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Output Capacitor Selection

Nidhi P Shetty
Intellectual 735 points
Other Parts Discussed in Thread: LM3481

Hello Sir

I'm using LM3481MM/NOB converter IC in my project. It is chosen to operate at 550KHz. with a inductor with a value of 6.8 micro henry and with a input supply of 5V and to boost it up to 12V. While selecting the output capacitor only about current requirement is mentioned in the datasheet. No parameters or equations are mentioned in the datasheet to find the output capacitor value. So what is the minimum capacitance required for this design. Please help me out in this concern.

Regards

Nidhi P Shetty

  • 0
    TI__Guru* 78270 points
    Hi Nidhi,

    Thank you for considering the LM3481. The datasheet describes the Cout selection by load current because the boost rectifier current is a pulse train and its average is the load current, and the pulse train determines the amount of charging and discharging current of the output capacitor, that in turn results in certain ripple voltage.

    Most designs including yours may have Vout ripple spec. Use the following to determine the minimum output capacitance:

    Co_min = Io * Dmax / (Fsw * Vripple)

    Where Vripple is the pk-pk ripple voltage, Fsw the switching frequency, Dmax is the max duty cycle, and Io is your max load current.

    You may also need to consider the holdup time by Co, namely when Vin is lost and how long the output capacitor need to hold up the voltage.

    Thanks,
    Youhao Xi