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LM73100: Startup fails when heavy load is present

Wei-Han Lai
Prodigy 50 points
Part Number: LM7310


Tool/software:

I observed this behavior on both the EVM and the board I designed. When I have a 4ohm load on the output and apply anything above 12.2V, the LM73100 fails during startup and the output stays off. I checked Iin, OVLO and UVLO pins and saw no unusual behaviors. Waveforms are below.

Another observation is, when I slow down the slew rate of the input power supply to less than 20V/s, then startup will be successful. I would prefer to solve this so that the part will startup successfully regardless of load condition or input voltage slew rate.

C1: OVLO, C2: Iin

C1: ULVO, C2: Iin

C1: Vout, C2: Iin

  • 0
    TI__Mastermind 39295 points

    Hi Wei-Han Lai,

    When you have high power dissipation at the device, the device will perform thermal shutdown to protect the FET. I think in your case, If your load have high capacitance, your inrush current will be very high which can trigger TSD during startup. If your resistive load is high, continuous current can trigger TSD during startup. 

    In case of high inrush current case, slowing the startup by increasing dvdt cap, will reduce the inrush current and allow proper startup.

    In case of high current drawn by resistive load, making the startup faster will help in proper startup.

    We have design calculator which can help you in calculating the dvdt cap for your load. 

    Design calculator: SNOR027 Calculation tool | TI.com

    Reply here for any follow up question or click resolved. 

    Best Regards,
    Arush

  • 0 in reply to Arush Gupta
    Prodigy 50 points

    For inrush, I measured input current and saw nothing unusual. As can be seen from the waveforms in the original post, the input current never rises above 1.5A before the part trips. For reference, my output capacitance is is 64uF and DVDT pin is set with 100ohm + 10nF.

    For high current draw due to resistive load, once I am able to successfully start the part, it will run indefinitely (tested up to 3 days) with the resistive load, so I don't understand how it will cause a thermal shutdown during startup if it can run indefinitely during steady state.

  • 0 in reply to Arush Gupta
    Prodigy 50 points

    I tried as you said where increasing the dvdt cap will allow startup with a high load capacitance but then startup with heavy resistive load gets worse. My guess is that the FET is operating in the linear region and so the more time it spends in linear region the more likely it is to get hot, but that is just my guess.

    Likewise I set the dvdt pin to open and saw that startup with heavy resistive load got better but then would fail startup with heavy load capacitance.

    A strange thing I saw was that I tried to speed up startup using a 100pF cap on the dvdt pin, but instead startup got incredibly slow. It took more than a minute for the switch to turn on. Is there a minimum cap value that I can put on the dvdt pin?

  • 0 in reply to Wei-Han Lai
    TI__Mastermind 39295 points

    Hi!

    Usually, power dissipation is higher during startup as Vout is yet to reach Vin. During steady state, Vout is almost near Vin unless any external event so power dissipation at device is less. 

    Wei-Han Lai said:
    Likewise I set the dvdt pin to open and saw that startup with heavy resistive load got better but then would fail startup with heavy load capacitance.

    Yes, this is expected. Dvdt cap open is the fastest startup. 

    Wei-Han Lai said:
    I tried as you said where increasing the dvdt cap will allow startup with a high load capacitance but then startup with heavy resistive load gets worse.

    This is also expected.

    Can you please share your operating conditions, load details. I can check suggest some dvdt value. Or you can also check this with design calculator shared above.

    Best Regards,
    Arush