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

[SOLVED] LDO Output resistance.

Status
Not open for further replies.

saqib.shah06

Junior Member level 1
Joined
Nov 29, 2009
Messages
18
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
India
Activity points
1,471
Hi everyone,
I have a question regarding the output impedance of a LDO, which I am trying to simulate in HSPICE.

It is a basic design with an error amplifier and a PMOS pass transistor. I am having the following doubt.

For any given LDO, the dominant pole lies at P= 1/RC (R = rds of the pass transistor and C=output cap ). My HPSICE simualtions confirm this. I am injecting the AC signal by breaking the feedback loop (this ahs been perfectly design - i have checked it multiple times). However when I do a .tf analysis in HSPICE, the .lis file says that my output impedance is 100milliohm. (the rds of the pass transistor is around 75 ohm). Can someone please explain this discrepancy?

btw I am using very large feedback resistors (100k each), and a current source load. The output current is around 100mA

Thanks a lot!
 

erikl

Super Moderator
Staff member
Joined
Sep 9, 2008
Messages
8,112
Helped
2,682
Reputation
5,344
Reaction score
2,283
Trophy points
1,393
Location
Germany
Activity points
44,152
... when I do a .tf analysis in HSPICE, the .lis file says that my output impedance is 100milliohm. (the rds of the pass transistor is around 75 ohm). Can someone please explain this discrepancy?
rds is a static (DC) parameter, the output impedance is a dynamic (small signal) parameter. Due to the LDO regulation mechanism with nearly full feedback, i.e. gain≈1, the (low frequency) output impedance -- in a 1st (coarse) approximation -- is rds/Aol -- Aol being the open loop gain of the LDO. So your .tf analysis result seems quite correct.
 

saqib.shah06

Junior Member level 1
Joined
Nov 29, 2009
Messages
18
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
India
Activity points
1,471
erikl , thanks for you reply. but I still have one query. When I simulate the LDO, I am breaking the loop gain by inserting a large (1meg) inductor and using a cap to inject the input signal, effectively making turning it into an open loop - in such a case the outpur R shouldnt be affected by the loop gain, should it?


rds is a static (DC) parameter, the output impedance is a dynamic (small signal) parameter. Due to the LDO regulation mechanism with nearly full feedback, i.e. gain≈1, the (low frequency) output impedance -- in a 1st (coarse) approximation -- is rds/Aol -- Aol being the open loop gain of the LDO. So your .tf analysis result seems quite correct.
 

erikl

Super Moderator
Staff member
Joined
Sep 9, 2008
Messages
8,112
Helped
2,682
Reputation
5,344
Reaction score
2,283
Trophy points
1,393
Location
Germany
Activity points
44,152
I am breaking the loop gain by inserting a large (1meg) inductor and using a cap to inject the input signal, effectively making turning it into an open loop - in such a case the outpur R shouldnt be affected by the loop gain, should it?
Also with 1MH in the feedback path you still have a DC gain of ≈1 , isn't it? At higher frequencies the output impedance will increase of course, due to decreasing Aol and feedBack.
 

saqib.shah06

Junior Member level 1
Joined
Nov 29, 2009
Messages
18
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
India
Activity points
1,471
Also with 1MH in the feedback path you still have a DC gain of ≈1 , isn't it? At higher frequencies the output impedance will increase of course, due to decreasing Aol and feedBack.
@erikl - perfect! I talked to my professor today and he gave the exact same explanation, I understand now :smile:. Thanks a lot! The whole .tf thing being about DC caused the confusion.
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top