LT3045 datasheet(19/32 Pages) LINER

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LT3045

3045fa

For more information www.linear.com/LT3045

applicaTions inForMaTion

parasitic LC resonance frequency. Besides, it is generally a

very common (and a preferred) practice to bypass regula-

tor 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 demo

board layout for achieving the best possible PSRR perfor-

mance. The LT3045 demo board layout utilizes magnetic

fieldcancellationtechniquestopreventPSRRdegradation

causedbythishigh-frequencycurrentflow—whileutilizing

the input capacitor.

Filtering High Frequency Spikes

ForapplicationswheretheLT3045isusedtopost-regulate

a switching converter, its high PSRR effectively sup-

presses any “noise” present at the switcher’s switching

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

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

LT3045’s bandwidth — associated with the switcher’s

power switch transition times will almost directly pass

through the LT3045. 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’s input can serve as an

LC-filter to suppress these very high frequency spikes.

Output Noise

The LT3045 offers many advantages with respect to noise

performance. Traditional linear regulators have several

sources of noise. The most critical noise sources for a

traditionalregulatorareitsvoltagereference,erroramplifier,

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 voltage

reference to allow for noise reduction by bypassing the

reference voltage.

Unlike most linear regulators, the LT3045 does not use a

voltage reference; instead, it uses a 100µA current refer-

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’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. Paral-

leling multiple LT3045s 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’s DC regulation. Capacitor leakage of even 100nA

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

LT3045 incorporates fast start-up circuitry that increases

the SET pin current to about 2mA during start-up.

As shown in the Block Diagram, the 2mA current source

remains engaged while PGFB is below 300mV, unless the

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