LT3083
12
3083fa
APPLICATIONS INFORMATION
When longer supply lines, filters, current sense resistors,
or other impedances exist between the supply and the
input to the LT3083, input bypassing should be reviewed
if stability concerns are seen. Just as output capacitance
supplies the instantaneous changes in load current for
output transients until the regulator is able to respond,
input capacitance supplies local power to the regulator until
the main supply responds. When impedance separates the
LT3083 from its main supply, the local input can droop
so that the output follows. The entire circuit may break
into oscillations, usually characterized by larger amplitude
oscillations on the input and coupling to the output.
Low ESR, ceramic input bypass capacitors are acceptable
for applications without long input leads. However, applica-
tions connecting a power supply to an LT3083 circuit’s IN
and GND pins with long input wires combined with low
ESR, ceramic input capacitors are prone to voltage spikes,
reliability concerns and application-specific board oscil-
lations. The input wire inductance found in many battery
powered applications, combined with the low ESR ceramic
input capacitor, forms a high-Q LC resonant tank circuit. In
some instances this resonant frequency beats against the
output current dependent LDO bandwidth and interferes
with proper operation. Simple circuit modifications/solu-
tions are then required. This behavior is not indicative of
LT3083 instability, but is a common ceramic input bypass
capacitor application issue.
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 LT3083
between two parallel conductors. In this case, the farther
apart the wires are from each other, the more the self-in-
ductance 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 conductors,
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 LT3083 also stabilizes the application.
As little as 0.1Ω to 0.5Ω, often less, is effective in damping
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 Output Capacitance
The LT3083 requires an output capacitor for stability. It
is designed to be stable with most low ESR capacitors
(typically ceramic, tantalum or low ESR electrolytic). A
minimum output capacitor of 10μF with an ESR of 0.5Ω
or less is recommended to prevent oscillations. 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 LT3083, increase
the effective output capacitor value. For improvement in
transient performance, place a capacitor across the volt-
age 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.
Give extra consideration to the use of ceramic capacitors.
Ceramic capacitors are manufactured with a variety of di-
electrics, each with different behavior across temperature
and applied voltage. The most common dielectrics used
are specified with EIA temperature characteristic codes of
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are
good for providing high capacitances in a small package,
but they tend to have strong voltage and temperature
coefficients as shown in Figures 3 and 4. When used with
a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an
