4 Implications of the LDO Noise

Let us take an example of a DC-DC Converter with integrated LDOs (such as TPS57140-Q1).

The bandgap noise that is internal to the LDO regulator becomes a limiting factor in the rejection of high frequency components. One of the example where its bad affect can be seen is when a a fast-falling input transient is applied at the input of the DC/DC converter. During the fast input falling edge, if the slew rate of the input is higher than a particular value, the internal LDO regulator of the device resets because of the power-supply rejection-ratio (PSRR) limitation. The fast transition corresponds to higher frequencies. The bandgap noise that is internal to the LDO regulator becomes a limiting factor in the rejection of high frequency components. For example, using the TPS57140- Q1 design simulation and bench test measurement, the resulting slew-rate value measured is 1.2 V/μs. If the slew rate is higher than this value, the device gets disabled and regenerates a soft start. Higher ESR of the input capacitor negatively affects the slew rate of the input voltage and the duration of this rate because of high current transient across the ESR according to ESR × C × dV/dt. Therefore, the use of a low-ESR ceramic capacitor is recommended.

Refer to Design Considerations for DC-DC Converters in Fast-Input Slew Rate Applications (SLVA693) for more information