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TPS25947: Power Good chattering with clean OUT and PGTH

dugan yoon
Prodigy 130 points
Part Number: TPS25947
Other Parts Discussed in Thread: TPS25946

I am testing a circuit where the PG output is chattering when a 3-phase brushless motor is driven from the same board. The motor runs off VBAT, which is also the input to the eFuse. The IN, OUT, EN, and PGTH pins are all clean with no glitches nor dropouts. There are many ceramic bypass caps on VBAT all located close to this circuit. There is no good reason that I can think of for this PG chattering to occur. Any ideas?

  • 0
    TI__Guru* 79001 points

    Hi Dugan,

    I will check and get back by tomorrow EOD on this.

    Regards

    Kunal Goel

  • 0
    Prodigy 130 points

    I gathered some zoomed in traces using my power rail probe to see VBAT at the input and VLED at the output of the TPS25947. 

     

  • 0 in reply to dugan yoon
    TI__Guru* 79001 points

    Hi Dugan,

    1. How LED EN is connected to eFuse?

    2. Can we probe pull up supply of PG pin? Lets see if it is fine.

    3. What is the reason for chattering in VIN?

    4. Only possible reason I think could be reverse current blocking(VOUT jumping above VIN)  or overcurrent.

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    Prodigy 130 points

    1. How LED EN is connected to eFuse?
    LED EN is statically driven to a logic high by a microcontroller. I have confirmed that it stays high and does not glitch nor toggle.

    2. Can we probe pull up supply of PG pin? Lets see if it is fine.
    I have already probed the PG pullup supply and verified that it is stable.

    3. What is the reason for chattering in VIN?
    VIN (or VBAT) is connected to the high side of a motor that is being driven as shown by CH 4 of the scope, signal MOTOR U.

    4. Only possible reason I think could be reverse current blocking(VOUT jumping above VIN)  or overcurrent.
    I think you may be on to something with reverse current blocking since there are caps on VLED that hold it up, but when the motor gets driven, VBAT dips lower than VLED and reverse current will tend to flow. With this hypothesis do the waveforms make sense? Can you describe the eFuse internal circuit functions that are causing the waveforms seen?

  • 0 in reply to dugan yoon
    TI__Guru* 79001 points

    Hi Dugan,

    It could be VIN falling below VOUT or VOUT jumping above VIN which is causing device to detect reverse current.

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    Prodigy 130 points

    I thought I responded to this but don't see it here now. This line of reasoning that VIN could be falling below VOUT seems likely. The motors are pulling current from VIN while there are caps on VOUT holding up that voltage, and the load at VOUT is light compared to the motor demands. Can you talk to the internal circuits that monitor current flow and how it reacts if negative current flow is detected? I want to understand the mechanisms in play here. Further, have other users encountered this situation and found ways to deal with it?

  • 0 in reply to dugan yoon
    TI__Guru* 79001 points

    Hi Dugan,

    This device has reverse current blocking. On detecting that pass FET will be turned off. Please read 8.3.7 Reverse Current Protection in datasheet.

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    Prodigy 130 points

    Hi Kunal,

    The turning off of the pass FET is not the main concern. The chattering of the PG output is problematic, especially since the OUT voltage never crosses the 7.5V falling threshold. Section 8.3.11 says PG goes L for reverse current condition, so it looks like the chip is behaving as specified. Would a capacitor across IN to OUT be worth trying?

  • 0 in reply to dugan yoon
    TI__Guru* 79001 points

    Hi Dugan,

    Cap across IN-OUT is not recommended. More cap at VIN side and some load like 100K from VOUT to GND should help in prevent VIN fall and VOUT jump.

    If you don't need reverse current blocking you can use p2p part TPS25946, so PG will not go low in reverse current scenario. 

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    Prodigy 130 points

    Ahh, yes of course! An eFuse that allows reverse current!