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LM3478: Rout specification in the gm amlifier

Chris Chang1
Prodigy 30 points
Part Number: LM3478
Other Parts Discussed in Thread: LM5155

I am following AN-1286 (SNVA067C) simulating the system of a LM3478. In page 4 Aea=gm*Rout. And it says the values of gm and Rout can be found from the datasheet of LM3478. However, I  can only find the gm value of 800 umhos in the datasheet. There is no mentioning of Rout throughout the datasheet. Can you confirm the value of Rout is 50 Ohms as described in AN-1286? What is the variation?

Chris

  • 0
    TI__Guru* 78400 points

    Hi Chris,

    First, thank you for considering the LM3478.  The Rout is 50k.  It can be obtained by referring to both gm in the ec-table and Figure 13.  The open loop dc gain of the error amp is basically Rout x gm = 50k x 800u = 40, or 32dB as shown in Figure 13.  Unfortunately the variation of Rout is not characterized.  Use 50k for the loop design.

    Best Regards,

    Youhao Xi, Applications Engineering.

  • 0 in reply to Youhao Xi
    Prodigy 30 points

    Dear Youhao,

      Thanks for the clarification. I also found a couple of errors in SNVA067C:

    - equation (29)  on page 7 the unit for wp2 should be rad/sec. 31Hz is correct but the unit is wrong.

    -  equation  (30) on page 8. I believe it's for wz3 instead of wz2. Also the unit should be rad/sec as above.

    Chris Chang

  • 0 in reply to Chris Chang1
    TI__Guru* 78400 points

    Hi Chris,

    Thank you for the comments.  Agree with you on your findings. This is an old documents (more than 15 years ago) and seemingly the 2013 updates did not catch these.  We will see how to correct this document.

    Best Regards,

    Youhao Xi

  • 0 in reply to Youhao Xi
    Prodigy 30 points

    Dear Youhao,

      I am working on a booster design with an input voltage of 28V and an output voltage of 200V~250V and a maximum output current of 0.3A. The WEBENCH Power Designer can not generate a design based on the input parameters. Is there a reason why? Does it has a limit on the maximum duty cycle? Does it mean it's impossible to come up with a design that meet the spec?

    Chris

  • 0 in reply to Chris Chang1
    TI__Guru* 78400 points

    Hi Chris,

    The Webench is a very good tool but it also has limitations. The Webench model references to an EVM, and your operating range exceeds the referenced circuit.  

    I tried Webench with the Lm5155, and it can generate a design. Please consider the LM5155.

    Thanks,

    Youhao