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output capacitor of LDO

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pugongying

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output noise ldo

What is the main contribution of LDO's large external output capacitor? Better transient response or low output noise? Or other reason?

What about some on-chip LDO with small (<100pF) onchip output capacitor,do they have sever problems?
 

Yes. Better response and better noise.
Only on-chip cap (<100pF) will lead to very bad response
 

Its better transient response and also noise. This cap also decides the stability and pole zero compenastion.
 

1. What is the main contribution of LDO's large external output capacitor? Better transient response or low output noise? Or other reason?

Ans: Use a large capacitor (normally external) to create a dominant pole and to provide an instantaneous charge source during fast load transient. Thus, it attains better transient response.

2. What about some on-chip LDO with small (<100pF) onchip output capacitor,do they have sever problems?
On Chip LDO (without external capacitor) requires an internal fast transient path to compensate for the absence of external cap.
 

Not need. Some new structure for LDO can be stable even with 10pF internal cap. But bandwidth is small and about 10~200kHz.
But load response will easily pull the output to ground.
 

Output capacitor is mainly used to reduce the transient load regulation
if the output capacitor is not to be used,then the bandwidth of the EA should be vast.


yes, this big load cap creates a pole at low frequency. hence compensation is reqd.
 

LDO output cap and output resistance make a pole. So it affect the close loop stability.
Larger cap value can make the pole move to dc, so noise filtering effect is better.
Larger cap value also reduce the output resistance at higher frequency, so LDO transient responce of spike current kick is better.
Larger cap value make a bigger output RC time constant, when LDO load a current step, the output slew rate is slow.
 

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