Hello All,

I have an interest in moving some of my LED lighting applications away from logic-level MOSFET designs and instead using step-up boost converters designed as LED drivers. I have to admit that once I started digging through datasheets and information about these little ICs, they seem more complicated than I first imagined, especially in trying to understand how to select the proper values for the inductor and the Schottky Barrier Diode. I have been reading documents from TI and Toshiba, among others on this topic and I appear to be having trouble with the math and cannot figure out why, so I am looking for a little help.
In the document from TI, titled Schottky Diode Selection in Asynchronous Boost Converters, which (obviously) applies to selecting an appropriate SBD, the following is given:


I am following these formulas using the given example values and the result I'm getting is not 0.749A. Here are my results:

\( D = \frac{V_{outmax} - V_{inmin} * efficiency}{V_{outmax}} \)

\( D = \frac{26.7 - 2.7 * 0.87}{26.7} \) \( D = \frac{26.7 - 2.349}{26.7} \) \( D = \frac{24.351}{26.7} \) \( D = 0.912 \)

\( \Delta I_L = \frac{V_{in} * D}{f_{SW} * L} \)

\( \Delta I_L = \frac{26.7 * .912}{1,000,000 * .00001} \) \( \Delta I_L = \frac{24.351}{10} \) \( \Delta I_L = 2.4351\)

\( I_{L(PEAK)} = (\frac{I_{LED(TOTAL)}}{1 - D} + \frac{\Delta I_L}{2}) \)

\( I_{L(PEAK)} = (\frac{.06}{1 - .912} + \frac{2.4351}{2}) \) \( I_{L(PEAK)} = (\frac{.06}{.088} + \frac{2.4351}{2}) \) \( I_{L(PEAK)} = (.6818 + 1.21755) \)
\(I_{L(PEAK)} = 1.9A \)

I'm certain I have to be doing something wrong in my math, I'm so far off from the example result. I want to get the same result as in the example so I'm confident in calculating these values for the circuit I'm currently working on.
Thank you

 

You look to have made an error around DeltaIL by using the output voltage instead of the input voltage in the numerator. By my calculation that would result in DeltaIL being 0.246A. Using the equations given i calculate Ilpeak to be 0.805A which seems a bit off to their calculation so would be worth checking via simulation

 

In relation to the ILPeak i'm pretty sure my calculation is correct. I think whoever did the datasheet calculations divided the ripple by 2 then did it a second time which gives a value of 743mA. Still worth a simulation but just letting you know

 

In relation to the ILPeak i'm pretty sure my calculation is correct. I think whoever did the datasheet calculations divided the ripple by 2 then did it a second time which gives a value of 743mA. Still worth a simulation but just letting you know

Thank you! I did redo the math correcting my error and I got the same .805 result you did.

 

I suggest visiting some of the IC manufacturer's websites. Texas Instruments may still have some excellent tutorials about designing switcher power supplies. Other manufacturers also do. I would totally avoid the cartoon channel because some of the stuff is totally false. The manufacturers sites have a stake in being correct and honest. That makes a big difference.