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LM317M: LM317 Feedback Resistor size and value

Kunal barot
Intellectual 605 points
Part Number: LM317M

Hi Support

Please help with below query regrading LM317M Design . attached Ref Circuit from datasheet.

Q1 ) How to decide R1 and R2 power rarting, 

Q2) Can 0402 work on R1 and R2.

Q3) Image show R1 value 240 ohm, can we select higher value in kohm.

Thanks

  • 0
    TI__Mastermind 44820 points

    Hi Kunal,

    The first thing to consider is to select R1 and R2 such that they maintain the minimum load current rating for stability which is 10mA. So to try to answer your questions:

    Q1 ) How to decide R1 and R2 power rating,  R1 and R2 should be selected such that the series resistance ensures the minimum load current of 10mA. So if VOUT is 5V, then the sum of the resistors R1 and R2 should be at a minimum of 5/0.01 or 500Ohm. So to meet this requirement R1 could be 120Ohm and R2 360Ohm. 

    Q2) Can 0402 work on R1 and R2. Assuming the 0402 power rating is 1/16W, for the example above yes. If your output voltage is 12V then no. 

    Q3) Image show R1 value 240 ohm, can we select higher value in kohm. *The 240Ohm results in the typical minimum load current required for stability. If you have an always-on load ensuring a total minimum load current fo 10mA then this can work. 

    What is your input and output voltage for your application? From these questions, It looks like you are wanting to design a small solution with lower total power dissipation. We may have a better part. 

    I hope this helps.

  • 0 in reply to JCHK
    Intellectual 605 points

    Hi

    We have input voltage is 14.5V +-10%

    Output voltage is 3.3V , current needed is 150mA.

    Since this is very low cost project , so can not use switcher here, with LDO there will be 11.2V*150= 1.68W. I select DPAK and provide 8x 8 mm copper pad on PCB.

    Thanks

  • 0 in reply to Kunal barot
    TI__Mastermind 44820 points

    Hi Kunal,

    If you want to stick to a large package for the best thermal performance we have the TPS7B81:

    If you want an even smaller solution we also have the TPS7B84:

    Both are available with a fixed 3.3V output and are very low IQ with no minimum load current. 

    Do you think either of these could work for your application?