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MAX3232: Charge pump ripple frequency and ripple voltage

Manoj Turpati
Intellectual 270 points
Part Number: MAX3232

Hi,

I am using MAX3232 IC with a 3.3V supply. I would like to reduce impact of change pump ripple on my 3.3V supply since it is used to supply other devices on the board. I would like to add an LC filter at 3.3V supply line to suppress the noise. Could you please tell me at what cut-off frequency should i design my filter and the current rating of the inductor that i have to select ?

Best Regards,

Manoj.

  • 0
    TI__Mastermind 48922 points

    Hi Manoj,

    We don't guarantee any specific frequency for the charge pump - and it will vary based on loading conditions - that being said - the signal on VCC should be pure DC - so you want to make your cutoff frequency relatively low - but that being said the decoupling capacitor already should help stabilize the VCC voltage. Adding an additional filter can be done - but I don't know if it would be necessary as the VCC pin really isn't going to be giving off too much ripple. 

    if you do use an inductor it doesn't need a very high current rating - probably around 250mA max would be more than enough - but once again we don't really see issues from ripple voltages coming off VCC. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Intellectual 270 points

    Thanks Parker,

    Whats the value of the Inductor that i have to select ? I want to put two 22uF capacitors in parallel and I would like to make LC filter to supress the noise on VCC. 

  • 0 in reply to Manoj Turpati
    Guru 304240 points

    The internal frequency is nominally 500 kHz, but the charge pump is regulated, so the actual frequency will be lower. A previous question showed a typical ripple frequency of about 15 kHz, but this might change depending on your actual load current.

    An LC filter with a −3 dB frequency of 5 kHz would require about 22 µH; for 2.4 kHz, 100 µH.

  • 0 in reply to Clemens Ladisch
    TI__Guru 73372 points

    Clemens,

    Thanks for the help here.

    Manoj,

    Parker is out of office today but will get back with a response tomorrow. Clemens response is correct though.

    Regards,

    Eric Hackett 

  • 0 in reply to Manoj Turpati
    TI__Mastermind 48922 points

    Hi Manoj,

    I apologize for the delay - but what has been stated by Clemens is correct w.r.t. to inductor sizing. 

    Best,

    Parker Dodson