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LT3045-1

30451fa

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

APPLICATIONS INFORMATION

regulator is placed too far away (conservatively more

than a couple inches) from the LT3045-1, with no input

capacitor present, as with any regulator, the LT3045-1

input will oscillate at the parasitic LC resonance frequency.

Besides, it is generally a very common (and a preferred)

practice to bypass regulator input with some capacitance.

So this option is fairly limited in its scope and not the most

palatable solution.

To that end, LTC recommends using the LT3045-1 demo

board layout for achieving the best possible PSRR perfor-

mance.TheLT3045-1demoboardlayoututilizesmagnetic

fieldcancellationtechniquestopreventPSRRdegradation

causedbythishigh-frequencycurrentflow—whileutilizing

the input capacitor.

Filtering High Frequency Spikes

For applications where the LT3045-1 is used to post-

regulate a switching converter, its high PSRR effectively

suppressesany“noise”presentattheswitcher’sswitching

frequency — typically 100kHz to 4MHz. However, the very

high frequency (hundreds of MHz) “spikes” — beyond the

LT3045-1’s bandwidth — associated with the switcher’s

power switch transition times will almost directly pass

through the LT3045-1. While the output capacitor is partly

intended to absorb these spikes, its ESL will limit its ability

at these frequencies. A ferrite bead or even the inductance

associated with a short (e.g. 0.5”) PCB trace between the

switcher’s output and the LT3045-1’s input can serve as

an LC-filter to suppress these very high frequency spikes.

Output Noise

The LT3045-1 offers many advantages with respect to

noise performance. Traditional linear regulators have

several sources of noise. The most critical noise sources

for a traditional regulator are its voltage reference, error

amplifier, noise from the resistor divider network used for

setting output voltage and the noise gain created by this

resistor divider. Many low noise regulators pin out their

voltagereferencetoallowfornoisereductionbybypassing

the reference voltage.

Unlike most linear regulators, the LT3045-1 does not use

avoltagereference;instead,itusesa100µAcurrentrefer-

ence. The current reference operates with typical noise

currentlevelof20pA/√Hz(6nARMSovera10Hzto100kHz

bandwidth).Theresultantvoltagenoiseequalsthecurrent

noisemultipliedbytheresistorvalue,whichinturnisRMS

summedwiththeerroramplifier’snoiseandtheresistor’s

ownnoiseof√4kTR—wherebyk=Boltzmann’sconstant

1.38 • 10–23J/K and T is the absolute temperature.

One problem that conventional linear regulators face is

that the resistor divider setting the output voltage gains up

the reference noise. In contrast, the LT3045-1’s unity-gain

follower architecture presents no gain from the SET pin

to the output. Therefore, if a capacitor bypasses the SET

pin resistor, then the output noise is independent of the

programmed output voltage. The resultant output noise

is then set just by the error amplifier’s noise — typically

2nV/√Hz from 10kHz to 1MHz and 0.8µVRMS in a 10Hz

to 100kHz bandwidth using a 4.7µF SET pin capacitor.

Paralleling multiple LT3045-1s further reduces noise by

√N, for N parallel regulators.

Refer to the Typical Performance Characteristics section

for noise spectral density and RMS integrated noise over

various load currents and SET pin capacitances.

Set Pin (Bypass) Capacitance: Noise, PSRR, Transient

Response and Soft-Start

Inadditiontoreducingoutputnoise,usingaSETpinbypass

capacitoralsoimprovesPSRRandtransientperformance.

Note that any bypass capacitor leakage deteriorates the

LT3045-1’sDCregulation.Capacitorleakageofeven100nA

is a 0.1% DC error. Therefore, LTC recommends the use

of a good quality low leakage ceramic capacitor.

UsingaSETpinbypasscapacitoralsosoft-startstheoutput

and limits inrush current. The RC time constant, formed

by the SET pin resistor and capacitor, controls soft-start

time. Ramp-up rate from 0 to 90% of nominal VOUT is:

tSS ≈ 2.3 • RSET • CSET (Fast Start-Up Disabled)

Fast Start-Up

For ultralow noise applications that require low 1/f noise

(i.e. at frequencies below 100Hz), a larger value SET pin

capacitorisrequired,upto22µF.Whilethiswouldnormally

significantly increase the regulator’s start-up time, the

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