For building a constant current LED driver, I'm looking at the TI LM3407 datasheet: https://www.ti.com/lit/ds/symlink/lm3407.pdf
For calculating the necessary inductor, there's several things I don't understand:
- Under
8.1.4 Selection of InductorThe datasheet lists ripple current as \$ I_{L(ripple)} = \frac{\left(V_{IN} - V_{OUT} - 0.198 \times \left(1 + \frac 1 R_{ISNS}\right)\right) \times V_{OUT}}{L\times V_{IN} \times f_{SW}} \$
What I don't understand is where the term with the sense resistor is coming from, my understanding of a buck converter is that it depends only on duty cycle, frequency and inductance, but not output current?
Edit: I guess that's the on resistance of the built in MOSFET - which is given as 0.77 Ohm typical, 1.45 Ohm max (at 80mA).
- On the next page, there's
Table 1. Suggested Inductance Value of the Inductor, which e.g. lists 22µH for driving 5 LEDs with 1MHz from 25V, with \$I_{L(ripple)} = 0.8 \times I_{OUT}\$. But inserting those values into above formula, I arrive at >250mA ripple current, indpendent of \$I_{OUT}\$.
E.g. with \$I_{OUT} = 100mA, R_{ISNS} = 2\Omega\$, that ripple current seems excessive? - The datasheet has some suggested output capacitor sizes, but no information at all for how output capacitor size relates to LED ripple current. What's a good resource to learn about this?
