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TPS25980: Soft-Short Overcurrent Response Anomaly

cfranAND
Prodigy 80 points
Part Number: TPS25980

Tool/software:

Regarding the TPS25980:
I am observing behavior not described in the datasheet for soft-short conditions near the current limit.

With the evaluation board TPS25980EVM in default configuration (4.5A current limit) (no modifications to the board, all jumpers in default position):
For 0.4–0.7Ω soft-shorts, TPS25980 either performs persistent current limiting or undergoes a single circuit-break operation. After the first retry attempt, the device enters persistent current limiting and does not attempt any further retries.

I also see this behavior on other custom boards and different current limits.

I expected the TPS25980 to perform this response from the datasheet: "Auto-retry 4 times with minimum delay between retries and then latch-off".
I did not expect it to enter persistent current limiting operation, and this is not desired for our application.

Is this behavior described in the datasheet? Or is it an issue with the part?

See attached slide:

  • 0
    TI__Mastermind 39045 points

    Hi Craig,

    The device performs a retry after fast trip (i.e. short circuit response). This retry happens in current limit mode since the output load will be very high.

    Here the device will remain in current limit as long as thermal shutdown is not triggered. I see that in the test cases which you mentioned above, the max power dissipation is around 7W which should cause TSD in 500ms. Since you are not seeing the expected behavior, I would request you to try some small experiments.

    Can you redo this test but with higher operating voltage like 12V or 8V. This will cause higher power dissipation and should trigger TSD. Another thing you can try is perform the test but with higher ILIM (like 8A).

    Best Regards,
    Arush

  • 0 in reply to Arush Gupta
    Prodigy 80 points

    The device performs a retry after fast trip (i.e. short circuit response). 

    To clarify: the device should be triggering normal trip, not fast trip. But normal vs fast trip does not matter. 

    This retry happens in current limit mode since the output load will be very high.
    Here the device will remain in current limit as long as thermal shutdown is not triggered. 

    This is the behavior not described in the datasheet.
    Is this true:
    Circuit breaker functionality is not active during start-up (including start-up during retries). Only thermal shutdown is active during start-up.
    ?

    If that is true, then the TPS25980 never exits start-up during a soft-short because the current limit will not allow the output to reach the start-up exit criteria. I do not know the start-up exit criteria, but I assume it’s the Power Good threshold (VPGTHD=0.326V). In this example with an 4.5A current limit, 3.3V input voltage, and soft-short of 0.5Ω: 4.5A*0.5Ω=2.25V will never reach 3.3V-0.326V=2.97V.

    Therefore the datasheet statement “Adjustable overcurrent protection (with adjustable blanking timer) - Circuit Breaker to protect against soft overload conditions” is misleading because the circuit breaker functionality does not remain active during persistent soft-shorts and the TPS25980 current limits instead. The circuit breaker only appears to operate effectively when the soft-short condition is transient and clears after the first retry. 

    I see that in the test cases which you mentioned above, the max power dissipation is around 7W which should cause TSD in 500ms. Since you are not seeing the expected behavior, I would request you to try some small experiments.
    Can you redo this test but with higher operating voltage like 12V or 8V. This will cause higher power dissipation and should trigger TSD. Another thing you can try is perform the test but with higher ILIM (like 8A).

    No need to redo the test with different conditions. I now confirmed the TPS25980 is most likely entering thermal shutdown for shorts <0.4Ω.

  • +1 in reply to cfranAND
    TI__Mastermind 39045 points
    cfranAND said:
    To clarify: the device should be triggering normal trip, not fast trip. But normal vs fast trip does not matter. 

    yes, you are right here. with 0.4Ohms, the max current will be less than 2*ILIM. Anyways doesn't matter here. (if important, you can quickly check with latch off mode settings) 

    cfranAND said:
    Is this true:
    Circuit breaker functionality is not active during start-up (including start-up during retries). Only thermal shutdown is active during start-up.
    ?

    This is correct. During startup, if the current exceeds the ILIM, the device will perform current limit startup (even for circuit breaker device). 

    cfranAND said:
    If that is true, then the TPS25980 never exits start-up during a soft-short because the current limit will not allow the output to reach the start-up exit criteria. I do not know the start-up exit criteria, but I assume it’s the Power Good threshold (VPGTHD=0.326V). In this example with an 4.5A current limit, 3.3V input voltage, and soft-short of 0.5Ω: 4.5A*0.5Ω=2.25V will never reach 3.3V-0.326V=2.97V.

    Not completely correct. The device will go into current limiting. The output will be equal to ILIM*R_load. PG signal is controlled using that PGTH comparator. That is completely independent from the actual control logic block. 

    I am adding an example waveform for similar conditions. Also, if the load is high during the soft short, the startup themself can fail due to higher power dissipation (thermal shutdown) providing even shorter glitches.

    cfranAND said:
    Therefore the datasheet statement “Adjustable overcurrent protection (with adjustable blanking timer) - Circuit Breaker to protect against soft overload conditions” is misleading because the circuit breaker functionality does not remain active during persistent soft-shorts and the TPS25980 current limits instead. The circuit breaker only appears to operate effectively when the soft-short condition is transient and clears after the first retry. 

    The circuit breaker response is present in steady state. The device will only perform current limited startup during retry so more of a recovery attempt (which can result to some duration of current limiting for the system) but this does not change the fact that during primary protection event it will limit the current.  I understand that your concern is circuit breaker should never limit and should immediately fail the startup is current is higher than the set current limit. 

    Best regards,
    Arush

  • 0 in reply to Arush Gupta
    Prodigy 80 points

    Arush,

    Thank you. That answers my question.
    I request that Texas Instruments update the datasheet to describe this behavior more clearly.