Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics 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.

Help me decide whether I need to add another buffer to a LDO regulator

Status
Not open for further replies.

alicia8283

Junior Member level 2
Joined
Mar 3, 2005
Messages
21
Helped
2
Reputation
4
Reaction score
1
Trophy points
1,283
Activity points
1,436
hi all,

i need to design a LDO regulator right now. the error amp for the regulator is only a simple one stage op-amp. Do i need to add another buffer for it?
 

pseudockb

Member level 5
Joined
Aug 4, 2004
Messages
89
Helped
8
Reputation
16
Reaction score
1
Trophy points
1,288
Activity points
908
Re: LDO question

It really depends on how large your power transistor is. If the power MOS is large, you might need to add a buffer stage to push the non-dominant pole further. And also, adding buffer will help to improve your power supply ripple rejection.
 

cinch

Member level 2
Joined
Aug 25, 2005
Messages
44
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,286
Location
India
Activity points
1,593
Re: LDO question

Try one stage folded cascode opamp.... Advantage with this architecture is, folded cascode opamp's stability improves as u increase load cap. Since power mosfet is usually big, it offers huge capacitive load to error amplifier. So u dont have much stability issues.
 

xxf_86

Junior Member level 1
Joined
Aug 31, 2003
Messages
19
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
127
LDO question

I think it just decided by the pass device, if the load current is much large, so the pass device will be very large and the unstable will be a big problem, so the buffer will be added!
 

renwl

Advanced Member level 1
Joined
Apr 26, 2004
Messages
453
Helped
27
Reputation
54
Reaction score
5
Trophy points
1,298
Location
shanghai,china
Activity points
1,805
Re: LDO question

pseudockb said:
It really depends on how large your power transistor is. If the power MOS is large, you might need to add a buffer stage to push the non-dominant pole further. And also, adding buffer will help to improve your power supply ripple rejection.

why adding buffer will help to improve your power supply ripple rejection ?
thanks.
 

pseudockb

Member level 5
Joined
Aug 4, 2004
Messages
89
Helped
8
Reputation
16
Reaction score
1
Trophy points
1,288
Activity points
908
Re: LDO question

The ability to suppress the power supply noise depends on the bandwidth of the loop gain. Once the loop gain starts to drop, the loop can no longer rejects power supply noise effectively. The 3dB bandwidth of the loop is determined by the pole at the output of the error amp. Adding a buffer after the error amp will push the 3dB bandwidth of the loop further.
 

holddreams

Full Member level 6
Joined
Aug 2, 2005
Messages
351
Helped
15
Reputation
30
Reaction score
7
Trophy points
1,298
Location
Shanghai
Activity points
4,237
Re: LDO question

cinch said:
Try one stage folded cascode opamp.... Advantage with this architecture is, folded cascode opamp's stability improves as u increase load cap. Since power mosfet is usually big, it offers huge capacitive load to error amplifier. So u dont have much stability issues.


How should the dc gain of the folded cascode opamp?
25dB to 45dB?

thanks.
 

liuyonggen_1

Full Member level 4
Joined
Dec 12, 2005
Messages
193
Helped
7
Reputation
14
Reaction score
3
Trophy points
1,298
Activity points
2,424
Re: LDO question

pseudockb said:
The ability to suppress the power supply noise depends on the bandwidth of the loop gain. Once the loop gain starts to drop, the loop can no longer rejects power supply noise effectively. The 3dB bandwidth of the loop is determined by the pole at the output of the error amp. Adding a buffer after the error amp will push the 3dB bandwidth of the loop further.



what's the dominant pole in the loop? if the output of error amplifier contribute a non-dominant pole?
 

holddreams

Full Member level 6
Joined
Aug 2, 2005
Messages
351
Helped
15
Reputation
30
Reaction score
7
Trophy points
1,298
Location
Shanghai
Activity points
4,237
Re: LDO question

liuyonggen_1 said:
pseudockb said:
The ability to suppress the power supply noise depends on the bandwidth of the loop gain. Once the loop gain starts to drop, the loop can no longer rejects power supply noise effectively. The 3dB bandwidth of the loop is determined by the pole at the output of the error amp. Adding a buffer after the error amp will push the 3dB bandwidth of the loop further.



what's the dominant pole in the loop? if the output of error amplifier contribute a non-dominant pole?
 

jonneylu

Member level 1
Joined
Dec 7, 2005
Messages
33
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,459
LDO question

if the op drive R,buffer is needed.
 

steve_guo

Member level 1
Joined
Sep 29, 2004
Messages
35
Helped
4
Reputation
8
Reaction score
2
Trophy points
1,288
Activity points
183
LDO question

I think your design relys on you specs.
Sometime,one stage op-amp is sufficient.
 

niezimei

Junior Member level 3
Joined
Dec 1, 2006
Messages
28
Helped
2
Reputation
4
Reaction score
0
Trophy points
1,281
Activity points
1,390
LDO question

If you have a huge Cout say at the order of uF, obviously the output pole is the dominate.
 

edajason

Member level 3
Joined
Nov 5, 2004
Messages
60
Helped
6
Reputation
12
Reaction score
5
Trophy points
1,288
Activity points
441
Re: LDO question

The simple one stage op-amp will do it. You need to choose the right type of the input transistor in order to get a good PSRR. Say, you are gonna use PMOS pass transistor to provide the current, use n-input pair for your amplifier. A good choice for the amplifier can be a mirror amplifier (better output swing).

alicia8283 said:
hi all,

i need to design a LDO regulator right now. the error amp for the regulator is only a simple one stage op-amp. Do i need to add another buffer for it?
 

fanrong

Banned
Joined
Oct 20, 2006
Messages
124
Helped
3
Reputation
6
Reaction score
1
Trophy points
1,298
Activity points
0
Re: LDO question

Why use a n mos as input stage transistor ?
 

mists

Full Member level 5
Joined
May 14, 2002
Messages
277
Helped
22
Reputation
44
Reaction score
9
Trophy points
1,298
Location
China
Activity points
1,281
LDO question

what is the mean of mirror amplifier ? thanks!
 

liuyonggen_1

Full Member level 4
Joined
Dec 12, 2005
Messages
193
Helped
7
Reputation
14
Reaction score
3
Trophy points
1,298
Activity points
2,424
Re: LDO question

fanrong said:
Why use a n mos as input stage transistor ?

in order to force the PMOS pass transistor operated in saturation using the larger output voltage of N-input amplifier.

Added after 2 minutes:

what's the mirror amplifer? the complemetary output amplifier?
 

baniliu

Junior Member level 1
Joined
Jan 11, 2007
Messages
19
Helped
1
Reputation
2
Reaction score
1
Trophy points
1,283
Activity points
1,373
LDO question

sometimes

Added after 1 minutes:

sometime the buffer may form another non-dominant pole in the gate of the buffer with first stage, if the loop gain is high enough and within the unity gain bandwidth
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top