Data Sheet

LT83201

analog.com

Rev. A 20 of 45

APPLICATIONS INFORMATION

Low Frequency Output Noise

The LT83201 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 LT83201 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 LT83201 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 typically 4nV/√Hz at 10kHz.

With the previously mentioned noise sources operating at such low noise levels, other noise sources become non-

negligible contributors to the output noise. Choosing a compensation network that achieves good transient

performance with good phase margin ensures optimal noise performance. The Applications Information section on

Frequency Compensation provides guidelines on how to choose appropriate compensation.

Filtering Switching Ripple and High Frequency Noise

The LT83201 is a switching regulator and also has the typical artifacts of a switching regulator at the output, namely

a ripple at the fundamental switching frequency as well as high-frequency spikes associated with the fast switching

edges. While the output capacitor absorbs some of these spikes, the capacitor 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 fast transient response, ferrite beads, a PCB trace

or feedthrough capacitors may be used. For feedthrough capacitors, ensure sufficient feedthrough capacitors are

paralleled to carry the required load current. Figure 49 shows an example where two 3A-rated feedthrough capacitors

are used for additional switching ripple suppression to deliver up to 6A at the output. In practice, this is limited to 1A

for the LT83201.

Figure 49. Additional Output Ripple Filtering Using Feedthrough Capacitors

LT83201

GND

SW

OUTS

2.2µH

22µF

22µF

OPTIONAL

FEEDTHROUGH

CAPACITOR

3A RATED

4.7µF

VOUT