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TPS25924: EN threshold drop with thermal retry

Mark Guastaferro
Genius 11095 points
Part Number: TPS25924

Using the TPS259241 and noticed that in a short circuit condition with the device operating in current limit mode, and after the device goes into thermal retry mode for a few minutes,  it seems that the falling threshold for the EN pin is dropping.  It looks to be tripping at 1V well below the 1.32V minimum.

The device is very hot during this event. If the device is allowed to cool down and operates into a normal load, the falling threshold is within spec.

Also, it was found that the only way to duplicate this was to have the device retry into a short and operate in overload for a few minutes.  A current overload condition (not a short) did not seem to change the threshold.

Is this normal? 

Vin = 8.4V nom (2-cell battery), Current limit set to 1.25A. 

  • 0
    TI__Genius 11095 points
    A little additional information:
    Was able to trigger the low threshold condition be simulating a short using a Schottky diode on the output (cathode side tied to ground)
    The behavior was observed on a second device to make sure this wasn't a bad part.
    Also, the threshold returns to the correct level if the power is cycled when in the thermal retry mode.
  • 0 in reply to Mark Guastaferro
    TI__Guru* 95911 points
    Hi Mark,

    The EN threshold should not be affected.
    Let me cross-check on EVM and get back to you. Please share the schematic you are working on.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Genius 11095 points
    Hi Rakesh,
    I'll try to get the schematic. I know the EN resistor dividers are set to divide the input by 4 (Vin_enable = 5.6V) , but I don't know the exact values. And Ilim = 1.25A, so RLIM is about 18.2K.
  • 0 in reply to Mark Guastaferro
    TI__Guru* 95911 points
    Hi Mark,

    Thanks for the additional information.
    One thing to check is the variation or droop in the input voltage during overload/ thermal retry mode. So, this happened while testing with battery as input source?
    While getting schematic from the customer, can you also request them for a test waveform with Vin, Vout, Iin, EN voltage

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Genius 11095 points
    Hi Rakesh,
    He is not using a battery for the test. He has a power supply on the input. When in the retry mode, he is lowering the input voltage until the device turns off. He sees this happening at around 4 V instead of a the expected range above 5.31 V.

    I am not sure if he is monitoring the input voltage at the device pin or the power supply, but if there was a voltage drop between the supply and the input, wouldn't that make the device turn off at a higher power supply voltage rather than lower?

    He also noted that he can trip the part into this mode by simply shorting the output momentarily, so it may not be thermal related. He has to power cycle to make it recover.

    Have you tried this on your eval board?
  • 0 in reply to Mark Guastaferro
    TI__Guru* 95911 points
    Hi Mark,

    I will prioritize and get back in couple of days with my observations for this scenario.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Guru* 95911 points
    Hi Mark,

    I will reply through email as it involves customer system details/specs.

    Best Regards,
    Rakesh