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:
- 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.
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!