OSD3358 PMIC startup problem

[NMatringe]

Hello

I designed and validated a board built around the OSD3358. I now have a new batch of 10 boards that all exhibit the same problem : when applying the 5 V supply, after 50 ms the SYS_RTS_1V8 output rises to 1 V then stops and slowly decreases down to 0 and nothing happens afterwards. It looks as though the PMIC shuts down.

It is possible I did something wrong when first powering them up though I have no idea what, and maybe I fried the PMIC. Any insight is welcome

Thank you
N. Matringe, Alciom

[Neeraj Dantu]

Matringe,

This behavior is in general, an indication of a short present on one or more of the power rails of the PMIC. Please take a look at the start-up sequencing in Section 2 of the Power Application note here: https://octavosystems.com/app_notes/osd335x-c-sip-power-application-note/ to understand the start up sequence of the SiP. In addition to looking for shorts on all the power rails of the PMIC, you can scope the power up sequencing to determine which power rail is failing to come up.

Please also take a look at https://octavosystems.com/app_notes/osd335x-design-tutorial/bare-minimum-boot/boot-process/ as it goes through basic board bring up procedure.

Best,
Neeraj

• This reply was modified 2 years, 6 months ago by Neeraj Dantu.

[NMatringe]

The 1.8 V is the first rail to power up (LDO1 output).

After investigations, the problem is related to the capacitors on the 5V input (generated by a DC/DC boost) and SYS_Vout. I put more capacitance on the 5V converter and less on SYS_Vout and now it works.
Strange it worked fine on the first batch, though…

Thanks
Nicolas

[NMatringe]

Hello
The problem has come back in another form, the PMIC won’t startup once reset. This is linked to a glitch in the 5V input (how much current does this module draw ? I’m amazed)
After lots of browsing through TI forums we found that the PMIC is sensitive to brown-out and can lockup, exactly as we observed. The case is also mentioned in the OSD3358-SM-RED User’s Guide on page 43.

Apart from adding insanely huge amounts of capacitance on the 5V supply, is there anything that can be done without any redesign ?

Thanks

N. Matringe, Alciom

[Neeraj Dantu]

Alciom,

Unfortunately, apart from preventing the input voltage from going out of the specified range of the PMIC, there is not a solution that will mitigate brownout in all scenarios. Adding capacitance can help. The new version of the OSD3358-SM-RED reference design does include this mitigation to prevent lockups dues to brownout.

Please take a look at this app note from TI: https://www.ti.com/lit/an/slva901/slva901.pdf that describes mitigation strategies for different use cases.

Best,
Neeraj

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