Data Sheet

LT83401/LT83402

analog.com

Rev. 0

21 of 44

APPLICATIONS INFORMATION

Low-Frequency Output Noise

The LT83401/LT83402 offers many advantages with respect to noise performance in the low-frequency range

(<100kHz). Conventional step-down regulators have several sources of low-frequency noise. The most critical noise

sources for a conventional regulator are its reference, error amplifier, noise from the resistor divider network used

for setting output voltage, and the noise gain created by this resistor divider.

Unlike most step-down regulators, the LT83401/LT83402 does not use a voltage reference. Instead, it uses a 100μA

current reference. One problem that conventional step-down regulators face is that the resistor divider setting the

output voltage gains up the reference noise. In contrast, the current reference architecture employed by the

LT83401/LT83402 allows unity-gain operation to avoid gaining up the noise from the reference to the output.

Therefore, if a capacitor bypasses the SET pin resistor, the output noise is independent of the programmed output

voltage.

The resultant output noise is primarily set by the error amplifier’s noise, typically 4nV/√Hz from 10kHz to 1MHz. With

the error amplifier operating at such low noise levels, the drive stage noise of the step-down regulator may become

a non-negligible contributor to the output noise. The loop gain of the regulator gains down the drive stage noise;

therefore, the choice of the compensation network determines the drive stage noise contribution.

In general, choosing a compensation network that achieves good transient performance ensures that the drive stage

noise contribution is minimized. The Frequency Compensation section provides guidelines on how to choose

appropriate compensation. When choosing compensation, it is essential to ensure that the loop has a good phase

margin to prevent peaking in the closed-loop response. Peaking in the closed-loop response gains up the noise,

which should be avoided.

Filtering Switching Ripple and High-Frequency Noise

The LT83401/LT83402 is a switching regulator that exhibits the typical artifacts of a switching regulator at the output,

namely a ripple at the fundamental switching frequency and high-frequency spikes associated with the fast

switching edges. While the output capacitor absorbs some of these spikes, the capacitor’s Equivalent series

inductance (ESL) limits its ability to do so at high frequencies. Additional filtering at the output in the form of

feedthrough capacitors, ferrite beads, or an additional LC filter stage is recommended to eliminate these high-

frequency spikes and significantly reduce switching ripple.

If additional switching ripple reduction is required while retaining a fast transient response, ferrite beads, a Printed

circuit board (PCB) trace, or feedthrough capacitors may be used. For feedthrough capacitors, ensure sufficient

feedthrough capacitors are paralleled to carry the required load current. Figure 47 shows an example where two 1.5A-

rated feedthrough capacitors are used for additional switching ripple suppression to deliver up to 3A at the output.

In practice, this is limited to 1A for the LT83401 and 2.5A for the LT83402.

Figure 47. Additional Output Ripple Filtering Using Feedthrough Capacitors

LT83401/

LT83402

GND

SW

OUTS

2.2µH

100µF

100µF

×6

OPTIONAL

FEEDTHROUGH

CAPACITOR

1.5A RATED

4.7µF

×2

VOUT