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TPS566247: Webench asks for large duty cycle support

Sebastian Ciesluk80
Expert 1121 points
Part Number: TPS566247

Tool/software:

Hello , I f I creat ea new design on the above part specifying: Vin 4.5-5.5V, U out =3.3V Iout=2.5A, Ta =70C once teh WEBENCH opens design it pops the following info:

"The TPS566247 can support the large duty operation, please contact e2e.ti.com/.../196 forum for application help."

The duty cyclke of 3.3/4.5=0.733 does not look like extremally large, the data sheet mentions Large Duty Operation
 by dropping switching frequency but for Vin/Vout<1.6 and I am having 4.5/3.3=1.51. Though it says dropping of switching frequency happends in order to extend Ton ( probably above minimum  of 50ns sp[ecified in teh dat asheet) So at 600kHz  we talking about times of 1.17us and 426ns far away from Toff, Ton minimum specified in the data sheet.

1) will switching frequency change in my design?

2) what is the reason for it ?

3) why I am getting the WEBENCH info related to large duty cycle

4) Is there anything else I shoudl be awaro of related to this info what can affect operation of this controller?

Kind Regards
Sebastian

  • 0
    TI__Mastermind 26770 points

    Hi Sebastian,

    Yes based on datasheet, Vin/Vout = 4.5/3.3 = 1.363 is lesser than 1.6 so the larger duty cycle operation should kick in and reduce the frequency in order to improve transient performance. So you may see a lower steady-state frequency than 600kHz in this condition. Given your input range is limited to below 6V, there may be other devices that may work here without changing the steady-state frequency. If you are interested in finding some other parts like this, let me know and I can loop in the right contacts.

    Thanks

    Amod

  • 0 in reply to Amod Vaze
    Expert 1121 points

    Hi Amod, thank you for prompt answer. Our plan is to use this device in 3 places

    4.5-5.5V-3.3V 2A (3.5A spikes)

    11.5V-16V->3.3V 2A (3.5A spikes)

    11.5V -16V->5V 2A (4.5A spikes)

    so Vin is 4.5-16V,

    at lowest possible cost hence the device choice, are you able to propose something instead ? If we stay with the current choice as long are there any caveats which would surprise up later? If I calculated correctly the frequency would vary only marginally (600k-560k) assuming linear scaling between 0.98 and 0.625% duty. Not sure what are teh negative consequences of using variable frequency ( sounds almost as spread spectrum) . Of course transient reaction will be slower but this is acceptable in our design.

  • 0 in reply to Sebastian Ciesluk80
    TI__Mastermind 26770 points

    Hi Sebastian,

    Got it. It is only steady state frequency that will be lowered and not by much as you said correctly. Other than that, there should not be any change in the operation of this device so you can continue with this device. If you want, you can always order an EVM and test your use case prior to make sure it meets your requirements for the 4.5V input case.

    Thanks,

    Amod

  • 0 in reply to Amod Vaze
    Expert 1121 points

    Thanks, I already have some evns on order as we are planning rather big production on these regulators.

    One thing which still makes me uneasy is what I might be missing , as it is unusual for Webench to warn about - correctly working circuit . Why such warning / info exists and points designers to forum, what shall be the outcome of discussing large duty cycle operation ?

  • +1 in reply to Sebastian Ciesluk80
    TI__Mastermind 26770 points

    Hi Sebastian,

    WEBENCH is pointing to the large duty cycle operation correctly so it shows a message pop-up. This will just result in a lowered steady-state frequency from the normal 600kHz expected value. Other than that, there should not be any changes in the normal operation. It is good to know that you have evms on hand and can test the use case to see if behavior is acceptable for your requirements.

    Thanks,

    Amod