Author Topic: LDO feedback Resistors parallel res and cap purpose  (Read 182 times)

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Offline electronxTopic starter

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LDO feedback Resistors parallel res and cap purpose
« on: January 21, 2025, 02:28:17 pm »
What is the purpose of the series res and capacitor connected to the feedback resistors here?

 

Offline magic

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Re: LDO feedback Resistors parallel res and cap purpose
« Reply #1 on: January 21, 2025, 04:29:48 pm »
Not sure, but it could be included to improve transient response or very high frequency noise, or maybe it was necessary for stability (counteracting input capacitance of the FB pin or simply adding phase lead in the feedback loop near some critical frequency).

It's similar to adding the ADJ capacitor to ground in LM317 applications, but in this case it is only effective at hundreds of kHz and up.
 

Offline AnalogTodd

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Re: LDO feedback Resistors parallel res and cap purpose
« Reply #2 on: January 21, 2025, 06:02:05 pm »
Magic has the right ideas on the use of it. Usually having a capacitor there is frequency compensation for when the loop gain is dropped by having a resistor divider to the FB pin. My guess would be that the part showed some stability issues (not surprising considering they used a 22uF ceramic in 0805 case) and that adding the phase lead helped stabilize things.
Lived in the home of the gurus for many years.
 


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