LT3045 datasheet(18/32 Pages) LINER

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LT3045

3045fa

For more information www.linear.com/LT3045

applicaTions inForMaTion

a ceramic output capacitor. Similarly, due to LT3045’s

ultrahigh PSRR, negligible output noise is generated

using a ceramic input capacitor. Nonetheless, given the

high SET pin impedance, any piezoelectric response

from a ceramic SET pin capacitor generates significant

output noise – peak-to-peak excursions of hundreds of

µVs. However, due to the SET pin capacitor’s high ESR

and ESL tolerance, any non-piezoelectrically responsive

(tantalum, electrolytic, or film) capacitor can be used at

the SET pin – although electrolytic capacitors tend to

have high 1/f noise. In any case, use of a surface mount

capacitor is highly recommended.

Stability and Input Capacitance

TheLT3045isstablewithaminimum4.7µFINpincapacitor.

LTC recommends using low ESR ceramic capacitors. In

cases where long wires connect the power supply to the

LT3045’s input and ground terminals, the use of low value

input capacitors combined with a large load current can

result in instability. The resonant LC tank circuit formed by

the wire inductance and the input capacitor is the cause

and not because of LT3045’s instability.

The self-inductance, or isolated inductance, of a wire

is directly proportional to its length. The wire diameter,

however, 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.

OnemethoddividesthecurrentflowingtowardstheLT3045

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

thewiresconnecttwoequalinductorsinparallel.However,

when placed in close proximity to each other, their mu-

tual inductance adds to the overall self inductance of the

wires — therefore a 50% reduction is not possible in such

cases. The second and more effective technique to reduce

the overall inductance is to place the forward and return

current conductors (the input and ground wires) in close

proximity. Two 30-AWG wires separated by 0.02" reduce

the overall inductance to about one-fifth of a single wire.

IfabatterymountedincloseproximitypowerstheLT3045,

a 4.7µF input capacitor suffices for stability. However, if a

distantly located supply powers the LT3045, use a larger

value input capacitor. Use a rough guideline of 1µF (in

addition to the 4.7µF minimum) per 6" of wire length.

The minimum input capacitance needed to stabilize the

application also varies with the output capacitance as well

as the load current. Placing additional capacitance on the

LT3045’soutputhelps.However,thisrequiressignificantly

morecapacitancecomparedtoadditionalinputbypassing.

SeriesresistancebetweenthesupplyandtheLT3045input

alsohelpsstabilizetheapplication;aslittleas0.1Ωto0.5Ω

suffices. This impedance dampens the LC tank circuit at

the expense of dropout voltage. A better alternative is to

use a higher ESR tantalum or electrolytic capacitor at the

LT3045 input in parallel with a 4.7µF ceramic capacitor.

PSRR and Input Capacitance

For applications utilizing the LT3045 for post-regulating

switching converters, placing a capacitor directly at

the LT3045 input results in ac current (at the switching

frequency) to flow near the LT3045. This relatively high-

frequency switching current generates a magnetic field

that couples to the LT3045 output, thereby degrading its

effective PSRR. While highly dependent on the PCB, the

switching pre-regulator, the input capacitance, amongst

other factors, the PSRR degradation can be easily over

30dB at 1MHz. This degradation is present even if the

LT3045 is de-soldered from the board, because it ef-

fectively degrades the PSRR of the PC board itself. While

negligible for conventional low PSRR LDOs, LT3045’s

ultrahigh PSRR requires careful attention to higher order

parasitics in order to extract the full performance offered

by the regulator.

To mitigate the flow of high-frequency switching current

neartheLT3045,theLT3045inputcapacitorcanbeentirely

removed -- as long as the switching converter’s output

capacitor is located more than an inch away from the

LT3045.Magneticcouplingrapidlydecreaseswithincreas-

ing distance. Nonetheless, if the switching pre-regulator

is placed too far away (conservatively more than a couple

inches) from the LT3045, with no input capacitor present,

as with any regulator, the LT3045 input will oscillate at the

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