Nowadays, high-efficiency low-noise power supplies for portable equipment such as cellular phones and personal digital assistance (PDA) are in great demand. Low-dropout regulators (LDRs) are usually used to obtain this stable and low-noise supply voltage. Therefore, the stability and the accuracy of the output voltage of the LDR are very important in the design of the power supply. Generally, a cascade error amplifier for achieving a high loop gain is commonly employed in the design of the LDR in order to obtain an accurate output voltage. However, as the input voltage of the portable equipment decreases, the supply voltage of the LDRs is also reduced. This will lower the design margin of the LDR and the cascade error amplifier can no longer be used. One way to solve this problem is to...[ Read more ]

Nowadays, high-efficiency low-noise power supplies for portable equipment such as cellular phones and personal digital assistance (PDA) are in great demand. Low-dropout regulators (LDRs) are usually used to obtain this stable and low-noise supply voltage. Therefore, the stability and the accuracy of the output voltage of the LDR are very important in the design of the power supply. Generally, a cascade error amplifier for achieving a high loop gain is commonly employed in the design of the LDR in order to obtain an accurate output voltage. However, as the input voltage of the portable equipment decreases, the supply voltage of the LDRs is also reduced. This will lower the design margin of the LDR and the cascade error amplifier can no longer be used. One way to solve this problem is to use the cascade error amplifier. Nevertheless, the poles introduced by the high impedance nodes of the error amplifier will cause the LDR to be unstable and make the frequency compensation of the LDR very difficult.

In this thesis, the loop-gain frequency compensation on LDRs with low supply voltage is discussed. A systematic loop-gain compensation method based on the nested Miller compensation (NMC) is introduced. Theoretical analysis on the proposed loop-gain compensation is studied thoroughly. Moreover, in order to increase the speed of the loop response, three compensation topologies based on NMC with the addition of 1) voltage buffer, 2) feedforward transconductance push-pull output stage and 3) null resistor are proposed. Simulation and experimental results reveal the performance enhancement by using these topologies. Moreover, simulations show that the stability and the accuracy of the output voltage of the LDR are greatly increased. By using the proposed topologies, the LDR is always stable for the whole range of the operating load current.