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LM3150: Design considerations on Input capacitor and output capacitor

Dong Shen
Prodigy 400 points
Part Number: LM3150


Hi, Team

My customer is using LM3150 for 12Vin, 5Vin @ 4.5V. Different from TI EVM, aluminum capacitors are used as Input capacitor and output capacitor instead of ceramic capacitor. Customer is confusing on below questions:

  1. Whether we need to consider aluminum capacitor’s ripple current parameter? Customer observed aluminum capacitor ripple current was about 3~5A, which is higher than aluminum capacitor’s rated ripple current 300~500mA. So customer wants to know whether this rated ripple current could be used to this application.
  2. What should be considered when replace ceramic capacitor by aluminum capacitor? Ceramic capacitor has low ESR which is suitable for input capacitor. So for aluminum capacitor, do we need to select the same ESR? For output capacitor, when using aluminum capacitor, do we need to select the same ESR as Ceramic capacitor too?
  3. For this COT device, when using aluminum capacitors as output capacitor, what should be considered?

Thank you.

  • 0
    TI__Guru**** 191445 points
    Yes you need to consider those parameters.
    Typically you will need to use a ceramic input bypass capacitor as close to the IC VIN and GND pins as possible. You may also include an electrolytic type in parallel with the ceramic, especially if you have the input supply located far away or if it is connected by wires with significant inductance. You probably will not easily find an aluminum capacitor with low enough ESR and ripple current rating for the input capacitor if using it without some ceramic as well.

    For the output you will need to consider the ESR and ripple current also. All the ac inductor ripple current will flow through the output capacitor. You should be able to size the inductor such that the ripple current will be in the range suitable for electrolytic types. The ESR forms a zero that will help to stabilize the loop response. I recommend that you use the on line Webench tool to design your circuit. Follow the component selection carefully.