AN025

Analysis of LDO oscillation issues AN025 Introduction Using low noise and stable power supplies is a trend for power application. And LDO is one of the simplest applications. Yet LDO oscillation results in voltage instability. The analysis below tells you how to prevent LDO from oscillation. Vin Vout Error Amplifier R1 + Co GND Vref LDO R2 Fig 1.a: LDO basic structure Vin GND S Rds VgsGp Rpar Cpar D G Error Amplifier Vref LDO Zo Vout R1 + Co Resr Cb R2 Fig 1.b: LDO ac effective circuit The LDO in fig. 1.a is composed of four parts - pass element, error amplifier, reference voltage, and feedback resistor. The series pass element, PMOS transistor, is formed into a small signal model. The parasitic parameters, Rpar and Cpar, are signified by the output impedance of the error amplifier and the input impedance of PMOS. An ac prototype of LDO 1 AN025 is shown as fig. 1b, which is a combination of an output capacitor Co with RESR and bypass capacitor Cb. Why does an LDO re...