I am currently designing a custom carrier board for NVIDIA Jetson Thor as part of my master’s thesis. This is my first time working with USB-PD and PMBus-controlled regulators so I am still trying to understand what is expected to work out of the box versus what requires firmware changes.
In NVIDIA’s reference design the USB-PD controller interfaces via PMBus to an MP8856 buck regulator. The MP8856 is still pre-release and not available for purchase and my design target requires up to 60 V input voltage I need to search for alternatives such as the MP8880A, LTC3889 or other PMBus-compatible buck converters.
My design goal is to support USB-PD EPR. At the very least I want EPR to work as a sink and support standard voltage range operation as a source.
My main question is about PMBus compatibility and expected behavior. From a high-level point of view
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Can I replace the MP8856 with another PMBus-compatible buck converter and simply configure the voltage levels in the USB-PD configuration tool
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Or does this typically require modifying PMBus registers or command sequences in firmware
More specifically
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Does the USB-PD controller firmware rely on MP8856-specific PMBus registers or command behavior
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Is it sufficient that the replacement regulator supports standard PMBus commands such as voltage programming telemetry and fault reporting
I am trying to understand whether PMBus in this context is effectively plug-compatible at the firmware level or whether the MP8856 is assumed in ways that make substitution difficult especially when targeting USB-PD EPR operation.
I am aware that NVIDIA only offers 5 V source operation in their reference design which would also be a valid approach for me. I mainly want to understand the general behavior and limitations before making this design decision.
Finally I would also be interested in recommendations for suitable PMBus-capable buck converters that are known to work well with USB-PD source designs and the CYPD8225 in particular especially for higher input voltages up to 60 V.
Any clarification would be very helpful especially from the perspective of someone new to USB-PD and PMBus integration. Thanks!
Hello Julian,
Apologies for the delay in this request.
I remember facing many unknowns when going through my own master’s thesis. Learning about USB-PD mechanisms and controls is something that I needed to understand further as I started full-time.
Based on what you have just described, the MP8880A seems like a fine substitution in place of the MP8856 from a pure input/output voltage current level.
I also see this is the block diagram you are referring to from the following link: Jetson_Thor_Module_Carrier_Board_Spec_SP-12533-001_v1.2.pdf
To my knowledge at a surface level glance, there shouldn’t be any issues here other than ensuring the source/sink PDOs are all satisfied here. From the PMBus implementation perspective, this will require further investigation.
Therefore, I am currently looking into this request. You’ll hear back from me soon.
I know since it has been two weeks there have perhaps been developments in your design, so in the meantime if there have been any updates or things you have learned, please do share!
Best,
Krishan
Hi Krishan,
thank you for looking into this! The block diagram you mentioned is exactly the one I am working from.
In the meantime I contacted both NVIDIA and Infineon to better understand the firmware and PMBus interaction.
NVIDIA stated that they cannot provide further details regarding the firmware implementation. I therefore assume that, apart from PDO configuration, no major custom adaptations were made specifically for the MP8856.
Infineon confirmed that when replacing the default buck controller the firmware usually needs to be adapted to the selected regulator and that if an advertised PDO voltage cannot be reached this can result in UVP faults in the PD log. I also requested reference designs or application diagrams for USB PD EPR operation with the CYPD8225, but I have not yet received a response.
Based on this, my main concern is whether the MP8856 in the reference design is controlled purely via standard PMBus commands or if there are device specific assumptions that would complicate substitution with the MP8880A.
If available, I would also appreciate any technical documentation or application notes from MPS regarding PMBus integration in USB PD source designs.
Best regards
Julian
Hi Krishan,
you mentioned you were looking into earlier.
I have not yet received an update and wanted to check whether there has been any progress.
Julian
