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LM3492HC Rfb1

Dick Veltman
Prodigy 75 points
Other Parts Discussed in Thread: LM3492HC, LM3492

For a LED backlight design a LM3492HC is used to drive 2 LED strings with 17 LEDs per string. The nominal LED current is 100 mA and the LED forward voltage is 2.91V at 100 mA. The input voltage for the circuit is 24V. When  using WEBENCH for making the design it advises to use a resistor of 309K for resistor Rfb1.
After building the circuit with the suggested components it was concluded that the output voltage was to low for passing 100 mA through the LED strings.
After changing Rfb1 to a higher value (390k) the ciruit worked fine.

Note : For the coil a value of 100 uH was used instead of 56 uH

Qestion 1
Can you give me a reason why the circuit with Rfb1 = 309K is not working properly ?


Question 2
It seems that here is a significant difference for using the LM3492 and the LM3492HC
For the LM3492 a value of 402K for resistor Rfb1 is advised by WEBENCH, while for the
LM3492HC a value of 309K is advised. What is the reason of this difference ?

  • 0
    TI__Mastermind 25500 points
    Webench models can have errors, you should always double check against the datasheet equations. The 309k it chose in error would give you a voltage too high on the FB pin so it would cycle in and out of over-voltage and you would not get full current. It looks like the standard LM3492 circuit is giving you the correct value and the other is not for some reason.