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LM3556 / input capacitance value

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: LM3556

Hello,

There are the 6.3V and 10V voltage ratings in the LM3556 datasheet.
Are these capacitors measurement conditions difference?

I think that the  actual capacitance is reduced at the input pins.
Could you please let me know the actual necessary capacitance value for input capacitors?
In the datasheet, there is the discription of "10uF ceraminc input capacitor works well".
However, actual capacitance of these recommended capacitors is decreased.



Best Regards,
Ryuji Asaka
Ryuji

  • 0
    TI__Intellectual 1940 points
    Hello,
    You are correct that there is a capacitance degradation with voltage on ceramic capacitors. The target capacitance should be around 4-5uF after taking into account the degradation. Keep in mind that as the capacitance drops, so does the input voltage ripple. If high current is driven, it is recommended to keep the input capacitance and output capacitance as high as possible to lower the input voltage ripple and keep the output current ripple to a minimum.
  • 0 in reply to Greg Lubarsky
    Guru 20090 points

    Hello,

    Thank you for the reply.
    I think that these capacitors were tested with LM3556 for adding to Datasheet.
    Are there any difference in the measument conditions? (Voltage , Current etc..)

    Best Regards,
    Ryuji Asaka

  • 0 in reply to Ryuji
    TI__Intellectual 1940 points
    Hello,
    Bench testing was done over the operating input voltage range, output voltage range, LED current range and temperature. This was done with typical capacitors mentioned in the datasheet as well has with lower value capacitors to check stability.