Datasheet

LT3030
13
3030fa
For more information www.linear.com/LT3030
applicaTions inForMaTion
Input Capacitance and Stability
Each LT3030 channel is stable with an input capacitor
typically between 1µF and 10µF. Applications operating
with smaller V
IN
to V
OUT
differential voltages and that ex-
perience large load transients may require a higher input
capacitor value to prevent input voltage droop and letting
the regulator enter dropout.
Very low ESR ceramic capacitors may be used. However,
in cases where long wires connect the power supply to the
LT3030’s input and ground, use of low value input capaci-
tors combined with an output load current of greater than
20mA may result in instability. The resonant LC tank circuit
formed by the wire inductance and the input capacitor is
the cause and not a result of LT3030 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
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 LT3030
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. How-
ever, when placed in close proximity to each other, mutual
inductance adds to the overall self inductance of the wires.
The most effective technique to reducing 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 LT3030,
a 1μF input capacitor suffices for stability. However, if a
distantly located supply powers the LT3030, use a larger
value input capacitor. Use a rough guideline of 1μF (in
addition to the 1μF minimum) per 8 inches of wire length.
The minimum input capacitance needed to stabilize the
application also varies with power supply output imped-
ance variations. Placing additional capacitance on the
LT3030’s output also helps. However, this requires an
order of magnitude more capacitance in
comparison with
additional LT3030 input
bypassing. Series resistance be-
tween the supply and the LT3030 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 higher ESR
tantalum or electrolytic capacitors at the LT3030 input in
place of ceramic capacitors.
Output Capacitance and Transient Response
The LT3030 is stable with a wide range of output capacitors.
The ESR of the output capacitor affects stability, most nota-
bly with small capacitors. Linear Technology recommends
a minimum output capacitor of 10μF/3.3μF (channel 1
/channel 2) with an ESR of 3Ω, or less, to prevent oscil-
lations. The LT3030 is a micropower device, and output
transient response is a function of output capacitance.
Larger values of output capacitance decrease the peak
deviations and provide improved transient response for
larger load current changes.
Ceramic capacitors require extra consideration. Manufac-
turers make ceramic capacitors with a variety of dielectrics,
each with different behavior across temperature and applied
voltage. The most common dielectrics specify the EIA
temperature characteristic codes of Z5U, Y5V, X5R and
X7R. Z5U and Y5V dielectrics
provide high C-V products
in a small package at low cost, but exhibit strong voltage
and temperature coefficients, as shown in Figure 2 and
Figure 3. When used with a 5V regulator, a 16V 10μF Y5V
capacitor can exhibit an effective value as low as 1μF to
2μF for the applied DC bias voltage and over the operat-
ing temperature range. X5R and X7R dielectrics result in
more stable characteristics and are more suitable for use
as the output capacitor. The X7R type has better stability
across temperature, while the X5R is less expensive and
is available in higher values.
Exercise care even when using X5R and X7R capacitors;
the X5R and X7R codes only specify operating temperature
range and maximum capacitance change over temperature.
Capacitance change due to DC bias (voltage coefficient)
with X5R and X7R capacitors is better than with Y5V and
Z5U capacitors, but can still be significant enough to drop