AN019

6

Figure 15: AIC1117-33

Abnormal

Oscillation

at

Output Polarity

Conclusion

Conclusively, the major concern of the ripple noise is

the power line length because the power line

influences the system reliability indirectly through the

inductor effect. Either locating the input capacitor as

close as possible to the input polarity of LDO or

adding some bypass capacitors on the power line

may

reduce

the

complications

and

completely

eliminate the ringing. The concern of the power line

length on the PCB has always been left out of

consideration when LDO is applied to a system.

Again, we would like to point out the importance of

using the shortest power line to get the best LDO

performance

and

reduce

the

disturbance

from

switching converter. It is always essential to minimize

the loop among VIN, ground, input capacitor, and

bypass capacitor.

Ripple rejection of LDO represents the capability of

filtering noise. Users must be fully aware of the

PSRR capability they need to select the proper ripple

rejection capability of LDO.

Therefore, we are suggesting the users not only

concern about the cost but also performance when

they are making a decision on selecting an LDO.

Ripple noise has always been a complicated problem

to some fields. And it is crucial to get the best

resolution

to

solve

abnormality

and

external

disturbance. This can be obtained by a close

attention to the layout and trace of PCB.

High ESR for output capacitor must be applied to LDO. Also employment of an input capacitor is highly

recommended at the input polarity. They may result in the best and most efficient performance of the low

dropout linear regulator.