AN025

Analysis of LDO oscillation issues

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.

Rds

VgsGp

Error

Amplifier

R1

R2

Vref

GND

Vout

LDO

+

Co

S

D

G

Error

Amplifier

R1

R2

Vref

Vout

LDO

+

Co

GND

Vin

Cpar

Rpar

Vin

Resr

Zo

Cb

Fig 1.a: LDO basic structure

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

impedance of the error amplifier and the input impedance of PMOS. An ac prototype of LDO

1