
®
1121-5
FROM
OPTOISOLATED
INPUTS
V
REF
GND
–
+
LT
®
1077
1
86
5
7
2
3
6
7
4
3
2
An Isolated 4mA to 20mA Process Controller