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LM3409HV / setting the operation point

Guru 20090 points
Other Parts Discussed in Thread: LM3409HV, LM3409, TPS92661-Q1


Please let me know about LM3409HV and TPS92661.

(1) Roff and Coff setting
Could you please let me know the setting method of Coff, Roff for wide Vout range ?
Should we calculate the Coff and Roff by maximum output voltage?
Conditions are the following.
Vout:about 2V ~ 42V
Iout: 1.1A

(2) LM3409 schematics in the TPS92661 user's guide
There is the LM3409 schematics In the TPA92661-Q1EVM user's guide page 17.
At this schematics , there is the 5V supply for COFF pin.
Could you please let me know the role of this 5V supply ?
Should we add this 5V supply for above consitions ?

Best Regards,
Ryuji Asaka

  • Ryuji:
    You can choose Roff 24.9K and Coff 470pF. At 42V out, the calculated Coff = 0.365uS, Fsw = 525KHz. At 2V out , the calculated Coff = 11.8uS, Fsw = 88KHz, Ton = 0.436uS which is > min on time.
    The 5V supply on the TPS92661 EVM is for LED short operation. With this 5V, the buck converter will operate with Toff less than the maximum off time even with the output short. In your appliction, the minimum Vout = 2V, you don't need this 5V unless you want the buck converter to switch at higher frequency.
    David Zhang
  • Hello David san,

    Thank you for the reply.
    Could you please let me know the efficiency in your calculation?
    At 42Vout, I think that you used 100% efficiency for your calculation.
    However , I couldn't understand the efficiency at 2V out.

    The minimum Vout=2V is at the condition of All LED off.
    The 2V is calculated by the Ron=1.8ohm of TPS92661-Q1 and Iled=1A.
    Should we supply this 5V for Coff pin in this case?

    Best Regards,
    Ryuji Asaka


  • Ryuji:


       I used the equation on page 10 of the data sheet to calculate Toff and use T=Toff/(1-D), D = Vout/Vin to calculate the period T. I did't use the equation with power efficiency involoved.


    David Zhang

  • Hello David san,

    I think that Duty is caluculated by "1-(Vo/η x Vin)" from the equation(12) on page 13 of the datasheet.
    Thus, we consider efficiency for calculating ton and fsw.
    In the design example on the page 23~27 of the datasheet, efficiency is used for calculation. 
    Since efficiency is varied on Vout However Vout on the design example is fixed value, we need the efficiency.

    I have additional questions.

    (1)Roff and Coff setting in Webench
    I couldn't select  the Fsw=525kHz in the webench of LM3409HV.
    Also I couldn't simulate the LM3409HV same external components for both of Vout=2V and Vout=42V.
    Could you please let me know this reason?

    (2) TPS92661-Q1 EVM
    I think that the operation at the Vout less than 5V is the following.

    Vout = Iout*Rallon of TPS92661-Q1
    Iout=1.1A (setting by Rsns)
    Fsw is not fixed.

    Thus, Vout and Iout are regulated to fixed value however , Fsw is not fixed.
    Is my understanding correct ?

    (3) R77,78 on the TPS92661-Q1 EVM
    Could you please let me know the calculation method of R77,78 ?

    (4) LED Driver requirement for TPS92661-Q1
    Are there any guideline of LED Driver requirement for TPS92661-Q1 ?

    Best Regards,
    Ryuji Asaka

  • Ryuji:
    You can use 90% as the efficiency to calculate the switching frequency. The equation gives you the estimated switching frequency. Unlike Toff, it is not as accurate. This equation assumes high power efficiency. If you get low power efficiency, you cannot use this equation to calculate the switching frequency.
    Webench generates designs for you. So the 2V design may be different than the 42V design. It won't generate the same design for both 2V and 42V. That is the limit for webench.
    For TPS92661 EVM, the switching frequency is not fixed. It switches at higher frequency with LED short.
    R77 and R78 are Roff resistors. They determine the off time of the converter. Toff = time to charge C46 to 1.24V. You pick the resistor values and calculate the off time.
    There is no speical LED driver requirement for this IC.David Zhang
  • Hello David san,

    For low power efficiency, should I use 90% as the efficiency too ? ( in webench,  efficiency is about 58% at the Vout=3.2V.)

    I understood that the switching frequency is not fixed at less than 5V output.

    The LED driver should be stabled at the wide Vout range for 1LED to 12LED. And generally dcdc converter output voltage is almost fixed.
    Thus, I think that there is the requirement for LED driver. 

    Best Regards,
    Ryuji Asaka

  • Ryuji:


       Yes, you can use 90% efficiency to estimate the switching frequency if you don't know the power efficiency. If you know the power efficiency, plug in the actual efficiency in that equation. The LED driver requirement is for the application not for the IC. There are different requirements for different applications. TPS92661 is designed for automotive head light application. If your application is automotive head light, then the LED driver has to meet all the automotive requirements.



    David Zhang