LT3045 datasheet(37/45 Pages) AD

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Data Sheet

LT3097

APPLICATIONS INFORMATION

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Rev. 0 | 37 of 45

STABILITY AND INPUT CAPACITANCE

The LT3097 is stable with a minimum 4.7 μF INP pin capacitor

for the positive regulator and 10 μF INN pin capacitor for the

negative regulator. Analog Devices, Inc., recommends using low

ESR ceramic capacitors. In cases where long wires connect the

power supply to the input and ground terminals of the LT3097,

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 of this

instability and not the LT3097.

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 465 nH of self-inductance.

Several methods exist to reduce the self-inductance of a wire. One

method divides the current flowing toward the LT3097 between

the 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 close 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) close. Two 30-AWG wires separated

by 0.02" reduce the overall inductance to about one-fifth of a single

wire.

If a battery mounted close powers the LT3097, a 4.7 μF/10 μF input

capacitor suffices for stability for the positive/negative regulator.

However, if a distantly located supply powers the LT3097, use a

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

addition to the 4.7 μF/10 μF minimum) per 6" of wire length. The

minimum input capacitance required to stabilize the application also

varies with the output capacitance as well as the load current. Place

additional capacitance on the output of the LT3097 to help with this

issue. However, this approach requires significantly more capaci-

tance compared to additional input bypassing. Series resistance

between the supply and the input of the LT3097 also helps stabilize

the application; as little as 0.1 Ω to 0.5 Ω suffices. This impedance

dampens the LC tank circuit at the expense of the dropout voltage.

A better alternative is to use a higher ESR tantalum or electrolytic

capacitor at the input of the LT3097 in parallel with a 4.7 μF/10 μF

ceramic capacitor for the positive/negative regulator.

PSRR AND INPUT CAPACITANCE

For applications using the LT3097 for post-regulating switching

converters, placing a capacitor directly at the input of the LT3097

results in AC current (at the switching frequency) flowing near the

LT3097. This relatively high-frequency switching current generates

a magnetic field that couples to the output of the LT3097, degrading

its effective PSRR. While highly dependent on the PCB, the switch-

ing preregulator, and the input capacitance, among other factors,

the PSRR degradation can easily be more than 30 dB at 1 MHz.

This degradation is present even if the LT3097 is desoldered from

the board because it effectively degrades the PSRR of the PCB

itself. While negligible for conventional, low PSRR, LDO regulators,

the ultra-high PSRR of the LT3097 requires careful attention to

higher-order parasitics to extract the full performance offered by the

regulator.

To mitigate the flow of the high-frequency switching current near the

LT3097, as long as the output capacitor of the switching converter

is located more than an inch away from the LT3097, remove the

input capacitor of the LT3097. Magnetic coupling rapidly decreases

with increasing distance. Nonetheless, if the switching preregulator

is placed too far away (conservatively more than a couple of

inches) from the LT3097, with no input capacitor present, as with

any regulator, the input of the LT3097 oscillates at the parasitic

LC resonance frequency. In addition, it is generally a common

(and preferred) practice to bypass the regulator input with some

capacitance. Therefore, this option is fairly limited in its scope and

not the most palatable solution.

To that end, Analog Devices, Inc., recommends using the LT3097

demonstration board layout for achieving the best possible PSRR

performance. For more details refer to the LT3097 evaluation board

user guide (EVAL-LT3097-AZ. The LT3097 evaluation board layout

uses magnetic-field cancellation techniques to prevent PSRR deg-

radation caused by this high-frequency current flow, while using the

input capacitor.

FILTERING HIGH-FREQUENCY SPIKES

For applications where the LT3097 is used to post-regulate a

switching converter, its high PSRR effectively suppresses any noise

present at the switching frequency of the switching converter, typi-

cally 100 kHz to 4 MHz. However, the high-frequency (hundreds

of MHz) spikes, beyond the bandwidth of the LT3097, associated

with the power-switch transition times of the switching converter

almost directly pass through the LT3097. While the output capacitor

is intended partly to absorb these spikes, its ESL limits its ability at

these frequencies. A ferrite bead or even the inductance associated

with a short (for example, 0.5”) PCB trace between the output of the

switching converter and the input of the LT3097 can serve as an LC

filter to suppress these high-frequency spikes.

OUTPUT NOISE

The LT3097 offers many advantages with respect to noise perform-

ance. 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 the 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.

LT3045 Datasheet(PDF) 37 Page - Analog Devices

LT3045 Datasheet(HTML) 37 Page - Analog Devices

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