Why is the error amplifer used in LDO usually an OTA?

[greenplanet]

which type of differential error amp used for ldo

Why is the error amplifer used in LDO usually an OTA? What are its advantages over other amps?

Particularly, why don't we use a class AB amplifier? What are its drawbacks?

 

[jutek]

ota has the current output which is needed to drive large capacity of pass transistor.

the more current, the pole related to ota's output and pass transistor capacity goes to higher frequencies. it's needed to ensure frequency compensation

regards

 

[greenplanet]

isn't that OTA has high output impedance and high gain, which leads to better line and load regulations? However, load transient would be bad.

for the pole at the ota's output and gate of the pass transistor, isn't that 1/RC; so as ota has high Ro, gate cap of pass transistor is significant as well, the corresponding pole is gonna be at low frequency!?

 

[ambreesh]

An amplifier is a OTA followed by buffer to supply current.
In the LDO circuit the error amp is used to control the gate drive on the pass transistor.
The gate of the pass transistor is a capacitor and would only need current for charging and discharging, but no continious current requirment which is from a amplifier.
At times to move the pole formed by gate of the pass transistor we do buffer the OTA output, but that is just to decrease the output resistance seen at gate of passtransistor.
In an LDO our problem is the output pole due to load cap and the internal pole from the passtransistor. The output ploe location changes depending on the load current, which i am sure you are well aware off.

 

[suria3]

OTA only will have high impedance either at the input or the output and as not in normal 2 stage opamp design. Thus, the number of poles involved is less and it minimize the work of compensating the LOD loop when it comes to loop stability.

Added after 57 seconds:

OTA only will have high impedance either at the input or the output and as not in normal 2 stage opamp design. Thus, the number of poles involved is less and it minimize the work of compensating the LDO loop when it comes to loop stability.

 

[lijianheng]

OTA has high gain performance, and full differential topology, better PSRR performance in close loop.

 

[ambreesh]

Hi lijianheng,

Can you kindly tell me if you are suggesting use of a fully differential circuit for ota, in this case.
Also one has to look for overall system PSRR not just the PSRR for the OTA.

 

[hung_wai_ming@hotmail.com]

OTA is voltage-in and current-out, so it drives capacitive loads only as it's without a output buffer.
Pass gate is a large cap PMOS or NMOS, so OTA is a good choice. However, OTA has high input and output impedance, so buffering the OTA output is commonly seen.
One of the beauty to change the transconductance of OTA is to change the bias current, so a lot easier for LDO to design. Also, OTA can be set wide bandwidth, so faster settling for LDO

 

[greenplanet]

Thanks very much for all of your replies.

So I got info on the advantages using an OTA, what about the ClassAB push-pull amp?

 

[hung_wai_ming@hotmail.com]

what about you tell me the advantages of ClassAB push-pull amp first,. While you are answering yourself, you find the answer already

 

[greenplanet]

umm.. I am not sure if I am correct. Class AB push-pull has a higher driving capability and higher efficiency. That's all I know, in terms of frequency compensation and transient response, does it have any advantages over using an OTA?

 

[luojiexiaojun]

good comments

 

[winsonpku]

First, thanks for your replies, those very important for us!
As for the classAB amp, i think they have the below advantages:
1) The slew rate of the error amp is not a problem becuase the symetric drive ability.
2)Reduce the response time of the LDO in the transient condition which is given by the smaller slew rate time.
3)Improvement of the current effiency when the load current is changed frequently,which is got by the larger bandwidth of LDO.
Drawback maybe the larger quesient current of the classAB amp.
What is your comments?

what about you tell me the advantages of ClassAB push-pull amp first,. While you are answering yourself, you find the answer already