Datasheet

LT3081
15
3081fa
For more information www.linear.com/LT3081
applicaTions inForMaTion
In LT3081 applications with long wires or PCB traces, the
inductive reactance may cause instability. In some cases,
adding series resistance to the input and output lines (as
shown in Figure 4) may sufficiently dampen these possible
high-Q lines and provide stability. The user must evaluate
the required resistor values against the design’s headroom
constraints. In general, operation at low output current
levels (<20mA) automatically requires higher values of
programming resistors and may provide the necessary
damping without additional series impedance.
If the line impedances in series with the LT3081 are
complex enough such that series damping resistors are
not sufficient, a frequency compensation network may be
necessary. Several options may be considered.
Figure 5 depicts the simplest frequency compensation
networks as a single capacitor across the two terminals
of the current source. Some applications may use the
capacitance to stand off DC voltage but allow the transfer
of data down a signal line.
For some applications, pure capacitance may be unaccept
-
able or
present a design constraint. One circuit example
typifying
this is an “intrinsically-safe” circuit in which an
overload or fault condition potentially allows the capaci
-
tor’s stored
energy to create a spark or a
rc. For applica-
tions where a single capacitor is unacceptable, Figure 5
alternately
shows a series RC network connected across
the two terminals of the current source. This network has
the added benefit of limiting the discharge current of the
capacitor under a fault condition, preventing sparks or
arcs. In many instances, a series RC network is the best
solution for stabilizing the application circuit. Typical resis
-
tor values will range from 100Ω to 5k. Once again, Linear
T
echnology strongly recommends testing stability in situ
for any LT3081 application across all operating conditions,
especially ones that present complex impedance networks
at the input and output of the current source.
If an application refers the bottom of the LT3081 current
source to GND, it may be necessary to bypass the top
of the current source with a capacitor to GND. In some
cases, this capacitor may already exist and no additional
capacitance is required. For example, if the LT3081 was
used as a variable current source on the output of a power
supply, the output bypass capacitance would suffice to
provide LT3081 stability. Other applications may require
the addition of a bypass capacitor. A series RC network
may also be
used as necessary, and depends on the ap-
plication requirements.
IN
SET OUT
+
–
LT3081
50µA
R
SET
R
OUT
R
SERIES
R
SERIES
LONG LINE
REACTANCE/INDUCTANCE
3081 F04
LONG LINE
REACTANCE/INDUCTANCE
Figure 4. Adding Series Resistance Decouples
and Dampens Long Line Reactances
Figure 5. Compensation from Input to Output
of Current Source Provides Stability
3081 F05
IN
SET OUT
+
–
LT3081
50µA
C
COMP
OR
R
SET
R
OUT
R
COMP
C
COMP