LT3063_15 Datasheet, PDF (14/20 Pages) Linear Technology – 45V VIN, Micropower, Low Noise, 200mA LDO with Output Discharge

LT3063
Applications Information
Input Capacitance and Stability
Low ESR, ceramic input bypass capacitors are acceptable
for applications without long input leads. However, appli-
cations connecting a power supply to an LT3063 circuit’s
IN and GND pins with long input wires combined with a
low ESR, ceramic input capacitor are prone to voltage
spikes, reliability concerns and application-specific board
oscillations.
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
LT3063 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 approximately
465nH of self-inductance.
Two methods can reduce wire self-inductance. One method
divides the current flowing towards the LT3063 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 connects two equal inductors in
parallel, but placing them in close proximity creates 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 a battery, mounted in close proximity, powers the LT3063,
a 1µF input capacitor suffices for stability. However, if a
distant supply powers the LT3063, 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 impedance variations.
Placing additional capacitance on the LT3063’s output
also helps. However, this requires an order of magnitude
more capacitance in comparison with additional LT3063
input bypassing. Series resistance between the supply
and the LT3063 input also helps stabilize the applica-
tion; 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 LT3063 input in place of
ceramic capacitors.
Overload Recovery
Like many IC power regulators, the LT3063 has safe oper-
ating area protection. The safe area protection decreases
current limit as input-to-output voltage increases and keeps
the power transistor inside a safe operating region for all
values of input-to-output voltage. The protective design
provides some output current at all values of input-to-
output voltage up to the device breakdown.
When power is first applied, as input voltage rises, the
output follows the input, allowing the regulator to start up
into very heavy loads. During start-up, as the input voltage
is rising, the input-to-output voltage differential is small,
allowing the regulator to supply large output currents. With
a high input voltage, a problem can occur wherein removal
of an output short will not allow the output to recover.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low. Com-
mon situations include immediately after the removal of a
short-circuit or if the shutdown pin is pulled high after the
input voltage has already been turned on. The load line for
such a load may intersect the output current curve at two
points. If this happens, there are two stable output operat-
ing points for the regulator. With this double intersection,
the input power supply may need to be cycled down to
zero and brought up again to make the output recover.
3063f
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