LT3088_15 Datasheet, PDF (12/26 Pages) Linear Technology – 800mA Single Resistor Rugged Linear Regulator

LT3088
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 modifica-
tions/solutions are then required. This behavior is not
indicative of LT3088 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 LT3088
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 LT3088 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 LT3088 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 LT3088, 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 LT3088 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 LT3088 without requiring
capacitors to frequency compensate the circuit. Figure 6
highlights the simplicity of using the LT3088 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 LT3088, external
3088f
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