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LM25085 COT Frequency Independence

Richard
Prodigy 70 points
Other Parts Discussed in Thread: LM25085, LM5000, LM2678, TPS5450

Is there a way to make the frequency of the LM25085 (or other COT based converters) frequency independent of the output voltage?  I have a design that needs a variable output and when Rt is tied to VIN it causes a significant shift in frequency.  This is for a standard buck converter design.

Thank you!

  • 0
    TI__Genius 11490 points

    The COTs in LM5000 series are made so that the switching frequency is independent of VIN. This results in a nearly fixed frequency in CCM for a given VOUT. Also since COTs don't need compensation it is very easy to make VOUT vary dynamically by injecting at the FB node without worrying about stability. Unfortunately though there is no easy way of making the switching frequency indepedent of VOUT.

    Regards,

    Vijay

  • 0 in reply to Vijay86929
    Prodigy 70 points

    Thank Vijay.  The lack of compensation and variance of voltage is exactly what them LM5000 series so perfect in this scenario.  Am I overlooking another part or do you have another recommendation for a part that is both easy to compensate and at least semi-fixed frequency?

  • 0 in reply to Richard
    TI__Genius 11490 points

    It is possible to change VOUT dynamically by varying Soft-start/TRK pin of ICs. The technique works for both PWM and for Constant On Time converters. But I will need to know your VIN and IOUT range to suggest an IC.

  • 0 in reply to Vijay86929
    Prodigy 70 points

    Thanks again for your help.  The design need is for two different rails both requiring 3A but potentially peaking to a max of 5A.  Voltages ranges for them are 1.7V-5.6V and 8V-14V.   Supply rail is 18V.  Ripple needs to be < 40mv at all voltages.  

    I initially prototyped a couple of variable NatSemi PWM converters that I've used before but had difficulty in compensating them over the variable voltages and did not have great transient response.

  • 0 in reply to Richard
    TI__Genius 11490 points

    Hello Richard,

    How fast are you changing the VOUT compared to the bandwidth of the circuit?

  • 0 in reply to Vijay86929
    Prodigy 70 points

    VERY infrequent.  No more then once a second.  

  • 0 in reply to Richard
    TI__Genius 11490 points

    LM25085 still seems like a good way to go at those current levels to avoid compensation trouble. However as you are changing VOUT very infrequently, it should be possible to go with PWM converters, e.g., LM2678 or TPS5450. Both of them have internal compensation and integrated FET. You may have to test them in your circuit conditions to see if the response satisfies your needs.

    If you need further help with LM25085 continue this post. If you need help with LM2678 or TPS5450 please use the corresponding part number in a new tag.

    Thanks and regards,

    Vijay

  • 0 in reply to Vijay86929
    Prodigy 70 points

    Thanks.  I've used the LM2678 before and it's much harder to get ripple down very low with it.  The couple of COT units I've used for other designs have worked very well but I'm concerned about the widely varying frequency that the variable output will result in with the 25085.