LP2985-N: Low voltage output for 3.3V regulator

Felipe Neves80

Part Number: LP2985-N
Other Parts Discussed in Thread: ADS1299, LP2985

Hello,

I have designed and manufactured the following circuit:

There I use a LP2985 LDO to power my ADS1299 with 3.3V, the input voltage of the LDS is 5V. However I am having trouble with the output voltage. It usually floats around 1.5-1.7V, instead of the 3.3.

I have tested the circuit with many different capacitors, both tantalum and ceramic ranging from 1uF to 10uF, and had problems with all of them, some values (1uF) resulted in an unstable voltage over 4V.

I have also tried to isolate the problem by placing only the LDO and the input, output and bypass capacitor on the board, but with similar resuld.

Am I doing something wrong? Is the design wrong?

Thanks in advance

over 5 years ago

0 Ryan Eslinger over 5 years ago

TI__Guru 55940 points

Hi Felipe,

Unfortunately I am unable to see your circuit. Can you post a screenshot of what you were attempting to show?

In order for me to help you debug, could you also provide scope shots of IN, OUT, and ON/OFF?

Please ensure that your capacitors meet the ESR requirements as depicted in Figure 22 of the datasheet:

Very Respectfully,

Ryan

0 Felipe Neves80 over 5 years ago

Prodigy 210 points

Hello, thanks for your quick reply.

I have attached a picture of my measurements on the IN and ON/OF (interconnected) and OUT. There is also a image of the circuit attached .

0 Felipe Neves80 over 5 years ago

Prodigy 210 points

I am not being able to upload the pictures here

drive.google.com/.../view
drive.google.com/.../view
drive.google.com/.../view

0 Ryan Eslinger over 5 years ago in reply to Felipe Neves80

TI__Guru 55940 points

Hi Felipe,

Thank you for the schematic and scope shots. I do not see any issues with your schematic. Since you show separate scope shots for Vin and Vout, could you confirm that Vin is still 5 V at all times while Vout is at 1.72 V?

For your future knowledge, here is a link to instructions for how to properly attach images on the E2E forums:
e2e.ti.com/.../149.4-3-inserting-an-image-on-e2e

Very Respectfully,
Ryan

0 Felipe Neves80 over 5 years ago in reply to Ryan Eslinger

Prodigy 210 points

Yes, it is at 5V and 1.72V all the time. At one of my tests I got the value of 3.3V very briefly and then it dropped to 1.7V again. I could not replicate this result to send a picture. I have also tried to mount the regulator in a separate board, with the 5V coming from a regulated DC supply but the results were the same.

0 Ryan Eslinger over 5 years ago in reply to Felipe Neves80

TI__Guru 55940 points

Hi Felipe,

This is strange. On your separate board with the bench power supply generating the 5 V input, what is your load condition? Could you also do a visual inspection to ensure that there are no unintentional shorts or current paths such as residual flux particularly around the BYPASS pin?

Very Respectfully,
Ryan

0 Felipe Neves80 over 5 years ago in reply to Ryan Eslinger

Prodigy 210 points

It was a very strange issue. I tried re-soldering and substituting the IC with no success.
To solve it I had to unfortunately change for a compatible IC (NCP511SN33) that worked properly without changing anything else.
I wonder the other ICs were defective.

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LP2985-N: Low voltage output for 3.3V regulator