The design of integrated linear regulators for battery applications is full of difficult compromises. Designs must deliver low operating current for long battery life, while supplying clean, well-regulated power in a sometimes noisy environment. This application note describes techniques for improving the rejection of AC artifacts while maintaining low operating current.
Regulator operating currents of less than 250µA limit the available gain bandwidth, making specifications such as noise, regulation, and power-supply rejection tricky to achieve. In portable communications, low-dropout linear regulators (LDOs) generate supply voltages for the RF circuitry that must be especially clean when powering the synthesizer and voltage-controlled oscillator (VCO). The supply that powers the regulator often includes wideband AC ripple superimposed on the DC. The LDO is expected to reject these artifacts. When an LDO is powered by a switching regulator, it must be able to cope with switching frequencies beyond 300kHz. Designers expect these capabilities without an increase in the LDO's quiescent current.
There are two specifications in the LDO data sheet that refer to the LDO's ability to reject the various forms of noise on the incoming supply. They are line regulation and power-supply rejection ratio (PSRR).
Line regulation measures the ability of the LDO to ignore changes of input voltage. Mathematically,