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UCC28910: Help on VDD capacitor size

Umberto Dinoi
Prodigy 20 points
Part Number: UCC28910

Hello,

I have some doubts on VDD capacitor sizing. At startup, the controller is off, so CVDD is charged with the High Voltage current source until VDDon is reached.

From that moment on device starts switching and regulation is achieved.

Increasing the capacitor value will lead to longer startup times.

The suggested value for CVDD is given by the following equation, according to the device data sheet:

May anyone comment this equation and give more details?

What if I have another device at the primary side that draws current (3 mA) from the rectified auxiliary? May I consider that contribution on top of IRUN_max?

Thanks.

  • +1
    Prodigy 20 points

    I think I found the answer myself Slight smile

    At startup, C_VDD is charged with the high voltage current souce (ICH1, ICH2, ICH3 depending on VDD).

    When C_VDD reaches VDD_on, the controller starts switching and the high voltage current source is switched off. This causes VDD to decrease due to the switcher operating current that discharges C_VDD.

    As soon as the switcher turns on, we can assume the converter is in CC regulation because the target regulated voltage is not achieved yet. C_OUT is charged with I_OCC

    VDD shall not go below VDD_off before the converter output reaches CV regulation.

    The equation for C_VDD comes from that of a capacitor charging/discharging at constant current.

  • +1 in reply to Umberto Dinoi
    TI__Guru 72325 points

    Hello,

    If you need further help with this let us know?

    Regards,