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MAX3232E: Cap selection

Kurumi Hasegawa
Expert 7891 points
Part Number: MAX3232E

Hi team,

 

Could you help selecting witch cap to be used?

 

On the Table 9-1, recommended capacitor values are shown.

 

I believe cap's capacitance changes depends on the condition, such as DC bias or temperature.

So, for example when Vcc=3V, I should choose cap for C1 that can guarantee at least 0.1uF under worst case scenario (including DC bias, temp, etc.), is my understanding correct?

Also, is there any concern using larger cap?

 

Best regards,

Kurumi

 

  • +1
    TI__Guru 74252 points

    Kurumi-san,

    I'd recommend reading this blog written by our RS-232 expert, Hao Liu. In it, he goes over the design of charge pump circuitry and how the passive component selection affects the rippled of the signal.

    I've also assigned this thread to one of our applications engineers to keep up in case you have more questions, so if you have any please don't hesitate to ask.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Expert 7891 points

    Hi Eric,

     

    Thank you for sharing the great blog, however I was having a difficulty finding the answer to my original questions.

    My questions is if I need to consider effect from DC bias, temp, etc. when choosing capacitors, or not.

    For example, below is DC bias characteristic of 0.1 uF cap. If DC bias is 10V, the capacitance would be reduced to 0.02~0.03uF.

    I should choose cap for C1 that can guarantee at least 0.1uF under worst case scenario (including DC bias, temp, etc.), is my understanding correct?

     

     

    For the second questions, it was mentioned on the blog that large cap doesn’t always mean the better ripple performance. So I’m guessing too large cap is not ideal. But if that was wrong, please let me know.

     

    Best regards,

    Kurumi

  • +1 in reply to Kurumi Hasegawa
    Guru 316690 points

    The capacitor values are nominal, but this device was originally designed for electrolytic capacitors without DC bias effects. So the acceptable tolerance is ±20%.

    If you are using MLCC capacitors, then you have to ensure that the DC bias effect is no worse than that. So you must use larger packages.

    Larger input/output capacitors make the ripple smaller (because the charge moved by the flying capacitor is smaller relative to the charge on the I/O capacitors). Larger flying capacitors make the ripple larger (and the ripple frequency lower).

    Larger electrolytic capacitors have higher ESR, which can make the ripple worse, but this is not a problem for ceramic capacitors.

  • 0 in reply to Kurumi Hasegawa
    TI__Expert 7891 points

    Thank you so much for the reply.

    I understand that it was originally designed for electrolytic capacitors, and when MLCC is used, larger packages required.

    Also, thanks for additional info on the capacitance value!

     

    Beset regards,

    Kurumi