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How to choose Co in LDO

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analog_cmos

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Hi
anybody give me some valuable suggestion for choosing of Co of LDO.
i tried Co=1/2*pie*P1*Ro_pass equation at 1KHz. Is it right way .Generally Co value is 4.7 or 2.2 uF ??????? i am waiting for reply
 

There is no general recommendation for Co.
As soon as you know the reason for including Co (time response to input voltage and load changes, loop stability) you will see that the value depends on several parameters (placement of poles and zeros) . And don't forget the ESR effect.
 

Thanks Lvw,
it means that initially i choose any random value of Co and Resr than find out effect of LDO stability pole/zero position , transient response??????///
 

analog_cmos said:
Thanks Lvw,
it means that initially i choose any random value of Co and Resr than find out effect of LDO stability pole/zero position , transient response??????///
Click to expand...

Mmmmh....this would be a "try and error" procedure. Not engineers-like.
The better way is as follows:
Create a BODE plot for the loop gain without any Co. From this plot you can derive all the information necessary to choose the right value for Co with the aim to ensure good stability.
 

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