How to set the value of the feedback resistors?

[liuxufeishuang]

The max output current of my LDO is 50mA, the output voltage is 1.2V,and the reference voltage is 0.725V,
β=0.6,how to set the value of the feedback resistor?

 

[goldsmith]

Dear liuxufeishuang
Hi
It is simple Vout= Vref*(1+(R1/R2))
Best Wishes
Goldsmith

 

[liuxufeishuang]

Dear liuxufeishuang
Hi
It is simple Vout= Vref*(1+(R1/R2))
Best Wishes
Goldsmith

I agree，but my means is how to set the value of the feedback resistors.
R1=18K,R2=12K ? or R1=72K,R2=48K ? or R1=180K,R2=120K? or others?
If the value of the feedback resistors are too low, For IR1=IR2=1.2V/(R1+R2), the higher IR1 will increase the power consumption. And the smaller IR1 ，the higher value of the resistors,there will be noise problems .Is there a standard for the value of the feedback resistors? How much is the right? thanks!

 

[goldsmith]

Select one of the resistors randomly , and the equation will give you the other one .

 

[checkmate]

It's a tradeoff between current consumption and acceptable noise levels.
It also imposes a minimum load, that may affect the response when a current load is suddenly removed.

 

[goldsmith]

acceptable noise levels.

Dear checkmate
Hi
I think you're talking about temperature noise , right ?
Best Regards
Goldsmith

 

[FvM]

you're talking about temperature noise , right ?

I think, resistor noise can be neglected for voltage regulators in a first order. It's rather succeptibilty to external interferences and possibly LDO input currents.

 

[checkmate]

Here's a reference from TI on noise sources for an LDO.
https://www.ti.com/lit/an/slyt201/slyt201.pdf

 

[FvM]

O.K. they say resistor noise must be considered.

 

[liuxufeishuang]

thkq！I understand！

 

[checkmate]

O.K. they say resistor noise must be considered.

Actually you are right to an extent. The rule is there more to prevent heroes from using Mohm-range feedback resistors.
Most of the time, the 4kTR noise is small enough for most applications.

I'd prefer to look at it from the current point of view. Make sure you have something well above the uA range.
But how this feedback load affects its transient response must be well accounted for.