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# about the LDO loop stability

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#### anqilo2

##### Newbie level 1
hello everyone:
for a long time, i have been trying to study the LDO loop stability by doing bode plot and load transient test. however, i can only find many reference information about the zero and pole analysis but not the method of measure or practical test plot.
attached please find the test circuit for bode plot, the reference voltage is 1.25V getting from adj pin to gnd. the output voltage is 2.5V, now i supply a ac source to the adj pin and sweep the frequency response at the ratio of Vo/Vac.
unfortunately, the test result is not perfect, even i do not kown if it is a right tendency.
so, if there is someone could help me, giving me some suggestions or any data, thank you very much.

---a layman for analog

Is it vout?
You try to plot V2/Vac,here "2" is another end of inductor.

Also,in your circuit,there are two node named "2".

anqilo2 said:
hello everyone:
for a long time, i have been trying to study the LDO loop stability by doing bode plot and load transient test. however, i can only find many reference information about the zero and pole analysis but not the method of measure or practical test plot.
attached please find the test circuit for bode plot, the reference voltage is 1.25V getting from adj pin to gnd. the output voltage is 2.5V, now i supply a ac source to the adj pin and sweep the frequency response at the ratio of Vo/Vac.
unfortunately, the test result is not perfect, even i do not kown if it is a right tendency.
so, if there is someone could help me, giving me some suggestions or any data, thank you very much.

---a layman for analog

the Vo is Vout
if i plot V2/Vac, the gain is close to 0dB from 0 to 100kHz

anqilo said:
the Vo is Vout
if i plot V2/Vac, the gain is close to 0dB from 0 to 100kHz

Can you draw your LDO schematic?
When doing AC analysis,I usually add AC=1,and measure the output of V2.
Also the C and L adding in the loop are big enough.

1.L=200mH maybe too small.
try L=1TH and C=1TF to cut off AC loop.
2.AC source change to AC=1,So only read node 2(here "2" is another end of inductor) voltage can get open loop gain and phase(bode plot).

b/r

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