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TPS25750: IC misbehaves when using active USB cable (active CC) - How to identify malfunctions?

user6466152
Prodigy 181 points
Part Number: TPS25750
Other Parts Discussed in Thread: TPS54JA20

Hello, 

We are working on a project which has to be maintainable in the future and I have to provide solutions for the repair department based on diagnostics. We need to replicate conditions where a client messes up with the electronics (opens the case and touches circuits).

We did ESD discharge on 5 samples (simulating average human-circuit interaction) and we want to know if it's the IC that malfunctions or whether the passive components got blown.

However, we are new to using TPS25750 and it's different than MOSFETs. 

Here is our circuit. In green is the USB-C line,  in Yellow is the EEPROM flashed with 15V 3A configuration. On the right side in PINK is a converter from 15V to 5V.

On the bottom in blue/cyan are the consumers that operate on 5V from the converter.

This is our setup. Now we have the following behavior from the IC.

When connected to a passive 5V USB-C line, both the 5V line and the (internally converted by TPS25750) 3.3V line are stable. PPHV outputs 5V.

However, when we connect a 100W USB-C PD charger - all lines fluctuate including the CC line as you can see in the captures below. What is more, a soft clicking noise can be heard every time a peak is produced in the oscilloscope reading. 

3V3

5V

PPHV

CC line

What we need to determine from this is whether the IC has been affected by the ESD or the passives around it (for example capacitors).

How do we do this? Do you recognize any of the listed behaviors? 

  • 0
    TI__Mastermind 32661 points

    Hello,

    I noticed in your schematic that you did not include the VIN_3V3 capacitor, this is a required component for the TPS25750 to guarantee operation, even when it is used as just a sink.  This is not likely the cause of the issue that you are seeing, but may be a possible cause.

    The soft clicking sound that you are describing most likely indicates that there is AC current is now flowing through a capacitor somewhere in your system.  Further evidence of this is the AC ripple that you are seeing on all of the internal supplies.

    I would start looking at the large Bulk capacitors in the system to see if they have been damaged.

    I also see that the TPS25750 is continuously attempting to reconnect the VBUS.  This can be caused by the device detecting an over current or under voltage condition on PPHV.  This also leads me to believe that a capacitor or the Buck has been damaged.

    Do you know if your Buck is compatible with 100% duty-cycle? 

    It is seeing a case where PPHV is 5V and the buck is outputting 5V, so the buck needs to support 100% duty cycle.

    I would also verify that the EEPROM has not been damaged.

    Regards,

    Chuck

  • 0 in reply to Chuck Branch
    Prodigy 181 points

    Hello - I did more investigating and I have the following additions to what was said earlier. It seems that there's a problem that affects all of our chargers and we can't get to the root of it. This post is to explain what is going on and to ask for help for where to go next.

    We have identified that an EEPROM fully flashed with NULL bytes will stop the above erroneous behavior in all devices. Technically, without the flashed EEPROM, the charger does not fluctuate the VBUS line or the CC line or the PPHV. Although this still means that the device is still not working, I think it's a starting point.

    Secondly, our buck converter was tested previously with a stable DC Jack supply. We believe that this should work, albeit it has different passive components to fit the 15V rating (unlike before tested with 20V). This is made by TI and it's the package TPS54JA20. This was designed using the WEBENCH from TI - and we arrived at this Schematic: 

    After we discovered the device was failing with EEPROM flashed we tried to do unit debugging. We left the LED attached to the output of the buck attached to the PCB and desoldered the buck converter chip and the inductor. The led was still turning on from a capacitor of some sort (and we tested for shorts). We don't really understand how this is possible while the 5V output line is isolated from the circuit, especially when the buck converter chip is removed.

    One more weird issue is that we managed to have the USB-C chip output 15V by mistake when one of us heated up the PCB and then plugged the cable while the PCB was still hot/warm. When tested with the multimeter, the PCB showed 15V and no fluctuations on the oscilloscope on any of the lines. This was done when the buck converter was removed. Oddly, when the PCB cooled down, it stopped outputting 15 volts and got back to the above behavior.

    Do you have any ideas why we would see this?

    What we will try next is to isolate the buck converter first and test it with a power supply to see whether it works correctly independent from the USB-C supply.

    Secondly, we will try to bodge a ceramic capacitor to the 3V3 line you mentioned earlier. I assume that this is the VIN3V3 label to the top of the chip in the schematic (PIN 38). Please correct me if I am wrong. I also assume that this should be installed as a decoupling capacitor.

    Many thanks for your answer and interest in our issue.

  • 0 in reply to user6466152
    TI__Mastermind 32661 points

    I believe that you are on the right track by removing the DC-DC and testing the TPS25750D.

    Here is the procedure that I would follow to bring up the TPS25750D.

    1.  Please make sure that R11 and R8 are not inserted.  This will result in ADCIN1 and ADCIN2 being grounded.  This will configure the TPS25750D into Safe mode.  This will allow the TPS25750D to boot from the EEPROM before it enables the power switch.

    2.  Please use the TPS25750D configuration GUI to configure the TPS25750D to support the sink PDO's that you wish to support.

    3.  Once you have done this, you can program the EERPOM using one of the I2C headers that you have provided.

    4.  Once the TPS25750 EEPROM has been programmed, you should be able to connect an type C DFP that supports the voltage ranges that you wish to test.  

    5.  You should now see the target voltage on PPHV.  This voltage can be anywhere between 5 and 20V.  

    You description of the capacitor is correct.

  • 0 in reply to Chuck Branch
    Prodigy 181 points

    Hello again,

    I am back with some results.

    I assembled just the buck (right side - TPS54JA20) and tested it with a 15V bench power supply. 

    At first, the indicative LED turned on and the BUCK output line was receiving 5V - as expected. After this, I attached some load (< 1A  at 5V drawn) which I was sure won't produce a short. After this event, my power supply reported a short.

    I disconnected the power, I waited 5 minutes and after this, I tried connecting the power again without the load. This time the short was detected instantly. I used a multimeter in diode mode and indeed, there was a short on the board now.

    Now I know that either my buck converter design is flawed or the components I'm assembling are underrated - maybe the power I'm putting through them is just burning them. With this I have a couple of questions:

    1. What happens when you fry a MLC capacitor? Does the current just flow through it with 0 capacitance and 0 resistance?

    2. Can capacitors get blown on the board if they are decoupling? (given the voltage is not exceeded)

    3. What happens when you fry a resistor? Infinite resistance (NC)?

    4. What would happen with TPS25750 in the case of shorting PPHV to GND? Does it detect it and disable PPHV or just get damaged?

    I'm sorry if some of my questions are n00b and thank you for your guidance. I was introduced to the world of electronics from TI datasheets.

    Here is the original design from WEBBENCH. I have similar components and the resistors don't have lower power ratings (>1/8W for most).

    Many thanks

  • 0 in reply to user6466152
    Prodigy 181 points

    Another day and I managed to answer some questions and going to ask another.

    I determined that the passive components on the buck don't short the circuit directly. I desoldered the faulty buck and apparently, PPHV is shorted to GND and to SW line internally. This is very curious and I am wondering what could have caused the damage to the chip.

  • 0 in reply to user6466152
    TI__Mastermind 32661 points

    If PPHV is shorted to ground, then it is likely to damage the internal FET of theTPS25750D.  The internal PPHV path does not have short circuit protection because it is not intended to be externally exposed.

    Based on the shorts that you are reporting, it is likely that both the BUCK and the TPS25750 have been damaged.

    Regards,

    Chuck

  • +1 in reply to Chuck Branch
    Prodigy 181 points

    Dear Chuck,

    I found the culprit. Although what you said is highly likely to have caused problems, this was not the main issue.

    The problem was i2c reverse voltage. 

    What happened is that at some point I forgot to power on the board while writing the configuration to the EEPROM. The programmer that I'm using has internal pullup resistors, thus driving the i2c lines high (3.3V). This, in turn, turns out to create a positive voltage that sinks through the <should-have-been-powered> pull-up resistors needed on the board. Those pull up the 3v3 line to ~2.5 volts and probably burn the 3V3 internal LDO and other components internally. 

    I am not 100% sure why the internal 3V3 regulator still works when no EEPROM is present.

    I tested my hypothesis by burning yet another chip just to be sure (in the same fashion).

    For the rest of the build - it's just pandemonium. Now I'm outputting 15V as expected - from the PD chip and I'm going to work upwards to determine if anything else has a problem. If I have issues with the buck IC I will post another question later.

    I wish this was better documented, but it's also my oversight and TI shouldn't be responsible for the EEPROM programmer I'm using.

    Would it be possible to rename my thread to "IC doesn't function after EEPROM Flash: How to not fry your USB-C PD chip"? I think is more relevant for others in the future.

    Thank you for your support.