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LT3045-1

30451fa

For more information www.linear.com/LT3045-1

APPLICATIONS INFORMATION

a ceramic output capacitor. Similarly, due to LT3045-1’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

TheLT3045-1isstablewithaminimum4.7µFINpincapaci-

tor. LTC recommends using low ESR ceramic capacitors.

In cases where long wires connect the power supply to

the LT3045-1’s input and ground terminals, the use of low

value input capacitors combined with a large load current

canresultininstability.TheresonantLCtankcircuitformed

by the wire inductance and the input capacitor is the cause

and not because of LT3045-1’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.

One method divides the current flowing towards the

LT3045-1 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 connect two equal inductors in parallel.

However, when placed in close proximity to each other,

their mutual 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-

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

if a distantly located supply powers the LT3045-1, 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 LT3045-1’s output helps. However, this requires

significantlymorecapacitancecomparedtoadditionalinput

bypassing. Series resistance between the supply and the

LT3045-1 input also helps stabilize 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 a higher ESR tantalum or electrolytic

capacitor at the LT3045-1 input in parallel with a 4.7µF

ceramic capacitor.

PSRR and Input Capacitance

For applications utilizing the LT3045-1 for post-regulating

switching converters, placing a capacitor directly at the

LT3045-1 input results in AC current (at the switching

frequency) to flow near the LT3045-1. This relatively high-

frequency switching current generates a magnetic field

that couples to the LT3045-1 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

LT3045-1 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, LT3045-1’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

near the LT3045-1, the LT3045-1 input capacitor can be

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

output capacitor is located more than an inch away from

the LT3045-1. Magnetic coupling rapidly decreases with

increasing distance. Nonetheless, if the switching pre-

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