MAX3232: Charge pump ripple frequency and ripple voltage

Manoj Turpati

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.

over 1 year ago

0 Parker Dodson over 1 year ago

TI__Guru 52462 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 Manoj Turpati over 1 year ago in reply to Parker Dodson

Intellectual 280 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 Clemens Ladisch over 1 year ago in reply to Manoj Turpati

Guru 317370 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 Eric Hackett over 1 year ago in reply to Clemens Ladisch

TI__Guru 74292 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 Parker Dodson over 1 year ago in reply to Manoj Turpati

TI__Guru 52462 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

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MAX3232: Charge pump ripple frequency and ripple voltage