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WEBENCH® Tools/LM3492: LM3492

Yuval Shalev
Prodigy 60 points
Part Number: LM3492

Tool/software: WEBENCH® Design Tools

I am using the LM3492 to drive 2 strings of 9 LEDs per string. the LED Vf is 3.2volts and string voltage is 28.8V

Input power is 12VDC. The current is 20mA per string

The Webbanch gave me  a design with inductor value of 120uH, Rf1= 402K, Rf2=16.2K, Rrt-226K. I made a sanity check based on the formulas of the LM3492 datasheet 

1. I see that the Inductor value is too big. 120uH inductor causes too low switching frequency. Based on the formulas provided by the datasheet (paragraph 9.2.2.2), 12uH looks more logical 

2. if Rf2 is 16.2K, and Rrt= 226K ->Rf1 should be 170K. (paragraph 8.3.8 - calculate output voltage)

Where is my mistake?

  • 0
    TI__Genius 14455 points
    Hi Yuval,
    Can you please share the design to my email g-dhanabalan@ti.com. You can share the design by clicking on the Share Design icon at the top and following the steps after that.

    Regards,
    Gerold
  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    Genius 4910 points

    Hi Yuval,

    Sorry for the delayed reply. Please find the response below.

    Input Condition :

    Vinmin : 12

    Vinmax = 12

    Vout = 28.8

    Iout = 0.04A(0.02A through each series driver)

    Fsw = 611.606kHz

    Regarding Inductor selection :

    The actual Vout calculated for the given condition is  Vout = 28.85

    From the datasheet equation (9.2.2.2 Page No: 21 ),

    Iin =0.04808 (Note that current through single branch is used in calculation)

    Based on Vout ,Vin and fsw here , we can calculate ton = 9.54954E-07 sec and L_tgt = 119.1623 uH.

    To reduce the inductor size either increase the frequency or reduce the number of LED branches to 1.

    Regarding feedback divider :

    As per the above statement from datasheet ,when VFB reaches VFB-OVP, the maximum output voltage of the boost converter VOUT(max) reaches its maximum, which is 65V.

    Rfb2 is being selected based on VOUT(max) and not Vout_User. You can find similar feedback resistor being used in datasheet (9.2 Typical Application. Page No 20) although Vout is 38V(10*3.8)

    Thanks and Regards,

    Atul