Datasheet

9
LTC1451
LTC1452/LTC1453
U
SA
O
PP
L
IC
AT
ITY
P
I
CA
L
An Isolated 4mA to 20mA Process Controller
Has 3.3V Minimum Loop Voltage
11451/2/3 TA04
3k
10k
1k
45k 5k
90k 5k
Q1
2N3440
R
S

10Ω
V
LOOP

3.3V 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
LTC1453
4N28
OPTO-ISOLATORS
3.3V
500Ω
LT
®
1121-3.3
FROM
OPTO-
ISOLATED
INPUTS
V
REF
+
–
LT1077
This circuit shows how to use an LTC1453 to make an
opto-isolated digitally controlled 4mA to 20mA process
controller. The controller circuitry, including the opto-
isolation, is powered by the loop voltage that can have a
wide range of 3.3V to 30V. The 1.22V reference output of
the LTC1453 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.