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LM7800: Effeciency calculation

Mounir Ben ali
Prodigy 10 points

Part Number: LM7800

I used the "LM7805MP_NOPB" LDO in one of my projects, it did work and now I im preparing the power tree, the total power consumption is 5V/50mA and the input voltage is 9V,when i searched for the efficiency formula in this datasheet(https://www.ti.com/lit/ds/symlink/lm340.pdf) i didn't find it.
did i miss something? 

Best regards,

  • 0
    TI__Expert 5200 points

    Hi Mounir, 

    Not every datasheet has an efficiency formula written in it. 

    The efficiency formula is going to be 

    1 - (Output_Power / Input_Power) = Efficiency 

    Output Power = VOUT * IOUT

    Input Power = VIN * IIN 

    Regards, 

    Eduardo Linares

  • 0 in reply to Eduardo Linares
    Prodigy 10 points

    Hi Eduardo,
    The ideal case is knowing the efficiency , Vout, Iout, Vin and the only missing variable Iin, but in my case, the efficiency and Iin are missing, so the calculation is not doable with that formula.
    Is there an other way to know the Efficiency,i have the board and the required equipments to measure the input voltage,input current,output voltage,output current, then calculate the Efficiency,then use the Efficiency i got for the formula you sent. 
    If im correct that would work right?
    Else do you have any other data that would skip all the hassle.
    Regards,
    Mounir Ben Ali

  • 0
    TI__Expert 5200 points

    Hi Mournir,

    if the ground current is negligible relative to the output current, you can replace approximate it with the output current. if not, we can approximate the input current by adding the output current to the expected ground current. 

    Based on the graph in figure 12, it looks like you should be hovering around 5mA, so you can approximate the input current as 55mA.

    You should be at around 49.5% efficiency. 

    Regards, 

    Eduardo Linares

  • 0 in reply to Eduardo Linares
    Prodigy 10 points

    Hi Eduardo,

    With these values:Iin=55mA,Vin=9V,Iout=50mA and Vout=5V,and using the formula you sent:

    1 - (Output_Power / Input_Power) = Efficiency 

    Output Power = VOUT * IOUT

    Input Power = VIN * IIN 

    Eff = 38.8888%

    So based on what i understood,my calculation are correct but the efficiency is far from 49.5%.

    Regards,
    Mounir Ben Ali




  • +1 in reply to Mounir Ben ali
    TI__Expert 5200 points

    Mounir,

    Can you post your math? This is what I calculated:

    input power = 9V * 55mA = 495 mW

    output power = 5V * 50mA = 250 mW

    250/495= 0.505

    1-0.505=0.495

    Regards, 

    Eduardo Linares

      

  • +1 in reply to Eduardo Linares
    Prodigy 10 points

    Hi Eduardo,

    I did the calculation again and your result is correct, there is a mistake somewhere that I didn't notice.
    Looks clear now, thank you for your support.

    Regards,

    Mounir Ben Ali