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LMR23630: Issue during startup phase of LMR23630ADDA

Fabio Tavares Barros
Prodigy 20 points
Part Number: LMR23630

Hello,

I’m currently working in a circuit that uses a PN LMR23630ADDA from TI.

The aim is to have a DC/DC converter with input voltage from 9 to 36Vdc and the output voltage of 3.5Vdc.

I’m facing a behavior that I don’t know if it is a normal behavior.

When I switch the board on, after waiting for at least 2 minutes, the converter initializes without any problem. If I switch the board off and switch it on in less than 90 seconds after, the output voltage stays in 1.7Vdc approximately.

The only way to the converter starts without this behavior is if I wait more than 90 seconds before switching the board on.

Just to inform, there are two capacitors of 3300uF and one capacitor of 1500uF in the output of the converter. I’m aware that if I remove these large capacitors this behavior will be minimized, but I’m afraid of doing this once these capacitors came from an old circuit where they were necessary because of the Modem characteristics. During some events the Modem may need almost 2Amps.

Have you ever face this kind of situation with this converter? Is there any kind of workaround so that we can use the circuit in the way it is today?

I did not find a way to attach the schematic, please let me know if you need more information.

Thanks in advance,

  • 0
    TI__Mastermind 23845 points
    Yes, i have seen this before and the reason is because the internal soft start time for the LMR23630 is within 1-2ms and to charge the large output capacitor with this short amount of time requires a large current. IC = C DV/DT. Therefore the converter hits a current limit and try to restart again. Would you like to try the LMR23630DRR version? because the QFN package option have a longer internal soft-start time.

    The other thing you can do is to use a load switch therefore in between the output of the converter and the large output capacitor. So that the converter dont have to hit a current limit during soft-start.

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 20 points
    We can try to change to LMR23630DRR, but we cannot take the risk of a new failure, because we are running out of time.
    Regarding the second option, I imagine we should add a resistor in the output of the converter. If I’m right, what current should I consider in the output?
    BR,
    Fábio