LM27762: Output voltage formulas and matching OUT+ w/ OUT-

Marato Gebremichael

Prodigy 60 points

Part Number: LM27762

Hello,

The formula for the output voltage is different in the datasheet and EVM User Guide (please see images below)

Datasheet:

EVM User Guide

Please confirm which formula is correct.

In our application we've used the matching formulas (due to the EVM user guide and the WeBench calculations) for both positive and negative outputs, however the outputs are not closely matching. I'm seeing <1% tolerance on the positive voltage rail whereas I get <4% on the negative rail. Please advise on this issue.

Thank you.

over 6 years ago

0 Chris Glaser over 6 years ago

TI__Guru*** 143050 points

Hi Marato,

The D/S formulas are correct. The -1.22V and 1.2V values come from the electrical table on page 5.

If you adjust the feedback resistors accordingly, is the matching ok?

Do you have the same load on each output?

0 Marato Gebremichael over 6 years ago in reply to Chris Glaser

Prodigy 60 points

Hi Chris,

Got it. I've adjusted the resistors and it's working as expected now. The load is the same on each output.

Thank you Chris!

I was playing with WeBench configurator for the same IC and it gave the following values for the resistors:

ex. R1 = 590k R2=549k and R3=549k R4=511k for Vin 3.30V and Vo +-2.50. Is there any particular reason for this sort of matching of resistors? And the formula that it is being used seems NOT to follow the datasheet as well.

Cheers

0 Harish K R over 6 years ago in reply to Marato Gebremichael

TI__Intellectual 2320 points

Hi Marato,

Currently the WEBENCH resistor divider selection doesn't seem accurate. Kindly use the resistor divider as per Data-sheet recommendation.

Ensure that both R2 & R4 > 50k.

We will make the WEBENCH resistor divider configuration more accurate & let you know once the changes are done.

Thanks and Regards,

Harish

0 Marato Gebremichael over 6 years ago in reply to Harish K R

Prodigy 60 points

Hi Harish,

Thank you for your feedback!

0 Harish K R over 6 years ago in reply to Marato Gebremichael

TI__Intellectual 2320 points

Hi Marato,

We have resolved the issue w.r.t resistor divider in WEBENCH. We have made it as per datasheet recommendation.

Thanks and Regards,

Harish

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