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TPS2660: Startup behaviour when coming out of Short Circuit

Hao Jie Chan
Intellectual 270 points
Part Number: TPS2660

Tool/software:

Hi TI,

I would like to confirm the behaviour of the device when coming out from short circuit.

The eFuse is implemented such that the output would see a momentary inrush due to capacitive load, and would trigger the short circuit protection. 

Then, when the eFuse is starting up, the output voltage would be charged up, but the rate is slower than what I expected.

I ran a simulation with a modified reference simulation file, adding the switch and a capacitive load to simulate the inrush

The result shows that the current charging the capacitor is only 133mA, when my resistor is set to 62kohm, which should give me ~0.2A.

I've tried with other resistor values, but the current is always limited to ~0.13A

 

From this plot, I would expect the current to be limited according to the resistor, am I doing something wrong?

Also another queston, the FLTB is still asserted even though PWR_OUT has reached 12V. Is this also expected? I expected the timer to stop once the fault (the limited current in this case) is cleared.

  • 0
    Intellectual 270 points

    TPS26600_TRANS_CAPACITIVE_LOAD.DSN
    i wanted to upload the zipped simulation but it won't let me. I don't know if a .DSN file would work....

  • 0 in reply to Hao Jie Chan
    TI__Mastermind 30731 points

    Hello Hao,

    Output slew rate and inrush current is adjusted by dVdT cap during start-up.

    You are turning On the eFuse first and then the switch U2? I don't see 62K resistor at output.

    Can you explain what you are trying to simulate? 

    Thanks

    Amrit 

  • 0 in reply to Amrit Jit
    Intellectual 270 points

    Yes, eFuse is already turned on, then the switch U2.

    In my bench measurement I measured the ~FLT signal got asserted, and I was wondering why it does not charge the capacitive load in a shorter expected time, so I was simulating this behaviour.

    62k resistor is the ILIM resistor

    -Hao Jie

  • 0 in reply to Hao Jie Chan
    TI__Mastermind 30731 points

    Hello Hao,

    Looks like inrush current from cap is exceeding the ILIM value set. So FLT is asserted.

    Since simulation model is a behavioral model, I don't think this test case is covered.

    Please try on with EVM for accurate device behavior. 

    Thanks

    Amrit 

  • 0 in reply to Amrit Jit
    Intellectual 270 points

    Do you have a description for the behaviour after Short circuit protection is triggered, and the FET is slowly turned on? What would happen to the current limit in that situation?

  • 0 in reply to Hao Jie Chan
    TI__Mastermind 30731 points

    Hello Hao,

    During a transient output short circuit event, the current through the device increases very rapidly. As the current limit amplifier cannot respond quickly to this event due to its limited bandwidth, the device incorporates a fast-trip comparator, with a threshold I(FASTRIP). The fast-trip comparator turns off the internal FET within 250 ns (typical), when the current through the FET exceeds I(FASTRIP) (I(OUT) > I(FASTRIP)) and terminates the rapid short-circuit peak current. The fast-trip threshold is internally set to 87% higher than the programmed overload current limit (I(FASTRIP) = 1.87 × I(OL) + 0.015). The fast-trip circuit holds the internal FET off for only a few microseconds, after which the device turns back on slowly, allowing the current-limit loop to regulate the output current to I(OL).

    Find the Ifasttrip and I(SCL) limits below.

    Thanks 

    Amrit Jit