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Original question:

LM5113: LM5113 Rev. H Datasheet Concerns

LM5113: LM5113 Internal Boostrap Diode Startup Time, Detailed

Aidan Phillips
Prodigy 140 points
Part Number: LM5113
Other Parts Discussed in Thread: LMG1205, LMG1210

Hello,

I believe I am up to date in terms of the recommendation to use an external boostrap diode to improve startup bootstrap times for the LM5113.  It's my understanding that it is not needed, but can help.  This topic is thoroughly covered here. 

What I do not understand, is exactly how poor the startup behavior is with just the internal diode.  What exactly happens in the chip that degrades functionality at startup vs during operation?  Is it simply that there is significant resistance in series with the internal diode?  We chose to use the internal diode because we wanted to have the protection of the internal clamp circuit, which we learned here, only functions when the internal diode is used.

We are now seeing that sometimes the high side drive takes ~5ms to start switching.  When operating normally, the bootstrap cap charges way faster than this.  There looks to be a dependence on what the switch node voltage is before switching occurs.  Can this long 5ms time span be attributed to the startup loss in performance by having an internal diode?

  • 0
    TI__Mastermind 23215 points

    Hello Aidan,

    I have looked into the recommendations for the LM5113 and it looks like the external boot diode and 20 Ohm resistor is recommended.

    There are newer GaN half bridge drivers, the LMG1205 and LMG1210 that have improved operation of the bootstrap charging and overcharge prevention. I would recommend looking at these devices to see if they will work in your application.

    I would have to confirm what is the limitation of just the internal diode only regarding the start times.

    Regards,

  • 0 in reply to Richard Herring
    Prodigy 140 points

    I suspect that many users are locked into the automotive grade of the LM5113, which is not nearing EOL, so it's not as simple for us to just move away from the part.  Please let us know about the limitations.

  • 0 in reply to Aidan Phillips
    TI__Mastermind 23215 points

    Hello Aidan,

    There are no current plans to EOL the LM5113 Q1 grade, and even when that is considered there is quite a long warning period. Regarding the questions on the startup time with the internal only diode, I will need to reach out to get some details of the IC to address.

    Give us a couple of days early next week to reach out and get details to address the question.

    Regards,

  • 0 in reply to Richard Herring
    TI__Mastermind 23215 points

    Hello Aidan,

    Any request looking into the LM5113 internally results in the definite recommendation to add the external boot diode and 20 ohm resistance. I think there are improvements in the newer GaN drivers to address the boot diode operation which we recommend looking at the newer devices for improved internal boot diode operation. I am not able to get concrete guidance on predicting the internal only boot diode regarding start times. The 20 ohm series resistance is confirmed to be a long enough time constant to address any boot cap overcharging issues.

    Regards,