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TPS25750EVM: Voltage on VBUS dropping as current drawn increase

Marc-Antoine Dion
Prodigy 40 points
Part Number: TPS25750EVM
Other Parts Discussed in Thread: TPS25750

Hi everyone,

I am currently testing the TPS25750EVM module with a standard setting of 5 volts 3 amps. The setup for testing is a BK PRECISION 8601 (Active LOAD) hooked directly on the VBUS. I then hook a CY4500 EZ-PD Protocol Analyzer to the USB-C connector to analyse de CC communication. What I realised is that, as the current drawn goes high, the voltage drops drastically on VBUS. Is this a normal behavior of USB-C? I was under the impression that VBUS must stay in between 5% of the desired voltage at all time to work, but the voltage drop to as low as 4 volts when 2.8 A is drawn.

I tried this test with both a PC USB-C and a wall charger and both gave me the same result.

Should I be worried about this behavior?

Thank you and have a nice day,

Marc-Antoine Dion

  • 0
    TI__Genius 12665 points

    Hi Marc-Antoine,

    A 1V drop on VBUS is not normal behavior in terms of sourcing. I was unable to reproduce this on my side with 25750EVM as the source. How are you setting up your current load test? Is the 25750EVM sourcing or sinking? Are you connecting the load to VBUS on the 25750 EVM or the PC USBC/Wall Charger side?

    Best,

    Alex

  • 0 in reply to Alex Liu
    Prodigy 40 points

    Hi,

    I am currently using the TPS25750 as a sink for a 5V 3A profile connected to either a USB-C port on PC or a USB-C wall charger. I tried both and got similar result from them. I even tried 2 different chargers to see if it could be the charger'S fault, but both chargers did the same voltage drop. The load is applied from an active load directly connected between the VBUS and GND connector next to the USB-C connector on the TPS25750EVM.

    Here is the TPS25750 Application Customization Tool setup (I am not using the battery charger for my test) :

    Here is a picture of the setup for the test (The green cable is the negative of the active load and the white cable is the positive of the active load) :

    In the meantime, I have tried to change the profile to a 5V 1.5A as a sink for the TPS25750EVM and got a similar result, but the voltage only dropped down to 4.5V this time at around 1.4A.

    I can see on the CY4500 EZ-PD Protocol Analyzer Utility that the profile was accepted by the source. Here's the exemple for the 1.5A profile :

    Thank you and have a nice day,

    Marc-Antoine Dion

  • +1 in reply to Marc-Antoine Dion
    TI__Genius 12665 points

    Hi Marc-Antoine Dion,

    When sinking with the TPS25750EVM, we only specify an undervoltage protection threshold for the PP5V signal on the 5V power path, which you are using. The threshold is around 3.8-3.9V. A 1V drop to around 4V is not uncommon on the sink side for VBUS, I also saw this in my testing. Since you are applying more current, the voltage drop increases naturally and is not unexpected. You will sink 4V instead of 5V but you should not have an issue in terms of PD function or UVP protection.

    Best Regards,

    Alex