• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 148 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2776: Output ripple calculation

Kei Kuwahara
Expert 5550 points
Part Number: LM2776

Hello team,

The output ripple measurement graph (Fig 1) is very different from the output ripple calculation equation (equation 4) on the datasheet.

The shape of the graph on Figure 1 is not linear. What is the condition of the circuit used in this measurement?

There seem to be several drop points Do you change the Cout value during this whole measurement (Iout: 0.0001 to 1A)?

I have substituded the below parameters to the equation 4 of the datasheet.

COUT = 2.4 uF @5V, ESRCOUT­­= 3.5 mΩ, fSW = 2 MHz

The Vripple vs IOUT graph drawn is below.

This graph is very different from the shape of the Figure 1 shown below.

Best Regards,

Kei Kuwahara

  • 0
    TI__Prodigy 550 points

    Hello Kei, 

    Thank you for reaching out. I will check and give you feedback tomorrow.

    Regards, 
    Natalia

  • +1 in reply to Natalia Usuga Salazar
    TI__Mastermind 21150 points

    Hello Kei, 

    LM2776 is a very old part and most likely an inherited one. Unfortunately, not much validation or characterization data is available to cross-check the test conditions for the d/s plots. We should rely on the d/s and it says the typical circuit was used for all measurements unless specified. Which means a constant Cout could have been used.

    However, what I think is relevant is the fact that the equation for ripple calculation is a generic one for any Charge Pump inverter. It does not take into account any parasitics on the PCB, effective Cout at any given condition or even the total output impedance on the board for eg. Depending on the load the fsw could also vary in the range specified in the d/s from sample to sample.

    So, in my opinion, the formula is only to estimate the total Cout that may be required to achieve a particular ripple performance. But on a real PCB, there could be variations.

    I would be curious to understand the background of your question. Do we have any customer issue?

    Thanks,

    Febin

  • 0 in reply to Febin
    TI__Expert 5550 points

    Febin-san,

    Thank you for your answer!

    The customer is going to use this part, and trying to calculate Vripple for their design.

    Best Regards,

    Kei Kuwahara

  • 0 in reply to Kei Kuwahara
    TI__Mastermind 21150 points

    Hello Kei-san, 

    Could you share more details of the customer & project?

    If required, please send me an email.

    Regards,

    Febin