LT3081
14
3081fc
applicaTions inForMaTion
The self-inductance, or isolated inductance, of a wire is
directly proportional to its length. Wire diameter is not a
major factor on its self-inductance. For example, the self-
inductance of a 2-AWG isolated wire (diameter = 0.26") is
about half the self-inductance of a 30-AWG wire (diameter
= 0.01"). One foot of 30-AWG wire has about 465nH of
self inductance.
One of two ways reduces a wire’s self-inductance. One
method divides the current flowing towards the LT3081
between two parallel conductors. In this case, the farther
apart the wires are from each other, the more the self-
inductance is reduced; up to a 50% reduction when placed
a few inches apart. Splitting the wires basically connects
two equal inductors in parallel, but placing them in close
proximity gives the wires mutual inductance adding to
the self-inductance. The second and most effective way
to reduce overall inductance is to place both forward and
return current conductors (the input and GND wires) in
very close proximity. Two 30-AWG wires separated by
only 0.02", used as forward and return current conduc-
tors, reduce the overall self-inductance to approximately
one-fifth that of a single isolated wire.
If wiring modifications are not permissible for the applica-
tions, including series resistance between the power supply
and the input of the LT3081 also stabilizes the application.
As little as 0.1Ω to 0.5Ω, often less, is effective in damp-
ing the LC resonance. If the added impedance between
the power supply and the input is unacceptable, adding
ESR to the input capacitor also provides the necessary
damping of the LC resonance. However, the required ESR
is generally higher than the series impedance required.
Stability and Frequency Compensation for Linear
Regulator Configurations
The LT3081 does not require an output capacitor for
stability. LTC recommends an output capacitor of 10μF
with an ESR of 0.5Ω or less to provide good transient
performance in linear regulator configurations. Larger
values of output capacitance decrease peak deviations and
provide improved transient response for larger load current
changes. Bypass capacitors, used to decouple individual
components powered by the LT3081, increase the effec-
tive output capacitor value. For improvement in transient
performance, place a capacitor across the voltage setting
resistor. Capacitors up to 1μF can be used. This bypass
capacitor reduces system noise as well, but start-up time
is proportional to the time constant of the voltage setting
resistor (R
SET
in Figure 1) and SET pin bypass capacitor.
Stability and Frequency Compensation for Current
Source Configurations
The LT3081 does not require input or output capacitors
for stability in many current-source applications. Clean,
tight PCB layouts provide a low reactance, well controlled
operating environment for the LT3081 without requiring
capacitors to frequency compensate the circuit. Figure 3
highlights the simplicity of using the LT3081 as a current
source.
Some current source applications use a capacitor con-
nected in parallel with the SET pin resistor to lower the
current source’s noise. This capacitor also provides a
soft-start function for the current source. See Quieting the
Noise section for further details. When operating without
output capacitors, the high impedance nature of the SET
pin as the input of the error amplifier allows signal from
the output to couple in, showing as high frequency ring-
ing during transients. Bypassing the SET resistor with a
capacitor in the range of 20pF to 30pF dampens the ringing.
Depending on the pole introduced by a capacitor or other
complex impedances presented to the LT3081, external
compensation may be required for stability. Techniques
are discussed to achieve this in the following paragraphs.
Linear Technology strongly recommends testing stability
in situ with final components before beginning production.
Although the LT3081’s design strives to be stable without
capacitors over a wide variety of operating conditions, it is
not possible to test for all possible combinations of input
and output impedances that the LT3081 will encounter.
These impedances may include resistive, capacitive, and
inductive components and may be complex distributed
networks. In addition, the current source’s value will dif-
fer between applications and its connection may be GND
referenced, power supply referenced, or floating in a signal
line path. Linear Technology strongly recommends that
stability be tested in situ for any LT3081 application.
