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LM5109B: LM5109B HB to HS voltage during High Side turning on

Akihisa Tamazaki
Genius 11465 points
Part Number: LM5109B

Hello team,

The customer has questions about HB to HS voltage of LM5190B. I attached the schematics and waveform during High Side turning on as the follwoing pdf.


/cfs-file/__key/communityserver-discussions-components-files/196/LM5109B.pdf

The customer has some questions. Would you please advise?

Q1) When LM5109B turns on High Side FET,  HB to HS voltage become  (increase)  25.6V  during the transient period (alomost 40nsec).  Although it  exceed absolute maximum (18V), is it acceptable for LM5109B?

Q2)  Would you please advise how we should improve HB to HS voltage  smaller than 18V?

Q3) Why do  HB to HS voltage increase at the moment?   Why is the HB voltage increasing faster than HS voltage?  Is it boot strap function?

Those 3 questions are translated from the questions in Japanese in the LM5109B.pdf. Your advise would be so appreciated.

Best Regards,
Akihisa Tamazaki

  • 0
    TI__Mastermind 21455 points

    Hello Akihisa,

    Thank you for supporting the LM5109B with your customer.

    I will look to answer the customer questions and make suggestions.

    For question1: We cannot advise exceeding the absolute maximum ratings in the datasheet. I will suggest changes to reduce the voltage spike or confirm if the measurement may have induced voltage above the actual value.

    For question 2: It looks like the HB-HS spike is occurring, starting, before the high side turns on. Although the scope plot does not show much negative HS voltage spike, it is possible in some cases when the low side FET turns off and the HS voltage becomes negative due to the body diode conduction, there can be a negative HS voltage, this negative HS voltage can overcharge the HB cap due to VDD being sourced thru the boot diode and HS going negative. One way to reduce the HB charging peak voltage is to increase the Rboot resistance value and/or increase the boot capacitor value. Confirm that the HB-HS capacitor is located close to the IC pins to reduce trace inductance. Confirm that the scope probes are connected close to the IC pins.

    Question 3: I answered this in the previous discussion. The usual likely cause is if there is a negative HS voltage, the HB-HS cap can be overcharged.

    Confirm if this addresses your questions or you can post additional questions on this thread.

    Regards,

  • 0 in reply to Richard Herring
    TI__Genius 11465 points

    Richard-san,

    Thank you for your comment.

    Best Regards,
    Akihisa Tamazaki