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LM5145: Output low at 7 VDC until inductor value increased from 3.3 uH to 15 uH

Daniel Stochel
Prodigy 10 points
Part Number: LM5145

6825.LM(2)5145, LM5146 contorller design tool - revA1.xlsm

See attached for our design tool. We had this working on a previous rev of our board but now our new board is unstable unless we change the output inductor from 3.3 uH to 15 uH. How come that makes a difference?

  • 0
    TI__Guru 74422 points

    Hi Daniel,

    The higher inductance moves the LC resonance to a lower frequency, thus helping stability.

    Is the Cout and ESR correctly characterized here? 2000uF seems very high for a 27V output. If it's a ceramic, e.g. a bank of 10uF/50V, it must be derated for applied voltage. And if it's an electrolytic, the ESR is likely higher than 2mΩ, which means a lower ESR zero frequency.

    Based on the inductor ripple, you do go with 2.2uH here.

    PS: check the current limit, as it's set lower than the full-load rating.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Hi Tim,

    When we use the 15 uH inductor, the output still isn't completely stable but it does initially help the output jump up to a higher voltage level.

    We do have some large capacitors for Cout. To be exact, there is a 100 nF to GND, a 150 uF to GND, then a diode in series, followed by 2 1000 uF capacitors to GND. For the purpose of the worksheet, should we not include the 2 1000 uF capacitors? Also I didn't change the ESR value, just left it as default. Should I modify that to reflect the 100 nF and 150 uF capacitor ESR's in parallel?

    I haven't tried testing a 2.2 uH inductor on the output yet, but can you explain more why you think that will help with stability?

    As for the current limit, are you talking about the maximum current limit compared to the required current limit set point?

    Thanks

  • 0 in reply to Daniel Stochel
    TI__Guru 74422 points

    Daniel,

    The 100nF and 150uF won't do much when effectively in parallel with the 2mF (the diode is a short for small signal). In the quickstart, enter the total Cout and an ESR value slight lower than the effective electrolytic ESR value (to account for the low ESR of the 150uF).

    The ESR zero frequency here is likely quite low, hence the stability issue.

    --

    Tim