• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 2 answers
  • Subscribers 64 subscribers
  • Views 70 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5177: Current limit using ISNSP/N and IMONOUT

Al Hollmann
Prodigy 40 points
Part Number: LM5177

Tool/software:

Hi dear experts,

We want to use four LM5177 in quad-phase configuration to achieve about 300W throughput. And to protect the input and output circuitry, we want to use the ISNS current limit feature for both input and output rails, as depicted in the document "Parallel Operation of the Buck-Boost Converters Using LM5177 Buck-Boost Controller", chapter 3.

However i have trouble getting enough information about how to calculate minimum and maximum current limit for a given R_ISNS:

1. In the LM5177 datasheet, chp. 7.3.13, equation (16), there is a constant called "gm". Is it meant to be the "Current sense amplifier transconductance" from the electrical characteristics table?

2. Figure 7-17 suggests to me that in current limit mode, we won't need a R_IMON, but i would guess that we will need this resistor in both monitor and limit mode. Please clarify.

3. Can i use some or all equations of the LM51772 datasheet, chapter 7.3.12.2, to do calculations for LM5177?

4. Can you provide a calculation example for how to calculate minimum and maximum current limit for a given R_SNS (and a given R_IMON if applicable) for LM5177?

Thanks and BR,

Al

  • 0
    TI__Genius 9905 points

    Hi Al,

    Thanks for reaching out.

    1. Yes, this is right.

    2. When using current limit mode, a RC compensation network is necessary at IMONOUT pin. So a single resistor like in monitoring mode is not enough. There has to be a capacitor in series. According to the datasheet, this loop should be 3 to 5 times faster than the voltage control loop.

    3. The LM51772 is working in a different way. Not all of these formulas does automatically apply for LM5177.

    4. RSNS=5mOhms, threshold(limit mode active)=50mV (min.49, max. 51.7)

    min. current limit = 0.049V/0.005Ohms=9.8A

    max current limit = 0.051V/0.005Ohms=10.2A    (assuming the resistor has no tolerance..)

    I hope this helps.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 40 points

    Hi Moritz,

    Thanks for your answer!

    This is what i understand so far:

    In current monitoring mode, the ISNS differential voltage is translated into a source current out of the IMONOUT pin with 0.9/1/1.1mS as the V/I translation constant. And then the R_IMON will translate that current into a voltage which can be measured by some external circuitry. - Please confirm.

    But in current limiting mode, no translation into a current is done. Instead, the differential voltage at ISNS will be compared internally with the 49/50/51.7mV threshold to perform the limiting. - Please confirm.

    If the above statements are correct, then i'm still confused about the 2nd transconductance specification value in the datasheet (170/200/220µS), which is marked with IMON_LIMITER_EN = 0b1 (current limiter activated, i guess). Is his value supposed to help me calculate the current limiter RC compensation network?

    Can you give one example for the compensation network calculation?

    Thanks and BR,

    Al

  • 0 in reply to Al Hollmann
    TI__Genius 9905 points

    Hi Al,

    Al Hollmann said:
    In current monitoring mode, the ISNS differential voltage is translated into a source current out of the IMONOUT pin with 0.9/1/1.1mS as the V/I translation constant. And then the R_IMON will translate that current into a voltage which can be measured by some external circuitry. - Please confirm.

    Yes, thats right.

    Al Hollmann said:
    But in current limiting mode, no translation into a current is done. Instead, the differential voltage at ISNS will be compared internally with the 49/50/51.7mV threshold to perform the limiting. - Please confirm.

    Yes.

    Al Hollmann said:
    If the above statements are correct, then i'm still confused about the 2nd transconductance specification value in the datasheet (170/200/220µS), which is marked with IMON_LIMITER_EN = 0b1 (current limiter activated, i guess). Is his value supposed to help me calculate the current limiter RC compensation network?

    Yes, this value then could be used for calculation the compensation network. However, we unfortunately do not have a guide on how to calculate yet. We recommend to have the loop 3 to 5 times faster than the voltage control loop.
    The RC values used in the EVM should be a good starting point. (82k 22nF)
    If the loop is not stable add a small cap in eries to the 82k. But this need to be checked in the lab.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 40 points

    Thanks a lot, Moritz. This resolved my issue.

    BR,

    Al