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LM2623: LM2623AMM/NOPB Vin 4.5V to 9V

Chiara Belfanti
Prodigy 40 points
Part Number: LM2623
Other Parts Discussed in Thread: TLV61047

Tool/software:

Hi TI,

sorry to bother you, it seems I cannot use your device properly.

I would like to obtain a 9V from a Vin that has a min of 4.1V and a max of 4.9V (as output of BQ24079QWRGTTQ1).
The components are the same one of the datasheet, aside from the resistors Rf1 and Rf2 that are respectively 316KOhm and 50KOhm.

I see an output of 7.8V (very stable) and the FB pin drop to a 1V voltage.
I tried changing the capacitor on the frequency pin and to change resistors but the output stay at 7.8V.

I noticed that I reach a 1.23V voltage on the FB pin only if I turn up the Vin to 7V (sometimes even to 8, depending on the resistors ratio).

Any idea on what's happening? Have I reached a current cap?

Thank you for your advices!

Chiara

  • 0
    TI__Guru 65390 points

    Hello Chiara,

    Please try doubling C24. It is possible that you trip the duty cycle or inductor current limit with this low value. See this part of the datasheet: www.ti.com/.../datasheet

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 40 points

    Hello!

    Thank you for your reply.

    Unluckly, I tried using 10pF, a 47pF a 100pF and then a 220pF as C24 capacitor (as TI talked about the difficulty in choosing the right mesure) but the results are always the same.

    I saw a little improvement by removing C8, something I found odd.

    To be more accurate, I leave here some results, if they have any meaning for you:

    • With C8 as written, C24 70pF, V pin FB = 0.940V and Vout=7.77V
    • Without C8, C24 70pF, V pin FB =0.984V and Vout=7.9V
    • With C8 as written, C24 220pF, V pin FB =0.9V and Vout=7.76V

    Thank you for your help

    Chiara

  • 0 in reply to Chiara Belfanti
    TI__Genius 17500 points

    Hi Chiara,

    Is there a load at output when you test? If the load is heavy, the Vout cannot reach the target.

  • 0 in reply to Shipeng Cheng
    Prodigy 40 points

    Hi!

    No, I have no load attached.

    I tried simulating a load with a resistor to see if something changed... But nothing happened.
    I'll try to test it today, on an old buy where it worked before. Right now I can only think of a problem with the pcb itself.
    I'll let you know if I see some results but I'll take any advice you have.

  • 0 in reply to Chiara Belfanti
    TI__Genius 17500 points

    Hi Chiara,

    Do you try C24 as 4.7pF or 3.3pF? And you can try R16+R31+R35 larger to get lower frequency.

    Besides, I recommend our new product TLV61047, which is easy to use.