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Currentsharing between DCDC-converter and external 3.3V supply

Niels Koll
Expert 1950 points

Hello,


I have a card that is supplied with 3.3V @ 1A and 5V @ 4A. Unfortunately, I need more than 1A of 3.3V in my new design.

I am not able to simply generate the full 3.3V on-board, as I need 2A of the 5V rail for other purposes.

Do you have any idea how I could implement a 5V-to-3.3V switcher that boosts the 3.3V rail to 4A?

Regards,
Niels

  • 0
    TI__Guru**** 191445 points
    Assuming 90% conversion efficiency, 3.3 V @ 4 A will require about 2.93 A from your 5 V supply. If you were to split your 3.3 V rail into two independent supplies, 3.3 V @ 1 A plus 3.3 V @ 3 A with the 1 A coming from your input ad the 3 A generated from 5 V, then you would still require 2.2 A additional from your 5 V line.

    Bottom line: You only have 3.3 W + 20 W = 23.3 W input power and require 23.2 W on your board. You would need > 99% conversion efficiency. You will need to reduce your load requirement.
  • 0 in reply to JohnTucker
    Expert 1950 points

    John,

    you are right. I am very close to the limit. But the values are all "worst case estimations", so I think I can reduce then a bit during development.

    Yes, splitting my 3.3V devices an supply them by two different 3.3V voltage rails is an option.

    Do you see any chance to realize it with one 3.3V rail?

  • 0 in reply to Niels Koll
    TI__Guru**** 191445 points
    Per my earlier calculation, you would need to reduce your 5 V current demand from 2 A down to 4 - 2.93 = 1.07 A. Then you would have the current required to get 3.3 V @ 4 A. Alternately, the more you reduce the 33 V load current, the more available 5 V current you will have. You will just have to balance the numbers.