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LM2621: Clarification with input and output capacitors

Part Number: LM2621
Other Parts Discussed in Thread: TPS61175, TPS61089, TPS61378-Q1

Hi Team,

We would like to ask for your assistance to the customer's inquiries below.

We are looking at the LM2621 as an option for a booster converter. Our design requirements are 5-11 V in, 11 V out @ 0.5 A max. I had two questions to clarify for implementation:

a) The WEBENCH and datasheet both recommend tantalum capacitors for input and output capacitors. I did not see a minimum ESR for stability or to generate output ripple. Can Ceramic capacitors be used for both the input and output? Is there a minimum ESR on the output required for regulation?

b) The WEBENCH tool suggests Vin = 11 with Vout = 11 will be acceptable. Is there a recommended Max Vin based on the target Vout of 11 V? If we exceed this Vin, can we expect the switcher to continue regulating?

Here is the Webench simulation.

https://webench.ti.com/power-designer/switching-regulator/export/102 

Thank you for your support!

Regards,

Danilo

  • Hello Danilo,

    For your two questions:

    1. Yes, it's ok to use ceramic capacitors. No special ESR is required. But the datasheet recommends a Cin/Cout capacitance.

    2. I need to check it on bench.

    Since LM2621 is a quite old device, I would suggest you select TI newer boost devices, like LMR62014.

    Do you really need a 11V Vout from 11V VIN? When Vin is raised close to Vout, the Vout ripple, DC level would be worse because of minimum on time limitation.

  • Hi Zack,

    I am the customer in question and can provide additional details:

    1. Thank you for confirming. Cin/Cout recommended values will be followed.

    2. Thank you in advance.

    Re: LM2621's age, we originally planned to use TPS61175 and encountered stock disappearing overnight and moved to the LM2621 to have more confidence in availability. It looks like LMR62014 may be hard to find in volumes as well.

    This may be a separate question, but is it possible to somehow determine the age of the part, other than checking the datasheet date? We'd like to use more modern options when available, but beyond filtering for "active" when exploring options, there doesn't seem to be an easy when to consider age of the chip when trying to trade off footprint size, efficiency, and all the other usual comparable metrics.

    11V Vout from 11V Vin will be a rare case (Vin < 10 V will be the majority condition), this would only be an exception case and we'd like to understand how the circuit might behave. Reduced regulation or even unregulated feed-through would be acceptable.

  • Hi Sean,

    The datasheet date actually shows customer when this part is designed. So LM2621 is designed in 2004. The TPS61175 is designed in 2008. LMR62014 is designed in 2011. 

    LMR62014, TPS61089, TPS61378-Q1 should all work for this application.