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TPS25940 - I(LIM) at short

GSL
Prodigy 50 points

Hello,

I want to use TPS25940A in following configuration:

Vin=5V

I(LIM) = 0.6A (Rilim=147k)

I was expecting that the circuit will source about 0.6A when the output is heavily loaded (shorted via ammeter).

However the behavior I observed is different: At 0.6A the output voltage droped down but the output current continued rising to about 0.94A that is much more that 8%.

Could someone explain that behavior?

Thank you.

  • 0
    TI__Intellectual 840 points

    Hi,

    Are you applying the load using an eLoad in CC mode? If yes, please try with CR mode. And if possible, please share the current and voltage waveforms.

    regards,

    -vishal

  • 0 in reply to Vishal Gupta
    Prodigy 50 points

    Hi Vishal,

    eLoad, resistive load or a simple short doesn't change anything.

    Here is a waveform of Vout/Iout. CH1=Vout, CH2 = Iout (100mV/A). As you can see, the current is not limited to 0.6A as expected but is about 1A

    Regards

  • 0 in reply to GSL
    TI__Intellectual 2800 points
    Hi,

    Can you please monitor voltage across IMON resistor under overload condition. If it does match with 1A, we may need to take a look at other aspects of design. Please double check the value of ILIM resistor as well.

    V(IMON) = (I(OUT) x GAIN(IMON) + I(IMON_OS) ) x R(IMON)

    Where
    • GAIN(IMON) = Gain factor I(IMON):I(OUT) = 52 μA/A
    • I(OUT) = Load current
    • I(IMON_OS) = 0.8 μA (typ)

    Regards,
    Venkata
  • 0 in reply to Venkata Nandam
    Prodigy 50 points

    Hi Venkata,

    The current limiting resistor has correct value.

    For the Imon output I'm using a value that gives me approximately 1V/A output. And yes, at that output the voltage stays limited to about 0.63V.

    So, it seems that the circuit is working ... somehow. In my opinion there is something common with the additional 0.375A (figure 46). Probably because the power dissipation in my case is below the 10W mentioned in that section.

    The problem is that the behavior is not well explained in the datasheet.

     

    Anyway, I don't have more time to spend in debugging. There are enough current limiting devices on the market.

     

    Thank you for your time guys.

    Best regards

  • 0 in reply to GSL
    TI__Intellectual 2800 points

    Hi,

    We apologize for the inconvenience. IMON current is proportional to the load current. With 1V/1A setting, IMON voltage and load current should match with good accuracy.

    Figure 46 shows how fast trip comparator protects the device under hard output short circuit conditions. (Refer to the section 9.3.4 Overload and Short Circuit Protection). Fast trip comparator threshold is set to (1.5 x I(LIM) + 0.375) Amps current level. It is only a coincidence that delta in current you are observing is also 0.375A.

    I have modified the evaluation board TPS25940XEVM-635 to see if I can repeat the issue. With 154k ILIM resistor, current is limited to 0.56A and consistent over input voltage range. See the scope-shot here.

    I would request you to send your schematic and layout for review.  Bad layout can affect any device operation.

    Regards,

    Venkata