lt3021es8-trpbf Linear Technology Corporation, lt3021es8-trpbf Datasheet - Page 12

• Manufacturer Part Number: lt3021es8-trpbf
• Description: 500ma, Low Voltage, Very Low Dropout Linear Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT3021/LT3021-1.2/
LT3021-1.5/LT3021-1.8
The LT3021 incurs no damage if OUT is pulled below
ground. If IN is left open circuit or grounded, OUT can be
pulled below ground by 10V. No current flows from the
pass transistor connected to OUT. However, current flows
in (but is limited by) the resistor divider that sets the output
voltage. Current flows from the bottom resistor in the
divider and from the ADJ pin’s internal clamp through the
top resistor in the divider to the external circuitry pulling
OUT below ground. If IN is powered by a voltage source,
OUT sources current equal to its current limit capability
and the LT3021 protects itself by thermal limiting. In this
case, grounding SHDN turns off the LT3021 and stops
OUT from sourcing current.
The LT3021 incurs no damage if the ADJ pin is pulled
above or below ground by 10V. If IN is left open circuit or
grounded and ADJ is pulled above ground, ADJ acts like a
25k resistor in series with a 1V clamp (one Schottky diode
in series with one diode). ADJ acts like a 25k resistor in
series with a Schottky diode if pulled below ground. If IN
is powered by a voltage source and ADJ is pulled below its
reference voltage, the LT3021 attempts to source its
current limit capability at OUT. The output voltage in-
creases to V
load current the LT3021 supports. This condition can
potentially damage external circuitry powered by the
LT3021 if the output voltage increases 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 LT3021 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 LT3021 only. If a resistor divider is con-
nected 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
12
IN
– V
DROPOUT
U
U
with V
DROPOUT
W
set by whatever
U
circuit. In the case where the input is grounded, there is
less than 1µA of reverse output current.
If the LT3021 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 LT3021 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 LT3021 is designed to be stable with a minimum
capacitance of 3.3µF placed at the IN pin. Ceramic capaci-
tors with very low ESR may be used. However, in cases
where a long wire is used to connect a power supply to the
input of the LT3021 (and also from the ground of the
LT3021 back to the power supply ground), use of low
value input capacitors combined with an output load
current of 20mA or greater may result in an unstable
application. This is due to the inductance of the wire
forming an LC tank circuit with the input capacitor and not
a result of the LT3021 being unstable.
The self-inductance, or isolated inductance, of a wire is
directly proportional to its length. However, the diameter
of a wire does not have a major influence on its self-
inductance. For example, the self inductance of a 2-AWG
isolated wire with a diameter of 0.26 in. is about half the
inductance of a 30-AWG wire with a diameter of 0.01 in.
One foot of 30-AWG wire has 465nH of self inductance.
The overall self-inductance of a wire can be reduced in two
ways. One is to divide the current flowing towards the
LT3021 between two parallel conductors and flows in the
same direction in each. In this case, the farther the wires
are placed apart from each other, the more inductance will
be reduced, up to a 50% reduction when placed a few
inches apart. Splitting the wires basically connects two
equal inductors in parallel. However, when placed in close
proximity from each other, mutual inductance is added to
the overall self inductance of the wires. The most effective
way to reduce overall inductance is to place the forward
and return-current conductors (the wire for the input and
the wire for ground) in very close proximity. Two 30-AWG
wires separated by 0.02 in. reduce the overall self-induc-
tance to about one-fifth of a single isolated wire.
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