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LDO Architecture question

Jeff zhang
Prodigy 150 points

Hi, my question is which architecture has higher power efficiency? and why?

Assuming I can not use switching/charging pump supply. 5V supply has 100mA load current. 3.3V has 350mA load current.

solution 1:

12Vin -> LDO 5Vout -> LDO 3.3V

solution 2:

12Vin -> LDO 5Vout

12Vin -> LDO 3.3Vout

Thanks,

  • 0
    Intellectual 515 points
    Hello Jeff,

    In terms of efficiency, there is no difference between the two LDO solutions.

    Power needed from the 12V supply = (12V x (100mA + 350mA)) = 5.4W
    Total power needed by the two loads = (5V x 100mA) + (3.3V x 350mA) = (0.5W + 1.155W) = 1.655W
    Total power lost in the LDO's = (5.4W - 1.655W) = 3.745W

    That's about 30.6% power efficiency (1.655W / 5.4W)

    Total LDO power dissipation (i.e. power loss) of the two solutions is as follows:
    Pd solution #1 : ((12V- 5V) x ((100mA + 350mA)) + ((5V - 3.3V) x 350mA) = (3.150W + 0.595W) = 3.745W
    Pd solution #2 : ((12V - 5V) x 100mA) + ((12V - 3.3V) x 350mA) = (0.700W + 3.045W) = 3.745W

    For either solution consideration should be given to where the power is being dissipated, and the thermal resistance of the package used. In either solution one of the LDOs will be dissipating over 3W, so that particular LDO package will need a low thermal resistance to ensure that the maximum operating junction temperature in not exceeded.
  • 0 in reply to Daddio
    Prodigy 150 points

    Thanks for reply.

    this is what I thought! no differences.

    someone told me the solution 2 will have better efficiency. So I just want to check here if I missed something.

    Thanks!