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LM3407: inductor selection

Phil54870
Prodigy 165 points
Other Parts Discussed in Thread: LM3407

Hello,

   I’m trying to dimension the inductor for the LCD backlight converter LM3407.

My input voltage can vary between 12V and 24V.
The output voltage should be 10V (3 LEDs with *3,3V typ.), max continuous output current is 140mA.
As the LM3407 is advertised with "output < 0.9 * V_in", that should be possible.

TI provides a tool (an excel sheet) to dimension the inductor:
http://www.ti.com/tool/lm3407design-calc

  • It seems the inductor value is limited between 22uH and 47uH. Is that correct? If so: why?

Table 1 of the datasheet gives different inductor values for different input voltages.
For the excel sheet, it's the same. However, I need to have 1 value to cover the entire range of input voltages between 12 and 24V.

  • What inductor value would I need to choose? How do I determine the value?

The LM3407 is basically a buck converter. Using a different tool (still from TI) to determine the inductor value, I get completely different results.
http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slvc092&fileType=zip

Values change from 60uH to >200uH. Other 3rd party tools for buck converters return minimum inductor values around 100uH depending on peak / ripple / saturation current.

  • Why do different tools for the same converter technology return completely different values?

The datasheet doesn't specify any maximum current for the internal switch.

  • Is that parameter irrelevant? Is it automatically limitting the current to a safe range?

I need to specify inductor parameters like inductivity, resistance, peak / saturation current.
The converter should work over the full input voltage range.

  • Is there a simple way to determine these parameters? How...?

Thank you very much!

Phil

  • 0
    TI__Mastermind 25500 points
    I can't really comment on the tools, I do not know who made them. But perhaps the 47uH maximum is to ensure enough switch current ripple to regulate properly. All current mode control devices do. But it is likely you can go higher without issues depending on the switching frequency. But if you have two different values showing for two different input voltages you should be plenty safe to pick the higher of the two. As for inductor resistance, the lower the better for efficiency but of course there is a tradeoff in size/cost to be considered. The saturation rating should just be higher than the peak inductor current. There is an equation for the inductor current ripple and the peak inductor current in the datasheet.