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LM5088: Calculation of sense resistor

HKH
Prodigy 105 points
Part Number: LM5088

Dear all,

I am having trouble understanding the datasheet of LM5088. How is the equation (11) for calculating Rs deducted?

Ignoring margin this is eq. 11:

Rs = Vcs / (Io.avg + Io.pp/2 + Vo / L / fsw)

I do understand that

Io.avg + Io.pp/2 = I.peak

which is also the current in the sense resistor at time of sample/hold.

But what is this part "Vo / L / fsw" and how is that deducted?

/HKH

  • 0
    Prodigy 105 points
    I must correct my self:
    Io.avg + Io.pp/2 = I.peak

    is NOT the current in the sense resistor at time of sample/hold.

    This should be closer:
    I.sh = I.peak - Vo / L * (1-Vo/Vi) / fsw

    it seems very similar to the expression for current, in the datasheet:
    (Io.avg + Io.pp/2 + Vo / L / fsw)

    except there is no (1-D) multiplication. Why is that?
  • 0 in reply to HKH
    TI__Mastermind 39980 points

    Hello HKH,

    The LM5088 measures the current (Sample and Hold) at the end of the 1-D period, which is the valley of the inductor current.  The equation is computing the peak current by adding Vslope for slope compensation (Vramp)  The Datasheet equation enables you to calculate the needed Rsense when taking into account the Vcs level, which has the Vramp factored into the equation.  The peak current is computed with the Vramp term assuming a k of 1 and a D of 0. Hence the Vout/L*Fsw term, which will accurately predict the peak current limit level with this architecture.

    I hope this helps?

    David.

  • 0 in reply to David Baba
    Prodigy 105 points

    Thanks David,

    I beleive that I understand. This simplification assumes that compensation corresponding to the full down-slope has been added.

    But only half of the down-slope is actually needed for compensation (in theory), and hence the simplified equation for R.ramp is also to be used with care.

    /HKH