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TPS2595EVM: FLT# signal behavior are different for 2A and 4A current limit settings with output shorted to Ground

Bala Iyer
Prodigy 190 points
Part Number: TPS2595EVM
Thanks for getting the TPS2595EVM to us on quick turn.
I am using VOUT1 (TPS259520) for the following experiment.
My input is from a bench power supply - 3.3V, current limit set to 5.0A.
I am setting the current limit on TPS259520 VOUT1 to 2A, 1K Ohm to GND to GND (jumper J2 set between position 5 and 6).
The Green LED is ON.
Test 1) When I do HARD short the VOUT1 to GND, I am NOT seeing the RED LED (D3) go ON, that means no FLT# is not active. The bench power supply is showing a current draw of 0.85A
In this scenario.
Test 2) When I move the Current limit of TPS259520 VOUT1 to 4A 487 Ohm to GND (jumper J2 set between position 7 and 8), I do see that the FLT# LED turns ON stays ON even if the short is removed. I recycle the power and the unit comes back to working condition

The I(OUT) > I(CB) condition does not apply in both cases as the R(ILIM) is NOT ZERO.

So why it is triggering for 4A setting and NOT 2A setting, even with HARD short to GND in both scenarios ?
Thanks
  • 0
    TI__Guru* 95911 points
    Hi Bala,

    The FLT# signal asserts only for thermal shutdown scenario during overload or short circuit test.
    For 2A setting with Vin 3.3V; the device may not go into thermal shut down and it sees much power stress at higher current limit setting pushing the device into thermal shutdown. Once it shutdown, the latched variant (TPS259520) remains in OFF condition till power / EN recycle.

    Can you share your use case details and end product.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 190 points

    Hello Rakesh

    I understood the FLT# not getting triggered on current limit condition, but getting triggered on the thermal shut down scenario. I also understood the latched behavior.

    With output short circuited, with a 2A current limit setting on the TPS device, why is the current falling to 0.9A ?

    Also, in TPS259521 device (re try one) after what time does the re try happen and will i see a pulse activity on the FLT# if load short condition persist?

    End application is to power QSFP28 modules.

    Thanks

    Bala

  • 0 in reply to Bala Iyer
    TI__Guru* 95911 points
    Hi Bala,

    Can you please share test waveform with 2A current limit case to look at. It should limit to 2A typ. Probe Vin, Vout, Iin, FLTb signal.
    Retry interval for TPS259521 device is 93ms.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 190 points

    Rakesh

    Will post the waveforms

    My application( using TPS259521) is to have a steady current of 1A to 1.25 A @ 3.3V. I like to have indication when there is a current draw that is in excess of 3A (2 x my nominal operating current). Can I connect the ILM pin to GND via 0 ohm resistor and operate it on a continuous basis ? Is this a recommended configuration for continuous use or it is recommended only for test scenario ?

    I want the device to be enabled by driving from a controller's GPIO pin. So I have a 4.7K pull down on the EN/UVLO pin so that it comes up disabled  Can I assume that the device will turn ON for any voltage higher than 1.27V applied to EN/UVLO pin ? As I said, my Vin = 3.3V .

    Thanks

    Bala

  • 0 in reply to Bala Iyer
    TI__Guru* 95911 points
    Hi Bala,

    Connecting ILIM to GND does not guarantee the continuous current support at 3A. Many of the lot may restrict current to <0.5A and 3A is typ value. So, is is not recommended configuration

    yes, >1.27V at EN/UVLO pin ensures device turn ON.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 190 points
    Thanks Rakesh
    Ok. from the documentation , i got the understanding that, only if the current is more than 3A, it will restrict and activate FLT#, when the ILM is connected to ground. That is what I observed with the eval kit also.But if you say that it will trip at even 0.5A, that is scary
    May be i can set the current to max 4A and that should be a sure indication correct ?

    I have around 100uF on the load side of the TPS259521 device. That will be a lot of inrush current, is it not ? Will I be able to charge this up with out triggering the 4A current trip point ?

    Thanks
    Bala
  • 0 in reply to Bala Iyer
    Prodigy 190 points

    Hello Rakesh,

    My test setup in the lab got disturbed. If you have the TPS2595EVM, could you please try the DEAD SHORT on the output with 2A current limiting setup ? Vin = 3.3V, driven with a bench power supply that can supply 5.0A. I am suing channel 1, TPS259520 device

    I want to see if you are seeing the same issue.

    Best

    Bala

  • 0 in reply to Bala Iyer
    TI__Guru* 95911 points
    Hi Bala,

    ILIM short to ground is a single point failure case from device side. In this condition, the device may trip anywhere from 0.5A to 3A. Part to part variation will be there and we ensure to restrict fault current to 3A under that single point failure case.
    Ilim short or open setting are not for normal device operation.

    The inrush current can be limited to desired value by setting cap on DV/DT. Please use design calculator www.ti.com/.../toolssoftware

    I have EVM, will check the behavior for this use case and let you know.

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

    Hi Bala,

    You are correct.. The device will limit to lower currents than the set current limit value in output hot-short condition. This is because of fold-back behavior of the device when Vout is close to the ground. This should not be a concern to the application as it is limiting to lower values in fault mode.

    For soft overloads, the device regulates to the set current limit. Please let me know if you have any further questions.

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 190 points

    Thanks Rakesh for quickly reproducing the issue and capturing the waveforms and explanation.

    I wish the FLT# output goes active in this situation.

    So with 2A current limit setting, there is a fold back protection that is not triggering the FLT#.

    But in a 4A current limit setting, and a dead short at output, looks like the thermal stuff is triggering before the 4A could take effect and hence the FLT# goes active.. Is my understanding correct ?

    Do you mean to say that if the VIN = 5V and the CL setting is 2A, a dead short may limit current to 2A and clamp the voltage to 3.8V ?

    Thanks

    Bala

  • 0 in reply to Bala Iyer
    TI__Guru* 95911 points
    Hi Bala,

    Your understanding is correct on the thermal triggered sutdown.

    For the case VIN = 5V and the CL setting at 2A; with a dead short at the output -> the output voltage will be at close to GND potential and the device will fold-back to currents lower than 2A current limit.

    Best Regards,
    Rakesh