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TPS54202: Vout goes high after Vin disconnected

Eng Lih Khaw
Expert 2000 points
Part Number: TPS54202
Other Parts Discussed in Thread: TPS65261,

Hi,

Customer of mine is using TI DCDCs in their design. The connection is as below:

SMPS (26V) --> TPS54202 (12V)--> TPS65261 --> loads

I have  some questions would like to seek your advise:

(1): For EN pin, what is the meaning of “float” (in datasheet pg 3)?

(2): When unplug AC, VIN (26V) to TPS54202 starts to drop. But we observed VOUT goes up again to VIN level. May I know what cause this happen?

The schematic of TPS54202 is as below. 

According to EN spec, EN max is 7V, EN falling threshold 1.19 hence by adjusting the EN voltage divider to make the EN voltage lower, the issue can be resolved.

But we don't think this is the proper workaround as it is difficult to make the EN pin level goes lower due to slow discharge of VIN after TPS54202 is fully off.

Please advise.

Thanks

  • 0
    TI__Genius 10070 points
    Hi,

    Float means there is no need to connect this pin.

    Could you please explain you issue clearly? For the waveform, I don't know where is the zero axis, it is hard for us to figure out the root cause.
    It will be helpful if you can provide clearer waveform, thanks.

    Shawn
  • 0 in reply to Shawn Nie
    TI__Genius 9550 points

    hi,
    Before "Vout suddenly high again", you can find Vin ramps up slowly. Check VIN voltage if it is ~4.2V. if yes, it is make sense. The device is enabled at that point. (VIN_UVLO Rising 4.2V typical, see datasheet page 5)

  • 0 in reply to Jason Wang (BSR-MV)
    Expert 2000 points

    Hi Shawn & Jason,

    Thanks for your prompt response

    The below waveform should gives clearer picture of the issue. Yes, the Vin is > 4.2V when the 'short pulse' at Vout. So is this scenario considered normal?

    if this is the case, what would be the workaround?  

  • 0 in reply to Eng Lih Khaw
    TI__Genius 10070 points
    Hi,

    From the schematic, you set Vin_on=6.7V, Vin_off=6.37V. From the waveform, when Vin drops to 6.37V, the device shuts down, and the voltage of input capacitors increases, because the ESL of input side or other reasons. When the voltage reaches to 6.7V, the device starts up again.
    You can zoom in the waveform and measure Vin when it shuts down and starts up.

    Best Regards,
    Shawn
  • 0 in reply to Shawn Nie
    Expert 2000 points
    Hi Shawn,

    Thanks for your explanation.
    Correct me if i'm wrong, the Vin_on you mentioned is referred to Vstart in Eqn (1) in datasheet whilst Vin_off is Vstop. right?

    if the required Vout is 12V, should the Vstart set higher than Vout?
    Thanks
  • 0 in reply to Eng Lih Khaw
    TI__Genius 10070 points
    Hi,

    Yes, you are right, you can use this equation to set the UVLO.
    If Vout is 12V, you can set the Vstart higher or lower than Vout, it depends on you. For this issue, you can set Vstart a litter higher than Vstop to avoid the restart.

    Shawn
  • 0 in reply to Shawn Nie
    Expert 2000 points

    Hi Shawn,

    Thanks for your advise.
    Customer of mine decided to use internal enable function. Their specs is 27Vin, 12Vout

    I've some questions pertaining the following waveforms

    (1) On start up time, it takes about 17ms to reach 12Vout, is this ok?


    (2) Once Vin dropped till 4.2V, switching last around 1.5s. is this ok?

    (3) Is the switching at the beginning stage (circled in red) considered normal?

    (4) there's a dip observed (circled in blue) (12V drop to 7.7V) on VOUT when VIN rise near to 26V, this is normal?

  • 0 in reply to Eng Lih Khaw
    TI__Genius 10070 points
    Hi,

    Please see my input:

    (1) On start up time, it takes about 17ms to reach 12Vout, is this ok?
    Becuase the input voltage increase slowly, so it takes too much time to reach 12Vout. I think it is Okay, what is the value of your input capacitors?

    (2) Once Vin dropped till 4.2V, switching last around 1.5s. is this ok?
    From the waveform, when Vin dropped to 4.2V, the device didn't shut down, it seems the TPS65261 shut down, so the load was removed, the input voltage discharged slowly, and the output voltage is equal to input voltage. It takes 1.5s for the input voltage dropping to the value of UVLO, then the device shuts down.

    (3) Is the switching at the beginning stage (circled in red) considered normal?
    I think the startup waveform is reasonable, what is your concern?

    (4) there's a dip observed (circled in blue) (12V drop to 7.7V) on VOUT when VIN rise near to 26V, this is normal?
    It seems the large load was added at this point, so the output voltage has undershoot. Could you zoom in the waveform to see what happened? I think it related to the next stage circuit.

    Best Regards,
    Shawn