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LM2678: LM2678 adjustable alternatives

Julien Galtié
Prodigy 50 points
Part Number: LM2678
Other Parts Discussed in Thread: LM2679, LM22679, LM22678, LM2677, LM22677,

Dear support,

On my adjustable output voltage application I currently use the LM2678SX-ADJ reference but I would need some alternative references.

Can the LM2677 LM22677 LM22678 LM22679 and LM2679 references be potential 100% replacable solution? 

My main question is regarding their options on pin 5. I would like to keep them Non Connected and to remain with the same LM2678 behavior and control.

I am also open to any suggestions, preferable pin to pin compatibles or with same or more 7A current limit and 0 to 26VDC adjustable output.

Thank you in advance for answer.

Best Regards,

Julien

  • +1
    TI__Mastermind 22299 points

    Hi Julien,

    The LM22678 looks like the same TO-263 package and should be pin to pin compatible with LM2678.

    For LM2679 I think you would need to have the RADJ resistor connected to the pin 5. 

    However for LM22679 the datasheet says its OK to float IADJ pin for the default setting.

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 50 points

    Hello Orlando,

    Thank you for your fast reply. I agree that all the mentioned references are pin to pin compatible with LM2678.

    I was questionning if there is some internal parameters that I have to take in consideration and which could affect my application with a different behavior.

    On my application, pin 5 is floating and pin 7 is connected to microcontroller for on/off function. Also LM2267x has different internal frequency and feedback voltage.

    Best Regards,

    Julien

  • 0 in reply to Julien Galtié
    TI__Mastermind 22299 points

    Hi Julien,

    Correct, the FB voltages are slightly different, 1.285V for LM22679 and 1.21V for LM2679.

    This is about 6% difference, so your output voltage will change by 6% or you will have to adjust the FB resistors.

    The switching frequency is significantly different, 500kHz (LM22679) vs 260kHz (LM2679); 

    This means the optimal inductance is roughly 2x smaller for LM22679.

    However the topologies are different, so it's best to calculate optimal inductance based on datasheet recommendations, equation 11 in LM22679 datasheet and figure 17 in LM2679 datasheet.

    The softstart function should be similar, so you should be able to leave that on the microcontroller.

    Hope this helps,

    -Orlando