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LM25149-Q1: LM25149-Q1: Design Review Request

Kyung Min Yang
Prodigy 130 points
Part Number: LM25149-Q1
Other Parts Discussed in Thread: LM25149, LM25148

Tool/software:

Hello Team

We want to use LM25149-Q1 for Automotive controller power. We designed the circuit with information obtained using WEBENCH.
We designed it to be able to output 5V voltage at 12V input and output up to 20A current, but we would like to confirm the design contents regarding the operating frequency setting, EMI filter, and AGND configuration.

1. The input power is supplied by the battery. So, for space efficiency, the EMI active filter at the input stage was not used, and only the SSC was activated. If it is more efficient to combine the SSC operation of LM25149 with the EMI active filter operation, would it be better to add it?

I have attached the content output from WEBENCH regarding this content. Please give me your opinion.

2. WEBENCH recommended a design that sets the frequency of LM25149 to 743kHz. Is this a value that considers the output power and current compared to the input? If not, is there a recommended value?

3. Since no EMI active filter was used, the grounding other than PGND was treated as AGND. Please review the attached circuit diagram.

I look forward to your reply.
Thank you.

REVIEW REQUEST LM25149-Q1 CIRCUIT.pdf

LM25149Q WEBENCH DESIGN.zip

  • 0
    TI__Mastermind 35190 points

    Hello Kyung,

    1. They can always try with and without the EMI active filter and see if it is required or not. It is hard to say if they can get away with SS alone.
    2. I am unsure to the significance of the suggested frequency from WB. For automotive applications, most customers opt for either 400kHz or 2.2MHz. The latter is often used if they want to preform a cost down design, while 400kHz for the highest efficiency and thermals at the expense of a higher BOM cost.

    It seems like you are uncertain on what switching frequency to use. Do you want to proceed with the current 700kHz design? I would first confirm with the customer, as my experience supporting automotive, off-battery applications, most customers would opt for a 400kHz design. 

    Here is a design almost identical to your customer's requirements. It is operating at ~400kHz.

    www.ti.com/.../tidmcd3.pdf

  • 0 in reply to Marshall_Beck
    Prodigy 130 points

    Thank you for your reply.
    Based on your advice, I will consider the frequency and whether to apply EMI filter according to the product design requirements.

    I have an additional question.
    If I do not use EMI active filter, should I leave AVSS pin (1pin) and REFAGND (24pin) floating (Open)? Or should I connect them to AGND? I am confused because the datasheet does not mention if the 2 pins are not used.

    I look forward to your reply.
    Thank you.

  • 0 in reply to Kyung Min Yang
    TI__Mastermind 35190 points

    Hello Kyung, you are welcome. The user can disable AEF by biasing CNFG low after startup. There is a p2p option that doesn't have the AEF functionality at all, at if you find the design doesn't need AEF at all, I would consider going with that orderable...LM25148. The difference between 48 and 49 is 49 has AEF and 48 doesn't.