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TPS40400(SLUS930D-April2011)

Other Parts Discussed in Thread: TPS40400

Q1)I would like to check for the calculation of IRIPPLE. (TPS40400(SLUS930D-April2011)

      May I know where to get the  dt value(i.e. 3.05u as indicated in the calculation)?



Q2) What is the formula for IAC? Why the denominator for calculation of  IAC shows 4.88/ Sqrt12?

  • Angie,

    Q1

    Eq 37 uses the formulae  Vo/L x dt.  This dt is the off time of the top fet or the on time of the synchronous fet since Vo is used. (voltage across the inductor during off time of top fet)

    In this case Vo=1.2V, Vin=12V.  Duty cycle is 10%. so off time of top fet (sync fet on time) is 90%.

    Fsw=300khz.  1/fsw =3.33us.  dt=0.9*3.33us =3us

    Another way to calculate is (Vin-Vo)/L x dt.  In this case dt would be 0.1*3.33us   since Vin-Vo is the voltage across the inductor when the top fet is ON.

    Either way will give you the same answer for the ripple current. By the principle of inductor volt second balance; (Vin-Vo)/L* dt(on time of top fet) =Vo/L*(1-dt)

    Q2. The formula ripple current/sqrt12 is a standard method of finding the rms current for a triangular waveform. There is a different formulae for different waveforms. the ripple current is a triangular waveform hence uses /sqrt12

    Regards,

    Mathew