Another hurdle to overcome is the increased self-noise in amplifiers that accompany lower I_Q biasing. The internal blocks seen in Figure 7 contributing the most noise in LDOs are the reference system (band gap), error amplifier and resistor-divider that scales the output voltage. Figure 8 shows a typical noise profile vs frequency. The two main types of noise generated from these blocks are:

• Thermal noise (also called 4kTR noise) is a particular concern for ultra-low-I_Q designs because it is linearly proportional to the resistance used. Both resistor-derived bias currents used in the error amplifier and reference blocks, and the resistance used in resistor dividers, are dominant contributors to thermal noise at frequencies >1 kHz.
• Flicker noise (also called 1/f noise) is a low-frequency noise <100 Hz, that can be mitigated by increasing the sizes of differential pairs in the reference system and error amplifiers. This larger sizing however creates obstacles for nano-power designs, as they increase self-induced leakage and add more capacitance, which slows down response times.

A simple method to evaluate the resultant noise for a given I_Q is to multiply the integrated noise over the frequency range of concern and the I_Q at the operating point of interest. You can usually find both numbers on device-specific data sheets.