8.2.2.2 Output Noise

The internal voltage reference is a key source of noise in an LDO regulator. The TPS795 has an NR pin which is connected to the voltage reference through a 250-kΩ internal resistor. The 250-kΩ internal resistor, in conjunction with an external bypass capacitor connected to the NR pin, creates a low-pass filter to reduce the voltage reference noise and, therefore, the noise at the regulator output. For the regulator to operate properly, the current flow out of the NR pin must be at a minimum, because any leakage current creates an IR drop across the internal resistor, thus creating an output error. Therefore, the bypass capacitor must have minimal leakage current. The bypass capacitor should be no more than 0.1 µF to ensure that it is fully charged during the quickstart time provided by the internal switch shown in Functional Block Diagrams.

For example, the TPS79530 exhibits 40 µV_RMS of output voltage noise using a 0.1-µF ceramic bypass capacitor and a 10-µF ceramic output capacitor. The output starts up slower as the bypass capacitance increases due to the RC time constant at the bypass pin that is created by the internal 250-kΩ resistor and external capacitor.