TPS65216: AM335x RTC-Only Mode Power Connections and Consumption

Hi Berkay,

Your setup for the RTC-Only mode looks to be able to work. In terms of the current pull, I can only estimate ~1mA of current would be consumed by the RTC supplies of the AM335x processor. You would want to keep that coin cell to a standard 3.3V or lower to minimize the drop across the LDO. In regard to your question about the internal oscillator, I will loop in the appropriate expert who will better be able to provide insight on that. 

Thank you for your response. Looking forward to hearing back regards internal oscillator accuracy. 

I'm trying to find the answer to your RC oscillator question, but fairly sure this oscillator has very large frequency variations and will not be good enough to operate a real time clock.

I have a concern with your plans to use a separate 1.8V regulator to power the RTC. In the normal connectivity topology, the PMIC always on 1.8V LDO is used to power the RTC and the RTC tells the PMIC when to turn on/off the other device power rails. The RTC automatically tells the PMIC to apply power to all rails on initial power-up. However, your topology is counting on the PMIC 3.3V rail or the battery to source your separate LDO which is used to power the RTC. I do not think it will be possible to power up your system when using this topology if the battery is dead.

Regards,
Paul

You may want to review the following E2E post that describes different RTC power supply options and expected power consumption of each.

https://e2e.ti.com/support/processors/f/processors-forum/933504/am3351-rtc-current-consumption/3451826#3451826

Regards,
Paul

Hi Paul, 

Thank you for your response. We were following the instructions in the TPS65216 User Guide Figure 5. TPS65216 does not have an always on LDO so we are trying to create our own always-on-LDO.  

Your comment: The RTC automatically tells the PMIC to apply power to all rails on initial power-up.

Are you referring to the PMIC_POWER_EN signal between the AM335x and TPS65216? If so, we plan to use the PMIC_POWER_EN signal only to initiate a power-down sequence when a safe power-down is needed.

Its our understanding that from an off-state, we could bring up the system by simply using IN_BIAS (connected to mains) and AC_DET (grounded) pins of the PMIC. This would then initiate the power-up sequence. Are we missing something? 

Please take a look at the below pictures for our connections between TPS65216 and PMIC and if you could comment on it I would appreciate.  

Regards.

Thank you, this is very good information. As a matter of fact we are trying to the Traditional RTC Power Solution here... Consumption data is very valuable, thanks. 

I'm not familiar with the PMIC functions. My concern was based on the assumption the PMIC only applied power to a always on 1.8V LDO that would power the AM335x RTC, then the AM335x RTC would turn on the other power rails via the PMIC_POWER_EN signal. My concern would not be valid if the PMIC automatically applies power to all power rails including the 3.3V rail you are using to source the RTC 1.8V LDO when it receives power for the first time. You may need to confirm this with Emily from the PMIC team.

I'm concerned with your reset source for RTC_PWRONRSTn since it should be asserted for at least 1ms to provide enough time for the internal RTC LDO output to reach a valid level before RTC reset is released. Simple RC reset circuits can be problematic.

Are you connecting the crystal circuit grounds to the system digital ground? If not, have you read the AM335x Silicon Errata? There is an advisory in the errata that describes how the initial connectivity recommendation for crystal circuit grounds changes from the original recommendation.

I also saw your comment about only using warm reset as an output. Therefore I wanted to make sure have seen the TRM section that shows an external circuit to assist warm reset output during power ramp if your application requires a valid reset as the AM335x IO power ramps.

Regards,
Paul

Based on our discussion, I am leaning towards changing the PMIC to TPS65218D0. I assume this would take care of all the always-on-LDO problems for the RTC since that seems to be the main difference between TPS65218 vs TPS65216. Would you recommend TPS65218D0 in our case and if we make the switch, will we still need to worry about the RTC-only mode and power sequencing, or is it pretty much straight-forward? 

Thanks for the information. So does it mean that the internal clock's frequency may change from 16 to 60 kHz? That's an extremely wide clock frequency. If that's the case what is the purpose of internal RC? Is it serving some other purpose for the AM335x internally and not intended to be used externally?  

I forgot to answer your PMIC questions. As mentioned before, I'm not familiar with the various PMICs or their functions. I will try my best to answer questions specific to AM335x, but you should ask Emily questions related to the PMIC. If you are done asking AM335x questions, I will assign this thread to Emily so she can answer your PMIC questions.

Regards,
Paul  

Yes, you were very helpful. Thank you!