Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

How much should the error amplifier'gain be?

Status
Not open for further replies.

jefferson

Junior Member level 3
Joined
Mar 17, 2006
Messages
26
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
shenzhen,China
Activity points
1,550
error amplifier op amp regulation

Hi,every.
You know that a LDO need an error amplifier(EA) to rgulate the Vout, the DC gain of EA and its bandwidth influence the line regulation and load regulation and PSRR and so on.
My LDO use an EA whose DC gain is about 60dB,UGB is 10M Hz. The whole loop DC gain is 68dB, but UGB is only 200K Hz.
The LDO has a bad line regulation and a bad load regulation.

What's the problem? the DC gain too low?or the bandwidth too small?

Any reply would be appreciated.
 

electronrancher

Advanced Member level 1
Joined
Mar 24, 2002
Messages
447
Helped
46
Reputation
92
Reaction score
11
Trophy points
1,298
Activity points
6,942
bad load regulation error amplifier

if your loop gain is really 60db, then the percent output error due to the error amplifier is 1/1000 of VOUT. Anything else is due to something else.

Try looking at the reference voltage during line and load sweeps. do you really see a difference in the voltage at the op amp terminals? if that voltage is greater than 1/1000 of the ouput error, either your amp does not actually have 60dB gain or you have another problem like headroom issues.
 

pfd001

Advanced Member level 1
Joined
Jul 23, 2005
Messages
410
Helped
45
Reputation
90
Reaction score
10
Trophy points
1,298
Activity points
3,214
For load regulation, maybe your gain is not enough when the LDO works in the load conditions
 

jefferson

Junior Member level 3
Joined
Mar 17, 2006
Messages
26
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
shenzhen,China
Activity points
1,550
electronrancher said:
if your loop gain is really 60db, then the percent output error due to the error amplifier is 1/1000 of VOUT. Anything else is due to something else.

Try looking at the reference voltage during line and load sweeps. do you really see a difference in the voltage at the op amp terminals? if that voltage is greater than 1/1000 of the ouput error, either your amp does not actually have 60dB gain or you have another problem like headroom issues.
The loop gain simulated is really above 60 dB(maybe infact it doesn't reach this number?).I can't see if there is a voltage difference at the opamp terminals.
You mean headroom problem?I think headroom problem is about dropout voltage.Even if the Vin is high,line regulation is no good.

A phenomena is that when the Rload is light (Iout is small),the line regulation is good.Maybe the loop gain is still no enough?
 

electronrancher

Advanced Member level 1
Joined
Mar 24, 2002
Messages
447
Helped
46
Reputation
92
Reaction score
11
Trophy points
1,298
Activity points
6,942
no, if you simulated the ac response correctly, and your gain is really 60db you should have no line/load reg problems.

to simulate ac response, put a 1v ac source in the FB line. Measure from the negative terminal to the positive terminal of the ac source. Let's say your resistor divider connects to FB, and your opamp terminal is called VN. Usually FB is attached directly to VN. Now we split it with an AC source. The gain is db(V(VN)/V(FB)).

Run an AC sim at several different loads. If your AC gain is really 60dB, you probably have a problem with headroom - ie: the PMOS gate needs to be pulled down to 1v, but due to some stacked cascodes, the error amp can only pull down to 1.5v. this will cause load reg problems.
 

szekit

Member level 5
Joined
May 25, 2006
Messages
89
Helped
8
Reputation
16
Reaction score
3
Trophy points
1,288
Location
Hong Kong
Activity points
1,828
your load is loading down your gain? trying testing it with different loads.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top