12

LT3021/LT3021-1.2/

LT3021-1.5/LT3021-1.8

3021fa

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-

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

APPLICATIO S I FOR ATIO

circuit. In the case where the input is grounded, there is

less than 1

µA 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 10

µA 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 3.3

µF 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.