Datasheet

LT3090
13
3090fa
For more information www.linear.com/LT3090
applicaTions inForMaTion
reduction depends on the guard ring width. Leakages as
small as 50nA into or out of the SET pin creates a 0.1%
error in the reference voltage. Leakages of this magnitude,
coupled with other sources of leakage, can cause significant
errors in the output voltage, especially over wide operating
temperature range. Figure 2 illustrates a typical guard ring
layout technique.
If guard ring techniques are used, then SET pin stray
capacitance is practically eliminated. Since the SET pin
is a high impedance node, unwanted signals may couple
into the SET pin and cause erratic behavior. This is most
noticeable when operating with a minimum output capacitor
at light load currents. The simplest remedy is to bypass
the SET pin with a small capacitance to ground – 100pF
is generally sufficient.
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
LT3090 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 connects two equal inductors in
parallel.
However,
when placed in close proximity to each other,
mutual inductance adds to the overall self inductance of
the wires. The second and most effective technique to
reduce overall inductance is to place the forward and
return current conductors (the input wire and the ground
wire) in close proximity. Tw o 30-AWG wires separated by
0.02" reduce the overall self inductance to about one-fifth
of a single wire.
If a battery, mounted in close proximity, powers the
LT3090, a 4.7µF input capacitor suffices for stability.
However, if a distantly located supply powers the LT3090,
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 stabi
-
lize the application also varies with power supply output
impedance variations. Placing additional capacitance on
the LT3090’s output also helps. However, this requires
an order of magnitude more capacitance in comparison
with additional LT3090 input bypassing. Series resistance
between the supply and the LT3090 input also helps stabi
-
lize 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 higher ESR
tantalum
or electrolytic capacitors at the LT3090 input in
place of ceramic capacitors.
Stability and Output Capacitance
The LT3090 requires an output capacitor for stability. It is
stable with low ESR capacitors (such as ceramic, tantalum
or low ESR electrolytic). A minimum output capacitor of
4.7µF with an ESR of 0.5Ω or less is recommended to
prevent oscillations. Larger values of output capacitance
Figure 2. Guard Ring Layout for DFN
Stability and Input Capacitance
The LT3090 is stable with a minimum of 4.7µF capacitor
placed at the IN pin. Low ESR ceramic capacitors can be
used. However, in cases where long wires connect the
power supply to the LT3090’s input and ground, the use
of low value input capacitors combined with a large output
load current may result in instability. The resonant LC tank
circuit formed by the wire inductance and the input capaci
-
tor is the cause and not because of LT3090 instability.
The
self inductance, or isolated inductance, of a wire
is directly proportional to its length. However, the wire
diameter 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
3090 F02
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