Datasheet

LT3042
17
3042fb
For more information www.linear.com/LT3042
APPLICATIONS INFORMATION
and ESL tolerance, any non-piezoelectrically responsive
(tantalum, electrolytic, or film) capacitor can be used at
the SET pin – although electrolytic capacitors tend to have
high 1/f noise. In any case, use of surface mount capacitor
is highly recommended.
Stability and Input Capacitance
The LT3042 is stable with a minimum 4.7µF IN pin capacitor.
LTC recommends using low ESR ceramic capacitors. In
cases where long wires connect the power supply to the
LT3042’s input and ground terminals, the use of low value
input capacitors combined with a large load current can
result in instability. The resonant LC tank circuit formed by
the wire inductance and the input capacitor is the cause
and not because of LT3042’s instability.
The self-inductance, or isolated inductance, of a wire
is directly proportional to its length. The wire diameter,
however, has less influence on its self-inductance. For
example, the self-inductance of a 2-AWG isolated wire
with a diameter of 0.26" is about half the inductance of a
30-AWG wire with a diameter of 0.01". One foot of 30-AWG
wire has 465nH of self-inductance.
Several methods exist to reduce a wire’s self-inductance.
One method divides the current flowing towards the LT3042
between two parallel conductors. In this case, placing the
wires further apart reduces the inductance; up to a 50%
reduction when placed only a few inches apart. Splitting
the wires connect two equal inductors in parallel. However,
when placed in close proximity to each other, their mu
-
tual inductance adds to the overall self inductance of the
wires — therefore a 50% reduction is not possible in such
cases. The second and more effective technique to reduce
the overall inductance is to place the for
ward and return
current conductors (the input and ground wires) in close
proximity. Two 30-AWG wires separated by 0.02" reduce
the overall inductance to about one-fifth of a single wire.
If a battery mounted in close proximity powers the LT3042,
a 4.7µF input capacitor suffices for stability. However, if a
distantly located supply powers the LT3042, use a larger
value input capacitor. Use a rough guideline of 1µF (in
addition to the 4.7µF minimum) per 8" of wire length.
The minimum input capacitance needed to stabilize the
application also varies with the output capacitance as well
as the load current. Placing additional capacitance on the
LT3042’s output helps. However, this requires significantly
more capacitance compared to additional input bypassing.
Series resistance between the supply and the LT3042 input
also helps stabilize the application; as little as 0.1Ω to 0.5Ω
suffices. This impedance dampens the LC tank circuit at
the expense of dropout voltage. A better alternative is to
use a higher ESR tantalum or electrolytic capacitor at the
LT3042 input in parallel with a 4.7µF ceramic capacitor.
PSRR and Input Capacitance
For applications utilizing the LT3042 for post-regulating
switching converters, placing a capacitor directly at
the LT3042 input results in ac current (at the switching
frequency) to flow near the LT3042. This relatively high
frequency switching current generates a magnetic field
that couples to the LT3042 output, thereby degrading its
effective PSRR. While highly dependent on the PCB, the
switching pre-regulator, the input capacitance, amongst
other factors, the PSRR degradation can be easily over
30dB at 1MHz. This degradation is present even if the
LT3042 is de-soldered from the board, because it ef
-
fectively degrades the PSRR of the PC board itself. While
negligible for conventional low PSRR LDOs, L
T3042’s
ultrahigh PSRR requires careful attention to higher order
parasitics in order to extract the full performance offered
by the regulator.
To mitigate the flow of high-frequency switching current
near the LT3042, the LT3042 input capacitor can be entirely
removed—as long as the switching converter’s output
capacitor is located more than an inch away from the
LT3042. Magnetic coupling rapidly decreases with increas
-
ing distance. Nonetheless, if the switching pre-regulator
is placed too far away (conservatively more than a couple
inches) from the
LT3042, with no input capacitor present,
as with any regulator, the LT3042 input will oscillate at the
parasitic LC resonance frequency. Besides, it is generally a
very common (and a preferred) practice to bypass regula
-
tor input with some capacitance. So this option is fairly
limited in its scope and not the most palatable solution.
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