Data Sheet

LT83203/LT83205

analog.com

Rev. B

20 of 44

APPLICATIONS INFORMATION

Low Frequency Output Noise

The LT83203/LT83205 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 LT83203/LT83205 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

LT83203/LT83205 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 a 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 LT83203/LT83205 is a switching regulator and also have 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 47 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 3A

for the LT83203 and 5A for the LT83205.

Figure 47. Additional Output Ripple Filtering Using Feedthrough Capacitors

LT83203/

LT83205

GND

SW

OUTS

0.47µH

100µF

100µF

×6

OPTIONAL

FEEDTHROUGH

CAPACITOR

3A RATED

4.7µF

×2

VOUT