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3.3V LDO unstable in few custom i.MX25 boards.

Question asked by torus1000 on Jul 27, 2016
Latest reply on Jul 28, 2016 by Artur Petukhov

Hi,

 

The following power-up sequence is recommended for i.MX25.

  1. Assert power on reset (POR).

  2. Turn on digital logic domain and I/O power supplies VDDn and NVCCx.

  3. Turn on all other analog power supplies

  4. Negate the POR signal

So our board designed as following.

(0) POR=L (1) LDO_1.2V on (2) LDO_3.3V on (3) LDO_1.8V on (4) POR release

 

Most boards(90%) started up with no problem and stable but rest of boards(10%) looked unstable.

Please see attached waveforms.

2010727_waveform00.png

LDO_1.2V (ch1: yellow) rise first then LDO_3.3V (ch2: cyan) rise next but oscillation waveform observed.

(Additionally saying, this oscillation seems suppressed if add delay to the enable pin of LDO_3.3V.)

 

Why this type of oscillation happened in few specific boards?

 

How can I fix it? Is there any hardware design guide documents?

 

Can anyone help me?

Thanks.

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