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PMP9766: Supercap calculation

user4979999
Prodigy 690 points
Part Number: PMP9766

Hi,

please check customer question below. thanks.

1. For PMP9766, VIN=5V (Vmain), and I need Vsys close to 5V when Vin drop. The output load current ~1.5A when VIN drop. can i change the R5=47Kohm? 

Vout = [(R4/R5)+1]*Vfb = 4.6V

 

2, i want the supercap to hold the Vsys regulation 5secs, is the Super cap calculation below correct?

 Cmin = 2*5(sec)*4.6(V) / 0.8*(4.7*4.7-1.8*1.8) = 3.05 (Farad)

 

  • 0
    TI__Intellectual 1525 points

    Hi,

    first, you need to calculate the amount of energy your load consumes:

    5V*1.5A=7.5W

    With a runtime of 5s you get

    7.5W*5s=37.5J

    Now you need to accommodate for losses in the switching converter. Lets assume 85% efficiency:

    37.5J*1.15=43.125J

    This is the minimum energy you need to store to meet your specifications. This amount of energy requires the following amount of capacitance:

    Cmin = 2*(43.125J/(4.7V²-1.8V²))=4,58F

    If you need to connect two capacitors in series, two 10F capacitors would be required without any aging or tolerances taken into account.

    As for setting the output voltage, your calculation is correct, 47k will result in about 4.65V.

    Best regards,

    Björn