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LM25148-Q1: what does the gm=1200us mean ?

? ?
Prodigy 30 points
Part Number: LM25148-Q1
Other Parts Discussed in Thread: LM25141, LM25143, LM5143, LM25148

Tool/software:

what does the gm=1200us mean ?  why not uA/V ?

Is it the reaction time?

  • 0
    Prodigy 30 points

    another question，why 10K~20K？

  • 0 in reply to ? ?
    TI__Expert 5330 points

    Hi ??,

    1. The gm spec is given in us, which is the same units as uA/V.

    2. 10-20k is a good region to start with to have a feedback network with large enough resistance to not draw much current from VOUT, but not too large to cause issues with regulation.

    Thanks,

    Dino Maslic

  • 0 in reply to Dino Maslic
    TI__Guru 73742 points

    Actually, the transconductance is given in units of µS (uppercase S for Siemens), not µs (lowercase s for time).

    And 10-20kΩ is a good starting point for the lower feedback resistor, as less than 5kΩ will result in the IC detecting FB as low at startup and setting a fixed 5V output. See the data sheet for guidance on this.

    --

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

    Hi：

    why 10-20kΩ is a good starting point for the lower feedback resistor ？

    less than 5kΩ will setting a fixed 5V output？ the data sheet has no guidance on this

    if the fb_bot resistance less than 10KΩ，what will happen？

    and if the fb_bot resistance larger than 20KΩ，what will happen？

  • 0 in reply to ? ?
    TI__Guru 73742 points

    What you have copied above are values for fixed output settings. Looks like there are no limits on the feedback resistances for an adjustable output voltage setting (unlike the LM25141 and LM25143, for example). As Dino mentioned, 10-20kΩ is good from a quiescent current standpoint. Higher than than is fine as well, as long as the upper resistor doesn't exceed ~200kΩ (at which point its impedance becomes unnecessarily high, thus increasing noise sensitivity, possibility of board contaminants affecting the behavior, etc.)

  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

    Hi：

      For the LM5143，if FB is connecting to gnd ，the output is 5V。In this spec，recommend RFB1//RFB2<5kΩ，but not only RFB2 <5kΩ，why that？Could it be that the low mos  is open at the IC power up?

  • 0 in reply to ? ?
    TI__Guru 73742 points

    The LM5143 is a different controller (not relevant to the LM25148) - apologies for the confusion by mentioning that.

    The LM5143 needs to see more than 5kΩ on FB, otherwise it will detect it as being low and set a fixed 5V output. The LM25148 does not have that concern, as the fixed output setting is determined only by a resistor to VDDA.

  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

     For the LM5143，recommend RFB1//RFB2<5kΩ，why not RFB2 <5kΩ？

  • 0 in reply to ? ?
    TI__Expert 5330 points

    It is the Thevenin equivalent of the network that matters, not just the bottom resistor.

    Thanks,

    Dino Maslic