AN025 Datasheet, PDF (1/6 Pages) Analog Intergrations Corporation – Analysis of LDO oscillation issues

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
R1
Error
Amplifier
+
Co
Vref
R2
GND
LDO
Fig 1.a: LDO basic structure
Vin
GND
S
Rds
VgsGp
Rpar ^Cpar
D
G
Error
Amplifier
Vref
LDO
Zo
Vout
R1
+
Co
Cb
Resr
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, R_par and C_par, are signified by the output
impedance of the error amplifier and the input impedance of PMOS. An ac prototype of LDO
1