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Original question:

LM5155: Are there any maximum +/- limits on bias current into FB pin 8?

LM5155: Protecting FB pin from damage

Robert Zwicker
Prodigy 100 points
Part Number: LM5155
Other Parts Discussed in Thread: DAC5571

The ABS Max rating of the FB pin WRT ground is 3.8V.  I would think that the pin has ESD protection or similar so that (for example) a milliamp of current trying to pull it higher would cause the voltage to be clamped and would not actually damage anything?  For example if the chip is otherwise biased normally but I connect 8.2 K ohms from the FB pin to a 12 V source; would that damage the IC?

My design has already taken a worst case interpretation of this into account.  I am using an external source to clamp both an input synch signal (me reverse engineering datasheet figure 29) and the FB pin voltage.  However, I am proceeding with this (new to me) IC in a somewhat unconventional power converter design, and the more I understand this chip the better...

Thanks, 

Bob Zwicker

  • 0
    TI__Mastermind 21720 points

    Hi Robert,

    Thanks for reaching out with your question and for considering the LM5155 in your design. 

    There is an ESD diode on the FB pin but it is not recommended to use this as a voltage clamp. This diode is intended to operate for a small amount of time as it is an ESD diode.

    What condition are you worried about when there would be no bottom feedback resistor? In that case is the LM5155 going to be enabled?

    Thanks

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 100 points

    Thanks, Garrett

    My circuitry feeding the FB pin is relatively normal (with a normal FB divider) except for two items:

    1) In addition to the divider, I have a 0-5V from a DAC5571 through a 10K resistor into the FB pin.  This so that the output voltage can be adjusted by software.  The FB pin works as a summing junction in this way.

    2) I have some RC "feedforward" in the FB divider in case I need to limit overshoot or need it to tweak the compensation somehow as in type 3 compensation..  Right now this feedforward RC is not populated but it is in the artwork in case I find it useful.   The pcb artwork is still being finalized and I hope to have a board in maybe 2 weeks.

    But the more general answer: I have been designing power for 40 years and I am concerned about something unanticipated.  I can think of no NORMAL way to get >3V8 on the pin but I did not want some unusual situation causing > 3V8 source voltage through >1.5K ohm limiting impedance to break the chip.

  • 0 in reply to Robert Zwicker
    TI__Mastermind 21720 points

    Robert,

    Understood. The only case I can think of when the FB pin is going to go above the absolute maximum voltage rating is going to be if the lower feedback resistor fails open. This would cause the FB pin to get pulled high. One way to work around this is to have two resistors in parallel on the low side feedback. If one fails open the other one will still be present. It is highly unlikely that both resistor will fail open at the same time.

    The other option is to add a zener diode to the FB pin to ensure that that the FB pin doesn't exceed the absolute maximum voltage rating.

    Please let me know if you have any questions.

    Thanks,

    Garrett