LDO feedback resistor divider

[deep_sea]

Hi,
I wonder what is the motivation behind taken a ratio of LDO output voltage through a resistive divider and feeding it with a reference? what are the disadvantages of comparing Vout to VREF directly? Is the availability of a particular reference voltage the main reason?
If the LDO output level is more convenient as a common mode voltage for the input of the error amplifier and the reference voltage is not available and will be part of the LDO design, would it be better to design a reference voltage similar to the LDO required output?

 

[danadakk]

Post pic of circuit you are questioning .....


Regards, Dana.

 

[betwixt]

If you mean within a regulator device, the problem with direct comparison is if you compare the output with the same voltage as you need, it mostly defeats the objective - you need the same accurate and stable voltage (although maybe with less current capability) and that would have to come from another regulator. By dividing the output it is still proportional to the actual voltage and the internal reference can be stabilized from the input voltage.

Brian.

 

[dick_freebird]

There is only really one good low voltage reference and it's going to give you about 1.2V. Most desired supply voltage outputs are something else. The feedback divider is how the user gets to set the value. Fixed feedback, you're stuck.

 

[crutschow]

Yes, the problem is making a stable and accurate voltage reference for other than certain fixed voltages.
Typically an accurate resistive voltage divider is used to get other than those voltages.

 

[deep_sea]

There is only really one good low voltage reference and it's going to give you about 1.2V. Most desired supply voltage outputs are something else. The feedback divider is how the user gets to set the value. Fixed feedback, you're stuck.

Thanks for your reply. This means that "most accurate" bandgap reference is the original with 1.2 V output. So it is in most cases better to use as accurate reference as possible and use resistive divider as required by common mode of the amplifier. Right?

--- Updated Jun 6, 2021 ---

If you mean within a regulator device, the problem with direct comparison is if you compare the output with the same voltage as you need, it mostly defeats the objective - you need the same accurate and stable voltage (although maybe with less current capability) and that would have to come from another regulator. By dividing the output it is still proportional to the actual voltage and the internal reference can be stabilized from the input voltage.

Brian.

Thanks for your reply. You are right. In most cases LDO required output is slightly less than VDD while the reference is more likely a bit lower than LDO output. For example from 1.8 V supply, it might be required to have LDO output of 1.5 V while the bandgap reference is 1.2 V or less.