LT3022IDHC datasheet(13/16 Pages) LINER

13 / 16 page

LT3022

3022f

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

voltagesourceandADJispulledbelowitsreferencevoltage,

the LT3022 attempts to source its current limit capability

at OUT. The output voltage increases to VIN – VDROPOUT

with VDROPOUT set by whatever load current the LT3022

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

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.

Ifabattery,mountedincloseproximity,powerstheLT3022,

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.

LT3022IDHC Datasheet(PDF) 13 Page - Linear Technology

LT3022IDHC Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL