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Hi,We have an issue with the TPS65910A31 start-up,the TPS65910x is used as a PMIC for AM335x and out of 5 custom boards 3 are not working properly. During power up the VRTC voltage is very low at 0.8V.After some testing and with the help of the below E2E we found that the problem was related to the capacitor value on VRTC pin.Even the below E2E mentions that After lots of debugging I found that if we change capacitor value on VRTC pin to 0.1uF all the boards were working perfectly fine.e2e.ti.com/.../320847But as per the TPSxx datasheet, the capacitor value for VTRC should be in the range of 0.8uF to 2.64uF. In our case out of 3 boards 1> Board 1 works if we change the VRTC capacitor value from 2.2uF to 0.1uF2> Board 2 works only if we remove the VRTC capacitor3> Board 3 doesn't work at all.Is this some kind of Errata of this device?Best RegardsKummi
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In reply to Kummi:
In reply to Richard Solarski:
This could occur if the input supply dips low enough to transition the PMIC into BACKUP state, is that applicable to your application?
Discussion on this topic can be seen here: e2e.ti.com/.../222327
The VRTC LDO can current limit itself in the BACKUP state for all TPS65910x devices, and since this LDO provides internal blocks it must be allowed to regulate to the proper 1.8V. If removing the VDDS_RTC connection fixes this issue I would start there.
Hi Richard,Thank you for the suggestions.I am sorry for the delay.The input supply seems to be OK and we believe the IC is not in BACKUP state.I think this scenario doesn't apply to this application.With regards to removing the VDDS_RTC connection, removing this connectionseems difficult because we have to cut the PCB to do that.With regards to the VBACKUP pin even though we are not using the BACKUP supply,VBACKUP is connected to 4.7uF capacitor. Does this effect the proper working of the PMIC?
Hi Richard,Thank you.Below are the oscilloscope shots of VRTC and VREF during power up.Ch1(Yellow)-VRTC (500mV/div settings)Ch2(Blue)- VREF (500mV/div settings)Ch3(Purrple)- Input voltage(VCC1 to VCC7) (2V/div)
Board 1 (Working after VRTC Capacitor change)
Board 2 (Working after VRTC Capacitor change)
Board 3 (Not working)
Those rails look fairly noisy, do we know if that is caused by coupling from the probe ground loop or if that is legitimate noise on the VRTC and VREF rails?
Does the REFGND have a dedicated trace to the device and reference cap, or are they both shorted to a PGND?
Hi Richard,I forgot to answer about the REFGND connection.The REFGND is connected to the Main Ground Net (DGND) similar to the AM335x EVM or StarterKit.
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