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TPS82084: VOUT drops when VIN matches setpoint

Dave Reynolds
Prodigy 50 points
Part Number: TPS82084
Other Parts Discussed in Thread: TPS62088, TPS62821, TPS62802

Hello,

I have a design that uses TPS82084, to generate a 3.3V. The schematic is below. The input "VSLOT" is typically 3.3V. This circuit is designed to provide safe 3.3V to FPGA, as the FPGA can only tolerate 3.4V.

Anyway, what I'm seeing is that when VSLOT is very close to 3.3V (like 3.32-3.36V), the output of the 82084 slowly drops to 0V (over about 2 minutes). These is an additional 50uF of capacitance on the output (not shown). The supply is powering LEDs and other stuff, so its unlikely that it completely shut off.

This only happens when I power VSLOT with certain supplies. When I power VSLOT with a linear supply, this doesn't happen. Is the 82084 more susceptible to noise when it is near 100% duty cycle?

Thanks in advance!

  • 0
    TI__Intellectual 1780 points

    Hi Dave,

    Thanks for reaching out to us and please accept my apologies for replying late because of the weekend.
    Please refer to this application note (www.ti.com/.../slyt747.pdf )to understand the 100% mode in our low-power DC/DC converters better.

    Furthermore, can you please tell me what is your typical output current in your application and if you can provide the waveform for the input and output voltage and the type of supply you mean when you say "certain"?

    Thanks!
    Omar

  • 0 in reply to Omar Hegazi
    Prodigy 50 points
    Hello Omar,
    Thanks for the quick response. I've attached scopeshots of VIN and VOUT, both AC and DC coupled. In all cases the scope shots were taken while the 82084 was malfunctioning.
    The input voltage to the 82084 was 3.332V. The initial output voltage of the 82084 was 3.310V, before it starts dropping.
    As far as output current, its pretty low. Its just running a few LEDs and a small fan- definitely under 1A. Probably closer to 200mA. I looked at the 82084 with a thermal camera and it isn't getting warm.
    Best regards,
    Dave
    VIN, AC coupled

    VIN, DC coupled

    The following are all VOUT of the 82084:

    After voltage has dropped to ~1.7V:

    Long term droop

    Vout, AC coupled:

  • 0 in reply to Dave Reynolds
    TI__Intellectual 1780 points
    Hi Dave,

    Thanks for the reply and the voltages waveform.
    When Vin is too close to Vout, the device is in 100% mode and even if the Vin is less than the Vin,min needed to give you the desired Vout, the device will connect the Vin to Vout and you should just see your Vin when you probe Vout. Additionally, with the loads connected, 2 minutes is such a long time to discharge any capacitance.
    Can you please make sure your input voltage is stable and constant. You may also want to add a 1uF capacitor in parallel to the 47uF capacitor at in the input side. Additionally, can ou describe the discharge behavior over the 2 minutes? Is it linear or does it have a certain point where it just goes to zero?

    Best regards,
    Omar
  • 0 in reply to Omar Hegazi
    Prodigy 50 points

    Hello Omar,

    Thanks for the response. I'm certain the Vin is stable and constant. It has the ripple (shown in the AC waveform) but thats it. I can capture a trace with VIN and VOUT simultaneously.

    I trued 0.1uF across VIN, it didn't change anything. I'll try 1uF.

    The discharge behavior is this: VOUT is steady at about 3.110V for a while (like 30s), then it starts to drop slowly, like 1mV/s, then it speeds up, say to 10mV/s, until it gets to about 1.7V or so, where it drops slower, probably because the LEDs aren't on anymore. I don't know if the output actually goes all the way to 0V, I can check. I haven't waited that long.

  • 0 in reply to Dave Reynolds
    TI__Intellectual 1780 points
    Hi Dave,

    Thanks for your prompt reply.
    Please try that and let me know.
    Additionally, is the PG pin connected to anything? I would love to see how the signal there behaves in this situation. Furthermore, Can you also probe the inductor and share the switching signal in this situation. Finally, how many boards do you have and how many of them has this issue?

    Thanks a lot in advance and best regards,
    Omar
  • 0 in reply to Omar Hegazi
    Prodigy 50 points

    Hello Omar,

    I probed both sides of the inductor- never saw any switching. As such, both sides of the inductor look just like vout. The part starts up in 100% mode, apparently. I have verified that when I give it more vin, it does regulate. PG is unconnected. And, I only built one of these boards- so 1/1 have the problem. I use the 82084/5 all over the place (we've probably used over 20K of these parts without issue), but I've never used one in 100% mode.

    Attached are more scopeshots. I'm showing vin and vout at the same time. Blue is vin, yellow is vout. Vin doesn't change, vout drops.

    Notice that once vout starts dropping, some ripple appears. Its about 700Hz, which is very low. It seems like the part is trying to work, but it just can't. Like there isn't any charge for the upper fet.

    Initial condition, at powerup. Vin has some ripple, but vout is the same and vin's average

    After about 30s, vout is dropping, and has that weird waveform

    After 1 minute:

  • 0 in reply to Dave Reynolds
    TI__Intellectual 1780 points
    Hello Dave,

    I am writing to let you know that I am still working on this thread.
    I, unfortunately, do not have updates for you yet. I would like you to update me also if there are any new updates on your side.
    I will get back to you the latest by the end of this week.
    Thanks and best regards,
    Omar
  • 0 in reply to Dave Reynolds
    TI__Intellectual 1780 points
    Hi Dave,

    I have been trying to figure out the possible causes of this issue but I could not come up with something. Therefore, I would like to ask you to go through our FA procedure, which has to happen through the local distributor who provided the parts. Usually this should take between weeks and a month or two, depending on the distributor. I know this might be a long time for you. Therefore, if you are looking for a quick fix, I suggest you check out the TPS62088 or the TPS62821. If your load current is below 1A, maybe you can take a look at the TPS62802. These are discrete devices (not modules) but they are smaller in size and some of them comes with a fixed 3.3V Vout versions, which can save you few components.
    Please let me know if you need any further assistance with your design!

    Best regards,
    Omar Hegazi