I am looking to configure an LED backlight driver circuit and had a couple questions I was hoping I could find some answers for. My application is a TFT LCD screen with a single anode/cathode input with two parallel strings of 9 LEDs that are in series. The given forward current and voltage are 40 mA and 28.8 ± 1.8V. I have a 3.3V (linear reg) and 5V (USB) voltage source available to feed into the circuit. I believe the MIC2289C driver from Microchip is suitable for my project (and it is actually available lol), as it has the following specs:
- Max output voltage: 34V
- Min/Max input voltage: 2.5V/10V
- Current output: 750 mA
I have determined from the LED current set formula on page 5 that a 2.375 ohm resistance will choose the 40 mA output, so I think a setup with this chip seems pretty straightforward. However, I am wondering about the significance of the number of LEDs in the series and how the parallel config affects things. I read in another post that the number of LEDs in series doesn't matter, but to me it looks like it does somehow because the recommended output capacitor changes in the datasheet. In the sheet, a selection for the components is given but it only shows recommendations for 8 LEDs max and as I am going through TI's How to Select a Proper Inductor for Low Power Boost Converter, I am still not sure what inductance/capacitance I should be looking to use yet... it appears some estimation is required to solve for inductor ripple current. A couple of equations are given to determine the inductor, but in another TI document Basic Calculation of a Boost Converter's Power Stage, it is indicated near equation 2 that usually the manufacturer gives a range of values to use and I should go from there.
So, my specific question is this: what is your process for choosing the inductor? How about the capacitor? It seems like from the equations that driving the circuit using the 5V is better because of how a higher input voltage appears to lower the current through the inductor, but do you guys have any intuition on if that greatly affects performance? I am getting from these guides that tuning generally happens after you receive the board to optimize your performance, so I am looking to at least be in the ballpark with the first design. Any thoughts you have on inductor selection, recommended values, if the LED number or configuration affects things, performance trade-offs, etc etc would be greatly appreciated.
