13
LTC1655/LTC1655L
TYPICAL APPLICATIONS
U
This circuit shows a digitally programmable current source
from an external voltage source using an external op amp,
an LT1218 and an NPN transistor (2N3440). Any digital
word from 0 to 65535 is loaded into the LTC1655 and its
output correspondingly swings from 0V to 4.096V. This
voltage will be forced across the resistor R
A
. If R
A
is
chosen to be 412Ω, the output current will range from
0mA at zero scale to 10mA at full scale. The minimum
voltage for V
S
is determined by the load resistor R
L
and
Q1’s V
CESAT
voltage. With a load resistor of 50Ω, the
voltage source can be 5V.
CLK
D
IN
CS/LD
0.1µF
V
CC
V
OUT
GND
5V
LTC1655
µP
1655/55L TA04
–
+
LT1218
5V < V
S
< 100V
FOR R
L
≤ 50Ω
Q1
2N3440
R
A
412Ω
1%
R
L
I
OUT
=
≈ 0mA TO 10mA
(D
IN
)(4.096)
(65536)(R
A
)
1
8
5
7
2
3
6
7
4
3
2
Digitally Programmable Current Source
