LM3485: Input Capacitance Guidance and Tradeoffs

Matt Calvo

Part Number: LM3485

Hey Buck Controller team,

I have a customer evaluating the LM3485 for a new power supply design and we are hoping to get some guidance from your team regarding input capacitor selection on Vin.

We see that the typical application circuit shows a 22uF capacitor on Vin but there are requirements in their system which require the capacitance on this node to be 0.27uF max.

Do you have any suggestions or comments on using, for example, two 0.1uF caps in parallel for the input and what potential impacts it might have on performance?

For reference, the input range is 9-28V, the output is 3.3V, and the current draw is 50mA max.

Thanks for all your help!

Best regards,

Matt Calvo

over 1 year ago

0 Henry Kou over 1 year ago

TI__Expert 3766 points

Hi Matt,

Input capacitors provide a bypass current path for ripple current that can stabilize VIN during a transient. I recommend reading the following technical article to determine the min CIN you can have for your system (eq 1), as well as editing a Webench simulation for your model here.

Regards,

Henry

0 Matt Calvo over 1 year ago in reply to Henry Kou

TI__Genius 14235 points

Thanks Henry!

I will pass this information along to my customer and notify them to reply back on this thread with any follow up questions as needed.

Best regards,

Matt Calvo

0 Henry Kou over 1 year ago in reply to Matt Calvo

TI__Expert 3766 points

Hi Matt,

Thanks and I'll mark the thread as resolved for now. Please reopen if any further issues arise.

Henry

Power management

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LM3485: Input Capacitance Guidance and Tradeoffs