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# How to properly do AC open loop LDO simulation?

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#### jutek

##### Full Member level 4
hello

could you tell me how to do AC open loop LDO simulation properly?

how the inputs should be polarized when i open the loop to pass device give me the current i want to

when i do transient and op analyses, everything is ok. vref is connected to - and output voltage through feedback divider to +.

i want the AC simulation to be adequate TRANSIENT simulation, to be sure how gain, UGF and pole/zero position influence transient response

regards

ldo ac simulation

jutek said:
hello

could you tell me how to do AC open loop LDO simulation properly?

how the inputs should be polarized when i open the loop to pass device give me the current i want to

when i do transient and op analyses, everything is ok. vref is connected to - and output voltage through feedback divider to +.

i want the AC simulation to be adequate TRANSIENT simulation, to be sure how gain, UGF and pole/zero position influence transient response

regards

Hi ,
In the attached pdf go to AC analysis ( page 17, Chapter 2). It clearly explains where the loop must be broken in order to do ac analysis.

#### Attachments

• ldo_book_140.pdf
3.2 MB · Views: 775

ac response ldo

hello

yes, i have this book, and i exactly break the loop in this place. but what should be connected to - after breaking? if i use only AC=0.1 and no DC value the result is not good cause opamp is not polarized well

ldo simulation setup to characterize the design

jutek said:
hello

yes, i have this book, and i exactly break the loop in this place. but what should be connected to - after breaking? if i use only AC=0.1 and no DC value the result is not good cause opamp is not polarized well

Add large R&C in your feedback path,you can build up the DC point and break the AC feedback path.

#oz

### #oz

Points: 2
ldo ac analysis

add a dc offset voltage at the input pin,
exmple:vin in 0 dc 1.25 ac 1

break loop pole/zero hspice

add a dc offset voltage at the input pin,
exmple:vin in 0 dc 1.25 ac 1

Yes, remerber to add a dc voltage.
The dc voltage would make the DC operation point accurate.

ldo simulation setup

in hspice there is a resistance that when dc the value is zero, but when ac, it is open. Can we just use the thing?

ldo current profiles

add a ac resister(dc=0.001Ohm, ac=1000,000kOhm) between the two break point;
feed ac signal in though a 1F capacitor;

ac analysis of ldo

I also confused it, sometimes, I use the AC source with ac=1V to break the loop.

sometime, I use the resistor with AC=1G , and a cap=1F . and sometime, I use

the ind=1G and a cap =1F.

the only diff thing I thought is the phase of it different. But offten, the gain plot is

also diff.

So, I use the method mostly equal what I think it should be.

how to do ac analysis of ldo

another way is add a large inductor on you feedback path, and a capacitor connect to ground from opamp side. this may help you to do AC simulation.

ac anslysis of ldo

alenhan said:
another way is add a large inductor on you feedback path, and a capacitor connect to ground from opamp side. this may help you to do AC simulation.

this maybe a good idea!
but it could make some other problem. when I connect a capacitor (c=10p) parallelled with resistor R3, it should bring a pole and a zero. But because the large capacitor(c=1KF) connected in the input so by hspice simulation the capacitor parallelled with the resistor R3 will not take some effect!

another way is to use two LDOs , one closed loop and the other is open loop , then use the closed loop LDO to get the DC for the open loop LDO.

ldo simulations

1/(C_OL*L_OL)^0.5 must be less than simulation min. frequency.

ldo openloop simulation

hi mark_nctu!
Why 1/(C_OL*L_OL)^0.5 must be less than simulation min. freq? Can you explain it for me?
Thanks.

ldo spice simulation

Can I use the resistor instead of the inductor for AC simulation.

coolroyy

myblues said:
alenhan said:
another way is add a large inductor on you feedback path, and a capacitor connect to ground from opamp side. this may help you to do AC simulation.

this maybe a good idea!
but it could make some other problem. when I connect a capacitor (c=10p) parallelled with resistor R3, it should bring a pole and a zero. But because the large capacitor(c=1KF) connected in the input so by hspice simulation the capacitor parallelled with the resistor R3 will not take some effect!

pls check the ac response in the VDIV

gavinchiu said:
Can I use the resistor instead of the inductor for AC simulation.

Yes,but the res must very large

LDO AC simulaton

You can break the loop using L (1G) and C(1F) values. AC signal can pass through the capacitor and inductor allows DC value and breaks the loop

Re: LDO AC simulaton

Please find the attach document for more clarification

#### Attachments

• f4_3269.pdf
149.2 KB · Views: 285

V
Points: 2
LDO AC simulaton

really good one,i just need this,support!

Re: LDO AC simulaton

i would like to ask how to get this book by free of charge

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