LT3022 LINER [Linear Technology], LT3022 Datasheet - Page 13

• Manufacturer Part Number: LT3022
• Description: 1A, 0.9V to 10V, Very Low Dropout Linear Regulator
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
The LT3022 incurs no damage if the ADJ pin is pulled
above or below ground by 10V. If IN is left open-circuited
or grounded and ADJ is pulled above ground, ADJ acts
like a 25k resistor in series with two diodes. ADJ acts like
a 25k resistor if pulled below ground. If IN is powered by a
voltage source and ADJ is pulled below its reference voltage,
the LT3022 attempts to source its current limit capability
at OUT. The output voltage increases to V
with V
supports. This condition can potentially damage external
circuitry powered by the LT3022 if the output voltage in-
creases to an unregulated high voltage. If IN is powered
by a voltage source and ADJ is pulled above its reference
voltage, two situations can occur. If ADJ is pulled slightly
above its reference voltage, the LT3022 turns off the pass
transistor, no output current is sourced and the output
voltage decreases to either the voltage at ADJ or less. If
ADJ is pulled above its no-load recovery threshold, the
no-load recovery circuitry turns on and attempts to sink
current. OUT is actively pulled low and the output voltage
clamps at a Schottky diode above ground. Please note that
the behavior described above applies to the LT3022 only. If
a resistor divider is connected under the same conditions,
there will be additional V/R current.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled to
ground, pulled to some intermediate voltage or is left open
circuit. In the case where the input is grounded, there is
less than 1µA of reverse output current. If the LT3022 IN
pin is forced below the OUT pin or the OUT pin is pulled
above the IN pin, input current drops to less than 10µA
typically. This occurs if the LT3022 input is connected to
a discharged (low voltage) battery and either a backup
battery or a second regulator circuit holds up the output.
The state of the SHDN pin has no effect on the reverse
output current if OUT is pulled above IN.
Input Capacitance and Stability
The LT3022 design is stable with a minimum of 10µF
capacitor placed at the IN pin. Very low ESR ceramic
capacitors may be used. However, in cases where long
wires connect the power supply to the LT3022’s input and
ground, use of low value input capacitors combined with
DROPOUT
set by whatever load current the LT3022
IN
– V
DROPOUT
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 LT3022 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
LT3022 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 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. Two 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 LT3022,
a 10µF input capacitor suffices for stability. However,
if a distantly located supply powers the LT3022, use a
larger value input capacitor. Use a rough guideline of 1µF
(in addition to the 10µF minimum) per 8 inches 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 LT3022’s output also helps. However, this requires
an order of magnitude more capacitance in comparison
with additional LT3022 input bypassing. Series resistance
between the supply and the LT3022 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 LT3022 input in
place of ceramic capacitors.
LT3022

3022f