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LMR38020EVM: problem with softstart in IBB topology

Yibo Wang
Prodigy 10 points
Part Number: LMR38020EVM
Other Parts Discussed in Thread: LMR38020

Hi, 

I have tried to use LMR38020EVM to generate -12V from 24V, load current is 0.5A. Schematic shown below:

I have found that the softstart of the chip brings big problem on the output.

If I start the chip with no load condition, the output curve looks as below.

I think it is inrush current triggers the overcurrent protection blanking time, but I do not understand why the capacitor kind of discharged after the voltage reaching the reference voltage.

If I connect the output with some load(120Ohm for example), the chip simply won't work, I saw a big jump in the input current from my power source, luckily the chip is not damaged, and the behavior is repeatable. 

I I connect the load after start up and the voltage is stable, it works well. I measured the close loop bode, the phase margin is 55 degrees which are perfectly fine. 

So it is really some issue with the soft-start, that kind of conflict with the IBB topology.

Can someone explain why it behaves like that?

Thank you very much

Best regards

Yibo

  • 0
    Prodigy 10 points

    Update:

    I read through the datasheet, found that the description on Hiccup mode is quite similar with the behavior in the osci.

    "in applications with a large amount of output capacitors and high VOUT, the inrush current is large enough to trigger the current-limit protection, which can make the device enter into Hiccup mode. The device tries to restart after the hiccup period, then hits the current limit and enters into Hiccup mode again, so VOUT cannot ramp up to the setting voltage ever"

    This is exactly what is happening, the output voltage first ramp up, then the hiccup mode is triggered by the inrush current, the capacitor discharges and restart again.

    But as in the datashee, the hiccup mode should be disabled by the blanking time, for whatever reason, it is not disabling the hiccup mode.

  • 0 in reply to Yibo Wang
    TI__Guru 73015 points

    Hello

    I will assign this post to the engineers who worked on this device.

    Thanks

  • +1 in reply to Yibo Wang
    TI__Mastermind 34875 points

    Hi Yibo

    Do you have current limit on your input power supply?

    What's the total output capacitor in your circuit? what's voltage rating of the cap you placed on VIN and GND_IN?

    can place 8.2K on R7 to startup at VIN=18V?

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 10 points

    Hi, Andy

    Thanks for the reply

    Yes, I set the current limit of the power supply to roughly 2A. I think higher values would damage the chip.

    The total output capacitor is 22uFx3, 100nF, as shown in the schematic of the evaluation board, the voltage rating of the cap is 100V, should be no problem with 36V voltage stress.

    will try that and give you the feedback.

    BR

    Yibo

  • +1 in reply to Andy Chen
    Prodigy 10 points

    Hi, Andy

    I just tried with R7 8.2k and Vin = 18V, the device is not gonna start because the voltage on the EN pin is not higher enough.

    But I tried that with Vin = 24V, then the problem seems to be solved.

    Now it start with 4ms soft start and no hiccup is triggered. I also tried with other load conditions, also looks fine.

    Could you explain why putting R7 with a resistor 8.2K helps? What I see is only it forms a divider on the EN pin, as the EN pin has only a threshold to enable or disable the device, it should has no effect on the start behavior.

    Best regards

    Yibo 

  • 0 in reply to Yibo Wang
    TI__Mastermind 34875 points

    Hi Yibo,

    The input inrush current or the HS current peak at turn-on Vin>18V (with EN divider)  is much smaller than at Vin=4.2V (with EN short to VIN).

    If you have current probe, you can measure VIN, SW, Inductor current and Vout at same time to see if abnormal startup is caused by input P/S current limitation or LMR38020 Hiccup.

    B R

    Andy 

  • 0 in reply to Andy Chen
    Prodigy 10 points

    Ok thanks a lot for the help!