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TPS40060 100% duty cycle

Jurijs Varnels1
Prodigy 40 points
Other Parts Discussed in Thread: TPS40060, LM5085, TPS54360, TPS54260

Hi!

I am developing buck converter, which is able to operate at true 100% duty cycle. The input voltage can change from 10 to 34 Volts, but ouptut voltage is 12V (or equal to input voltage, when below 12V). As result I have chosen TPS40060 IC, because it has P-Fet, which provides mentioned d=100% operation.

At the moment I have made protoype of circuit on PCB, and with disabled UVLO and feed-forward function it's funcitioning OK from 14V to 34V input voltage. When input voltage drops below 14V, output voltage is decreasing. For example: at Vin=12.7, Vout=10.9; at Vin=11.6, Vout=10.0V; at Vin=10.5, Vout = 9.0V; At Vin=10V Vout=8.6V

How is it possible to obtain desired d=100% operation with TPS40060 IC ? (Should I do just readjusting of compensation network components, or there are possible other problems?)

(I already have design, which is properly operating at 100% duty cycle, based on LM5085, but unfortunately for higher power applications it's not suitable because LM5085 is not synchronous)

  • 0
    Prodigy 40 points

    Maybe the design circuit with its components will be also helpful:

  • 0 in reply to Jurijs Varnels1
    TI__Mastermind 35125 points

    Hi Jurijs,

    Although it has a PFET for the high side switch, the TPS40060 does appear to have a maximum duty cycle limitation. Please see below. I expect the actual maximum duty cycle will fall somewhere within this range depending on the characteristics of the supply (such as switching frequency, input to output ratio, inductor, FETs, etc.).

    To double check this is what is happening, can you show a screenshot of the switching node under the max duty cycle condition?

    What is the output current of the design? I may be able to help you find another solution if this will not work.

    EDIT: Picture didn't come through the first time, should be below now.

    Thanks,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 40 points

    Hi Anthony,

    Now I see it - I haven't paid attention to that table in datasheet, because I was sure of 100% maximum d, when I saw this number at product page:

    speaking about switching node waveform at maximum duty cycle:


    The desired output power is 50W. (about 3Amps through freewheeling diode or bottom mosfet, at maximum input voltage)

  • 0 in reply to Jurijs Varnels1
    TI__Mastermind 35125 points

    Hi Jurijs,

    Sorry for the confusion there. I'll get the max duty cycle listed on the product page corrected.

    I'll also let you know some options for your 100% duty cycle application later today.

    Regards,
    Anthony

  • 0 in reply to Anthony F
    TI__Mastermind 35125 points

    Jurijs,

    After taking a deeper look I am unaware of a synchronous option with 100% duty cycle. The LM5085 may end up being the the best fit at this moment.

    Another option to consider is TPS54360 with an external charge pump to provide gate drive for the high side FET. But again this will be non synchronous.

    Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 40 points

    Ok, then I'll use LM5085 - (and I'm thinking now that actually it will be even more efective than synchronous solution, when it'll run at lower input voltages or/and half-load...) . Thanks for help and advices =)

    Btw. I know that there is one method considered in application report SLVA444 - "Providing Continuous Gate Drive Using a Charge Pump" (although it says that "true 100% duty cycle operation cannot beobtained using this method") - do you think it could be a good solution for duty cycle close to 100%  in future, if I'll need more powerful converter?

  • 0 in reply to Jurijs Varnels1
    TI__Mastermind 35125 points

    Hi Jurijs,

    The charge pump method given in SLVA444 is certainly a good option to consider in the future for higher power options for any controller/regulator using a similar BOOT regulator for the high-side gate drive. Note, the method it is referring to when saying "true 100% duty cycle cannot be obtained" is the standard BOOT regulator used in the TPS54260. With this, the high-side FET must turn off at some point to refresh the charge on the BOOT capacitor keeping it from reaching 100% duty cycle. When the charge pump is used the BOOT capacitor remains charged and the high side FET can stay on.

    I have not personally worked with it, but the LM5085 may still be better in the end because you can optimize your FET for the specific application. Any other controller which does not have a minimum off time limitation on the high-side FET can also be considered.

    Regards,
    Anthony