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LM5143-Q1: Design calculation question

Takeshi Sasaki
Expert 4480 points
Part Number: LM5143-Q1

Hi team,

Could you give me comment on following customer questions?

Q1. Could you share equation how to calculate Rcomp, Ccomp, CHF in dual phase single output case? Eq. (43), (44), (45) looks dual output case, but it looks different from dual phase single output case based on the result of the quick start calculation sheet.  

Q2. Based on the quick start calculation tool, the customer understands minimum Cout requirement is as follows. Is it correct understanding? 

Dual phase single output: Cout1: 11uF, Cout: 11uF

Dual output: Cout1: 22uF, Cout2: 22uF

Q3. For minimum input capacitance requirement, the customer calculates Cin = 7.8uF based on the datasheet equation at dual output. Taking DC bias and the # of channels, Cin = 40uF [=(10uF x 2) x 2ch] is required. Is it the same requirement for dual phase single output? Or the minimum input capacitance requirement is a half of the dual output case of the Cin? 

Best regards,

Takeshi Sasaki

  • +1
    TI__Mastermind 49647 points

    Takeshi-san,

    1. The per-phase equivalent circuit can be derived with Cout-per-phase = Cout-total / (Number of phases) - so in this case Cout-per-phase = Cout-total/2. Also, Rload-eff = Vout/Iout-per-phase = Rload * (Number of phases). The load pole is still 1/(2*Pi*Rload*Cout) as usual.

    2. Minimum Cout in the quickstart is just based on the output ripple spec. In practice, it should be chosen based on the load transient spec. Note you can use WEBENCH to also assist with component selection.

    3. The input capacitance for a two-phase circuit is lower as ripple current cancellation occurs due to the phase shift between phases. For a 2-phase circuit, duty cycles close to 50% achieve the most cancellation, whereas it's not as great at 25% and 75%.

    Regards,

    Tim