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LM5069-1 resets automatically

Florian Brendel
Prodigy 20 points
Other Parts Discussed in Thread: LM5069

Hello,

we have some Problems with the LM5069-1, which shouldn't restart automatically. When the Current rises above Ith, the LM5069-1 cuts off for about 7ms and turns on again for several times.

Our Configuration: PWR is not connected, Rshunt is 0.61mOhm so Ith is between 80-90A, UVLO is tied to VIN over 91kOhm so UVLO should be disabled, OVLO is set to 61V

Here you can see the measurement we did. (Orange is the Current, pink the InputVoltage VIN, red is PGD Pin, Cyan is the Output Voltage)

Detailed Cut Off:

Why does that happen?

  • 0
    TI__Genius 12621 points
    Hi Florian,

    For reference, we have an LM5069 design tool available online at www.ti.com/hotswap --> "Tools & Software".

    This is the same design tool we use to create new designs, perform a schematic review, or help troubleshoot an issue.

    I highly recommend you try using the tool to analyze the design above. We have an application note at www.ti.com/hotswap --> "Technical Documents" --> "Robust Hot Swap Design' which discusses the equations and methodology used by the design tool.

    We also have step-by-step video tutorials at www.ti.com/hotswap --> "Support & Training" in case you have any questions on how to fill out a step in the tool.

    After using the tool, feel free to attach it here for us to help analyze.

    I had trouble reading the values from the waveform but I noticed the retry period may have been around 40ms in the waveform? Can you please confirm the timescale?

    If so, that timing does not match the LM5069 insertion time nor restart time based on a 333nF Timer capacitor.

    Can you test with T3 removed to ensure it is not affecting the ON/OFF behavior?

    Also, what resistor values are R15 - R18?

    Note that there should be 0ohm on the VIN line especially, otherwise it could affect behavior. The LM5069 does not use a separate kelvin sense for VIN. The VIN pin will draw 1.3mA typical, so having resistance on that pin will throw off sense resistor measurement which is in mV.

    Also, we have seen issues occur with any hot swap controller (not specific to LM5069) if parallel MOSFETs are used without gate drive resistors. We noticed you have placeholders of 0ohm on the schematic (R19 and R20). That is great you had prepared for that. We would recommend using a value of 10-22ohms.

    Thanks,
    Alex
  • 0 in reply to Alex Triano
    Prodigy 20 points

    Hi Alex,

    Thanks for your reply. We did some changes yesterday and tested that. Yes the period is about 40ms. The Resistors R15-R18 are only to select different shunts on the PCB and fitted with zero ohm resistances.

    - we removed the second parallel MOSFET

    - removed T3 -> no improvement

    - reduce Timer capacitor to 22pF -> the retry period reduces to 10ms

    - 100nF on VIN to GND 

    - RC-Snupper over MOSFET 33Ohm 33nF -> no improvement

    - 10 Ohm Gate resistance -> no improvement

    Finally i added a capacitor from GATE Pin on the LM5069 to GND 6,8nF, this solved the Problem.



  • 0 in reply to Florian Brendel
    TI__Genius 12621 points
    Hi Florian,

    I'm glad to hear you found a solution. That is probably ok to use, so long as the capacitor is relatively small (6.8nF is comparable to MOSFET capacitance, should be ok).

    Just don't try adding >10nF, otherwise you will slow down the short circuit response a lot and could also risk dumping energy into the IC when it pulls down on the gate.

    If it's a loop stability issue, we generally recommend placing the cap from GATE to SOURCE of the MOSFET rather than GATE-GND. That may be worth trying if it's a simple test, but otherwise if it's working well from GATE-GND in your system then by all means pick what works.

    Thanks,
    Alex