Part Number: TPS65910
Hi,We have TPS65910A31 start-up (VRTC related) issue with one more customer.The TPS65910x is used as a PMIC for AM335x.Similar to the below posts, few boards are working after reducing the loadon VRTC LDO by changing capacitor value on VRTC pin to 0.1uF. e2e.ti.com/.../1843272e2e.ti.com/.../2026890Out of 30 custom boards 5 are not working even with 0.1uF on VRTC.During power up the VRTC voltage is very low at 0.8V.VRTC is connected to VDDS_RTC and RTC_KALDO_ENn as per the reference document (TPS65910Ax User's Guide for AM335x).With so many issues related to VRTC start-up, we believe this LDO should not be used in any use case, even if the load is smaller or not.Please do let us know if this is a kind of errata in TPS65910A31?Meanwhile with one of our customer, we have asked them to remove all the loads from VRTC and connect VDDS_RTC to available LDO like VDAC as shown below.Could please let us know if this is the proper way to reduce load on VRTC?And if there is any other recommended design to reduce load on VRTC..
It will be very helpful if you change the reference diagram in the user guide(SWCU093E) which will show that VRTC should not be used in any application.Best RegardsKummi
If you tie RTC_KALDO_ENn to VDDS_RTC you are disabling the internal RTC LDO on the AM335x. If so, you should have CAP_VDD_RTC tied to an external 1.1V power supply. Can you confirm this is the case? Otherwise, if RTC_KALDO_ENn is tied low then the internal RTC LDO will supply the necessary source. Also, as you are aware the recommended minimum output capacitor is .8uF so we can't guarantee the performance of VRTC LDO with a .1uF capacitor.
In reply to Raymundo Hernandez-Toscano:
In reply to Kummi:
Hi Kummi, Is there a reason that the customer boards have the RTC LDO on the AM335x disabled? Can you confirm that when VRTC is tied to VDDS_RTC only no other external loads, RTC_KALDO_ENn is pulled/tied low, that the boards still don't work? Also, can provide scopeshots of VDDS_RTC, CAP_VDD_RTC, and VREF? Connecting VDDS_RTC to VDAC is not the recommended connection as per SWCU093E. If possible, would you mind sharing a schematic of the board over a private message or email. -Raymundo Hernandez-Toscano
Hi Raymundo,Thank you.>Is there a reason that the customer boards have the RTC LDO on the AM335x disabled?->This is as per the recommended schematics, as per the AM335x Schematics checklistif RTC is not necessary we need to make the connections described in "RTC feature disabled" section.processors.wiki.ti.com/.../AM335x_Schematic_ChecklistThis application is not using RTC feature, hence the feature disabled.>Can you confirm that when VRTC is tied to VDDS_RTC only no other external loads, >RTC_KALDO_ENn is pulled/tied low, that the boards still don't work?->VRTC is tied VDDS_RTC, RTC_KALDO_ENn pins.And also Boot1 pin of TPS65910 same as the AM335x EVM.With a different customer we have seen below behavior.1> Few Boards work if we change the VRTC capacitor value from 2.2uF to 0.1uF2> Few Boards work only if we remove the VRTC capacitorWe have around four customers, with many boards which have issue in TPSxx start-upand in most of the case it is with the VRTC, VRTC output below 0.8V.>Also, can provide scopeshots of VDDS_RTC, CAP_VDD_RTC, and VREF? ->I will check if we can get the scopeshots from the customer.But you can find more details in my previous posts below.e2e.ti.com/.../501127e2e.ti.com/.../554074We have already cross-checked the schematics in below E2E. e2e.ti.com/.../1774426>Connecting VDDS_RTC to VDAC is not the recommended connection as per SWCU093E.->I have cross-checked this connection with Richard through below E2E and got the comment saying.."This is fine from the PMIC perspective"e2e.ti.com/.../501127Actually two of our customers are using this workaround already.With respect to changing capacitor value on VRTC pin to 0.1uF,we have comment from TI below saying 7 out of 25 boards were not working which they solved by changing to 0.1uF.e2e.ti.com/.../1814908One of my customer was facing similar issue of startup. They have made 25 boards & out of 25, 18 boards were working & rest of the 7 boards were not working.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.
If this issue was with only one customer or one application I would agreethat they may be something wrong in the schematics,but we are seeing this issue with many customers with many applicationswhich are designed as per the manuals and EVM schematics.Best RegardsKummi
Hi Kummi, Richard is correct, you can connect VDDS_RTC to VDAC LDO. However, keep in mind that VRTC on the TPS65910A31 can supply full load to VDDS_RTC and remain on when the PMIC is in a off state, VDAC does not. If you do not need the VRTC functionality ( always on option) then I don't see an issue with this. There are many parameters that can affect the performance of VRTC outside scope of the capacitance. Also, can you confirm that you in fact have the TPS65910A31 spin of the TPS65910xx and not the TPS65910A or TPS65910A3? If the PMIC is entering a low power mode (.1mA max load from VRTC) then this would not suffice for the AM335x RTC domain. -Raymundo Hernandez-Toscano
Hi Raymundo,Thank you.The device used is TPS65910A3A1.As I mentioned we are not using any low power modes. AM335x is always ON.And we are not using any RTC. This problem occurs only during the power up.I can understand VRTC MAX load should be less than 0.1mA during low power modebut this application is not using low power mode, so I don't understand where does the 0.1mA limit comes into picture.Best RegardsKummi
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