ADP151 datasheet(29/47 Pages) AD

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Data Sheet

LT3046

analog.com

Rev 0

29 of 47

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 0.02in reduce the overall inductance to about one-fifth of a single wire.

If a battery mounted close powers the LT3046, a 4.7μF input capacitor suffices for stability. However, if a distantly

located supply powers the LT3046, use a larger value input capacitor. Use a rough guideline of 1μF (in addition to

the 4.7μF minimum) per 6in 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 LT3046 to help this issue. However, this method requires significantly more output capacitance compared to

additional input bypassing. Series resistance between the supply and the input of the LT3046 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 the

dropout voltage. A better alternative is to use a higher ESR tantalum or electrolytic capacitor at the input of the

LT3046 in parallel with a 4.7μF ceramic capacitor.

PSRR and Input Capacitance

For applications using the LT3046 for post-regulating switching converters, placing a capacitor directly at the input

of the LT3046 results in AC current (at the switching frequency) to flow near the LT3046. This relatively high-

frequency switching current generates a magnetic field that couples to the output of the LT3046, degrading its

effective PSRR. While highly dependent on the PCB, the switching preregulator, and the input capacitance, among

other factors, the PSRR degradation can be easily more than 30dB at 1 MHz. This degradation is present even if the

LT3046 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 LT3046 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 LT3046, as long as the output capacitor of the

switching converter is located more than an inch away from the LT3046, remove the input capacitor of the LT3046.

Magnetic coupling rapidly decreases with increasing distance. Nonetheless, if the switching preregulator is placed

too far away (conservatively more than a couple inches) from the LT3046, with no input capacitor present, as with

any regulator, the input of the LT3046 oscillates at the parasitic LC resonance frequency. In addition, it is generally

a common (and a preferred) practice to bypass the regulator input with some capacitance. Therefore, this option is

fairly limited in its scope and not the optimal solution.

To that end, Analog Devices recommends using the LT3046 evaluation board layout for achieving the best possible

PSRR performance (refer to the LT3046EVK#DFN evaluation board user guide). The LT3046 evaluation board layout

uses magnetic-field cancellation techniques to prevent PSRR degradation caused by this high-frequency current

flow, while using the input capacitor.

Filtering High-Frequency Spikes

For applications where the LT3046 is used to post regulate a switching converter, its high PSRR effectively

suppresses any noise present at the switching frequency of the switching converter, typically 100kHz to 4MHz.

However, the high-frequency (hundreds of MHz) spikes, beyond the bandwidth of the LT3046, associated with the

power-switch transition times of the switching converter almost directly pass through the LT3046. 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, 0.5in) PCB trace between the output of the switching

converter and the input of the LT3046 can serve as an LC filter to suppress these high-frequency spikes.

Output Noise

The LT3046 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 the output voltage, and the noise gain created by this

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