LM27761: Feedback resistor effect of making lower value?

Hi Gregor,

My colleague Febin will look into this and come back to you by Wednesday.

Best regards,

Varun 

Thanks Varun for your support.

Hi Gregor,

LM27761 is a very old device and the best practice would be to adhere to the d/s recommendations.

The device is characterized and tested with R2>50K. Using lower feedback resistances could reduce the efficiency.

There is also a possibility that the LDO stability would be impacted. Since VFB is fed to the LDO. And VFB is dependent on the choice of resistors.

So, I suggest you should try this on an EVM first and confirm that the device meets all the system requirements.

Additionally, I would make sure the layout is properly designed as well. 

I hope this helps.

Regards,

Febin

Hi Febin

Thanks for the answer.

We did additional tests and found that the VFB line is very sensitive to be disturbed by even small humidity and dirt impact. Even 10M parasitic pickup from the switching capacitor will affect the LDO and it gives anything between 1 and 5V. Of course we use PCBA washing with ionic test and coating but that does not help to solve the problem. We realize that Layout is not that good as it could be and FB resistors are too close to CPout and this is the path for pickup garbagage.

Short term solution for using the alreay produced boards is to disable LDO and use CPout directly by hardwire short and putting FB to GND as suggested in another thread  "LM27761: LM27761 using only charge pump function" from 7 yers ago.

Long term solution is to design out LM27761 with new layout.

--->>>What type of newer DC/DC converter do you recommend? Product will be in production for minimum 15 to 30 years from now on and therefore we need good sourcable device. Requirement is +5V in (4.8 to 5.3) / Out typicaly -3.4V (-3.4 to -5V is working as well) / about typically 10mA for supplying 3x OPA192 plus some resistors (total less than 15mA). We need solution for very limitted board space, hight does not matter. Product is for harsh industrial environment, -40 to +85degree celsius and high EMC environment like several 1000A and several 1000V switching nearby.

Thanks very much for your help.

BR, Gregor

Hi Gregor,

As you might have noticed, E2E was under maintenance last week and I could not reply earlier.

However, our FAE had reached out to me and I hope you received an email already with my recommendations.

Regards,

Febin

Thanks for confirming.