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LM5170-Q1: Disabled UVLO behavior and IPK accuracy issue

hua chang
Prodigy 20 points
Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

I have several questions regarding the LM5170-Q1.

1. In buck mode, if the UVLO is set above 2.5V, what would LM5170-Q1 behave if there is a surge current (larger than IPK setting) existing on the load side? Does LM5170 have any transient capability or does it just work strictly as a current mode controller?

2. How accurate is the IPK setting is going to be? From the datasheet, I do notice that there is a big variance on the "IPK internal current source" rating and the rest associated parameters. Is this solely temperature dependent or varying a lot from device to device? 

  • 0
    TI__Guru 68605 points

    Hi Hua,

    Thank you for posting.  Answers to your questions:

    1. The LM5170 regulate the inductor dc current per ISETA setting.  If you have a fixed ISETA signal, the inductor current would basically remains the same but the actual output current may change under your load surge current supported by the output capacitor.  If you have the voltage loop enabled, it will response to the load current change and automatically adjust ISETA signal to catch up your load need.

    2. The accuracy is as presented in the EC-table. Note that the IPK function is provided as the secondary level protection incase the first level protection failed somehow.  The first layer is implemented with ISETA clamp, which can be done with a resistor divider at the ISETA signal line tied to the voltage loop error amplifier.  With the ISETA clamp, it determines the max current through the inductor, and the performance is pretty accurate.

    Hope this clarifies.


    Thanks,

    Youhao Xi, Applications Engineering

  • 0
    Prodigy 20 points

    Hi Youhao,

    Thanks for your feedback. Please see following comments.

    1. I am using a TYPE II feedback circuitry to regulate the output voltage, therefore the ISETA is determined by the controller. My concern is that if ISETA is over IPK setting, so I assume the IPK will clamp inductor peak current.  Is the limit transition happening immediately or is there any delays?

    2. Are you suggesting that IPK is just a fail-safe protection, not designing for precise protection?

    Thanks!

    Hua

  • +1 in reply to hua chang
    TI__Guru 68605 points

    Hi Hua,

    Answers to your questions:

    1. You should set IPK limit to be higher than ISETA.  However if IPK threshold trips first, it will directly cut down the duty cycle right away.

    2. Yes. Because the LM5170 inner current loop basically achieves an accurate current source (dc current proportional to ISETA), therefore the first layer of protection should use ISETA clamp.  The IPK is thus the second layer protection and it is just a fail-safe protection.

    Thanks,

    Youhao