5 How to Reduce the LDO Noise?

The primary noise source in the LDO is the band gap. The noise in LDO can be reduced using two methods. The following discussion explains both the methods.

One method to reduce the noise is by reducing the bandwidth of the LDO. This can be done by lowering the bandwidth of error amplifier inside the LDO. But if we reduce the bandwidth of error amplifier then it reduces the transient response of the LDO.

Another method is by using a low-pass filter (LPF). As we know, the most dominant source of noise in an LDO is the internal band gap. A LPF can be inserted between the band gap output and the input of the error amplifier. This reduced the band gap noise before it is gained up by the error amplifier. Typically this LPF is formed with a large internal resistor and an external capacitor. The cutoff frequency of this filter is set as low in frequency as possible to filter out nearly all of the noise coming from the band gap.

There is always the question “Why the huge power pass element (mostly FET), which takes up most of the total die area, is not a primary noise contributor?” the answer is the lack of gain. The primary noise source, the band gap, is connected to the input of the error amplifier and thus amplified by the gain of the error amplifier. As we know, the procedure to find the output noise is first refer each noise contributor to the op-amp input; so to find the noise from the pass FET you would first divide the noise contribution by the open-loop gain that exists between it and the error amplifier input. This gain is very large; therefore, the noise contribution from the pass FET is usually negligible.

To summarize, the LDO noise and PSRR both are important specifications to be taken into account when selecting the LDO. There are two ways the LDO noise is specified, and the user should look for the appropriate specification for their application.