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Original question:

LM5157: Non-isolated buck converters up to 60W

LM61460: Alternative to LT8643S

Arijav
Prodigy 125 points
Part Number: LM61460
Other Parts Discussed in Thread: LMR51450, LM62460

Tool/software:

Hello again,

I would like to revisit this topic. In the end for the first iteration of my design I had to go with the LT8643S, as I did not find any way to properly control the output voltage of the LM61460 via a PWM. The FB pin should not be used for that matter according to other posts and in any case in a negative buck setup it would increase highly the design complexity. In the LT8643S I can adjust the output voltage thanks to the TR/SS pin via a PWM. I would like however, if possible, to move to a TI IC, as the LT parts are very expensive. Is there any equivalent solution to the LT8643S in TI which allows a positive or negative output buck design with PWM controlled output voltage?

Thanks,

Javier

  • 0
    TI__Expert 5220 points

    Hi Javier,

    Just so I understand properly, you'd want to do configure a single device that could output either a positive or negative voltage and be changed "on the fly" via a PWM signal?

    Could you also give me your  power requirements (VIN, VOUT, IOUT, FSW) so I can get a better idea of what you are trying to do?

    Thanks,

    Dino

  • 0 in reply to Dino Maslic
    Prodigy 125 points

    Hi Dino,

    sorry, I realize I should have posted more background information from the original post. The setup is as follows. I have an isolated DCDC and at its secondary side output (+25V) I have 3 Buck Converter (3x LT8643S). One of them provides out of the +25V rail a positive voltage ranging from around 0V to +24V and the other two provide negative voltages ranging from around 0V to -24V each. The ground is a common one for the whole secondary side.

    To sum-up the requirements:

    1- VIN = +25V.

    2- Vout = 0 to +25V for the first Buck Converter. The control of the output voltage shall be PWM based.

    3- Vout = 0 to -25V for the second and third Buck Converter independently. The control of each output voltage shall be PWM based. A different interface (f.i. I2C) is something I could consider in a future iteration, so if there are parts that fit and have another kind of digital interface do not hesitate to proposed them too.

    4- Iout = 6A max. FSW is currently 1MHz with the LT8643S. What I need to comply with is the voltage ripple requirement in point Nr. 6.

    5- I am able to go down to 0V outputs with the LT8643S. I could get into some compromises (f.i. a minimum of 0.8V) if this brings a price advantage. It would be good however to get an option which can really go down (or up) to 0V.

    6- With the LT8643S at simulation level (the boards need to arrive) I get very clean signals (much better than with the LT8640S which I used originally). I would like to keep with an output capacitance of 50uF a voltage ripple of not more than 15 to 20mVpp in the whole voltage range.

    Thanks,

    Javier

  • 0 in reply to Arijav
    TI__Expert 5220 points

    Hi Javier,

    Here are some devices that should meet your requirements. 

    LMR51450: 36 VIN, 5 A IOUT (0.8V min)

    LM62460: 36 VIN, 6 A IOUT (1V min)

    Thanks,

    Dino

  • 0 in reply to Dino Maslic
    Prodigy 125 points

    Hi Dino,

    thanks. If I understand it correctly, in both cases I would have to use the FB pin directly for setting the variable output voltage, correct?:

    Example from "Methods of output-voltage adjustment for DC/DC converters":

    Regards,

    Javier

  • +1 in reply to Arijav
    TI__Expert 5220 points

    Javier,

    Correct, this is a very simple feedback network to use to alter the VOUT voltage. My personal favorite method due to its simplicity.

    Thanks,

    DIno