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LM2671: Design help and clarifications

ArTzy
Genius 14949 points
Part Number: LM2671
Other Parts Discussed in Thread: LM2675

Hi Experts,

Good day. Seeking your assistance on these clarifications with buck converters:

We are having availability issues with the LM2671-5.0/NOPB but the design (pcb layout) also allows for the feedback resistors for the LM2671-ADJ/NOPB part. Unfortunately, we are also experiencing availability issues with this. Questions are:

1) Is it possible to use LM2671-3.3/NOPB with a 3K6 feedback resistor to give 5.0V ?

2) Is it possible to use LM2675-5.0/NOPB without changing the inductor (we still only need 500mA output) ?

2a) Is it possible to use LM2675-3.3/NOPB with feedback resistor ? (as 1)

3) Is it possible to use LM2675-ADJ/NOPB ( comment as for (2) ) ?

My concerns with using a feedback resistor are that the data sheet  may not be telling the whole story in respect of FB pin
impedance and availability on fixed voltage devices, and possible impact on stability. We can tolerate increased Vout up to 5.5V.

For your assistance please.

Regards,
Archie A.

  • +1
    TI__Guru 59555 points

    Hello

    You are correct about using the fixed 3.3V for a 5Vout with an external resistor; this is not the best option.

    The absolute resistance of the internal voltage divider can vary as much as +/- 30%.  So, your external

    resistor will never match well with the internal resistors.  If you can tolerate a large variation in output 

    voltage, both high and low, then maybe you could make that work for you.

    However, I believe that either the fixed 5Vout or ADJ option are your best plans.

    You can use the LM2675 device.  It is best if your inductor is rated for the larger current

    of that device, even though you only need 0.5A.  Can you give me the part number of your inductor ??

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Guru 59555 points

    Hello

    I will close this post due to inactivity.

    You can create a new post if you have further questions.

    Thanks