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3.6-12V to 5V Buck Boost Design Based on LM25118MH

Suresh Kumar Kannan
Prodigy 50 points
Other Parts Discussed in Thread: LM25118

Hi,

We have designed Buck Boost converter where Input and Output data are as per follows:

IC : LM25118MH

Input : 3.6-12V DC

Output: 5V,4.5A

Issue:

When we are applying 3.6V at input UVLO is at GND. When we are increaseing input voltage from 3.6 to 4.8V, Buck Boost Converter start giving 5V output where UVLO Pin is 2.5V.

We want to generate 5V from this buck boost when input is 3.6V. Can you please look in to attache design and suggest what can be done here?

Reference:

Attached design for quick reference.7357.Buck_Boost.pdf.pdf

  • 0
    TI__Guru 58571 points

    Hi

    You need an external bias supply.

    Operating rating of VIN pin is from 3V to 42V, but VIN>=5V is required to initially start the controller. Please refer the Operating Rating section of LM25118 datasheet.

    Regards,

     

  • 0 in reply to EricLee
    Prodigy 50 points

    Hi,

    Please find below our actual scenario:

    1) VCC_BAT is our input voltage point. We are powering the Buck/Boost converter (LM25118) with 12V input and found LM25118 generating 5V.

    2) Our Input voltage range is 3.6V to 12V. We are initially powering the buck/boost converter with 12V only.

    3) Case1: We tried reducing the 12V input voltage to 3.6V. We found that LM25118 is stopped generating 5V when the input voltage is below 4.6V. In this case, we initially powered the LM25118 with 12V but LM25118 is not working at 4.6V and less input voltages.

    Note: During this 4.6V input at LM25118, the pin2 of the IC(LM25118) pulls the pin to GND. The pin2 releases the GND when the input voltage is above 4.6V and it pulls the GND when Vin is less than 4.6V.

    4) Case2: We powered LM25118 with 12V and went to 3.6V and found LM25118 is not generating 5V output.

    5) Please brief about external bias supply which you mentioned. You mean to say the pin 17. We have connected the pin17 to GND. Should we need to connect to 5V output. Please clarify.

    We are awaiting for your valuable reply.

    Thank You.

     

  • 0 in reply to Suresh Kumar Kannan
    TI__Guru 58571 points

    Hi

    If the initial input voltage is higher than 5V and you want the 5V output at 3.6V input aftert start-up, you can connect the 5V output directly to VCCX pin.

    In this case, the 5V output will supply LM25118 after start-up when the input voltage is below 5V.

    Regards,