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LMS3655: efficiency ask

Alan Hong
Prodigy 50 points
Part Number: LMS3655

when use webbench to run simulaton,

24V to 5V , I see under 0.5A , like as 10mA , 20mA , I can see the very high efficiency performance , reach 90% above alomost.

But , when change design to 24V to 12V  

I see the efficiency just remain about 50~60% at under 0.5A.

So , what does the main matter about the performance decrease so much ?

High side mos Ciss and Rdson ?  Or LMS3655S control scheme can not reach duty=50% and keep high effieiency?

  • 0
    TI__Mastermind 22299 points

    Hello Alan,

    I ran both Webench simulations and I don't see such a low efficiency at 0.5A between the two designs? Can you please share them with me?


    Regarding the losses at low Iout, they are highly dependent on the switching losses in the FETs and also the AC losses in the inductor (Core Losses). Seeing as the FETs are internal and the same in each case, the only variable in each is the core losses in the inductor. Even though at an Iout of 0.5A they are low relatively speaking in each case 0.12W for 12V out and 0.08W for 5V respectively; this as a percentage of Pout out when the current is low will be significant in terms of efficiency. I believe this is the reason for your large discrepancies in each case above. You do have the option to select a different inductor for the 12V out.
    Hope this helps?

    Orlando.

  • 0
    TI__Mastermind 22299 points
    Alan,

    The LMS3655 'BIAS' pin drives the internal LDO of the chip. This LDO powers much of the internal control circuits and in the datasheet is recommended to be powered off of the output.

    At 12V output, the internal LDO must regulate 12V down to internal logic level of around 3.3V, which is less efficient than regulating 5V down to 3.3V. This is why the efficiencies for 12V are lower than 5V for the range of current you are looking at. The IC losses are more visible at lower power.

    I believe this causes the issue of low efficiencies at lower current in addition to the AC inductor core losses described before.

    If you have a 3.3V rail available in your design, try connecting that to BIAS.
  • 0 in reply to Orlando Murray
    Prodigy 50 points
    So, conclution your response ,

    1. Choke DCR reduce and choke value how do your recoomand for u value ? we just can use SMT moding type ,can not use wiring circle DIP
    type.

    2. BIAS pin do not connect to output to avoid 12V to 3.3V LDO conversion, since that 5V to 3.3V will get more higher performance.

    Above 1, and 2, could you use TI webbech to run a simulation , to let me how improve degree , and send pdf result for me.
    Thanks.
  • 0 in reply to Alan Hong
    Prodigy 50 points
    So, conclution your response ,

    1. Choke DCR reduce and choke value how do your recoomand for u value ? we just can use SMT moding type ,can not use wiring circle DIP
    type.

    2. BIAS pin do not connect to output to avoid 12V to 3.3V LDO conversion, since that 5V to 3.3V will get more higher performance.

    3. If we have another 3.3V , how to connect the pin for BIAS and RESET pin , could you give me a refernce circuit to pass the LDO conversion ?

    Above 1~3, could you use TI webbech to run a simulation or give me a refernce scheme , to let me how much for improve degree at
    extreme light loading, and send pdf simulation file result for me.


    Email: alan.hong@quantatw.com


    Thanks.
  • 0 in reply to Alan Hong
    TI__Mastermind 22299 points
    Alan,
    Sorry for the delay, here are my responses:

    1) To minimize AC core losses use a higher inductance to get a lower ripple current.
    If you plan on using 12V out I suggest 22-47uH inductor.
    At 5V out the datasheet recommends 6.5uH -20uH.

    2)That is correct.

    3) The bias can be directly connected to any 3.3V rail. The datasheet suggests a 3Ω resistor Rbias.
    The /RESET pin is a power-good indicator. If the output is too high or too low, /RESET goes LOW. When the output is regulated correctly, /RESET is floating, and with a pullup resistor, can be pulled HIGH to any voltage less than 8V.
  • 0 in reply to Orlando Murray
    TI__Expert 6165 points
    Hello Alan,

    I also work with the LMS3655. We do not expect low efficiency in light load when converting 24 V to 12 V. Could you share your Webench projects with me as well? If you would like data taken in our lab, please let me know.

    Regards, Robert (Applications engineer at TI)
  • 0 in reply to Robert Blattner
    TI__Expert 6165 points
    Hello Alain,

    It appears that you have answered your question since there has been no reply for nearly a month. If you would like more information, please contact me directly or open a new thread.

    Regards, Robert