# LT3966 LED driver design

I have 2 strings of LED which have 6 LEDs in series. Initially, I considered using one CC boost module and a matrix dimmer, or a single driver with multichannel load switches to control the strings individually but after much searching up I found LT3966 and started the schematic design around it. The Vin would range from 5 to 12 V.

But I have a few doubts regarding this design. Consider an LED string consisting of 6 LEDs in series, and the typical VF of the LED is 2.75 V and the VF max is 3.5 V. The LED forward current is IF = 1000 mA for the entire string consisting of 6 LEDs.

Based on these parameters I have calculated the values of the following components: RISP - (0.25 V / 1 A) = 0.25 ohm

For FB resistors I chose (VFmax*No of LEDs) i.e. (3.5 * 6) = 21 V and used this to get R1 = 10k, R2 = 600.

RT resistor is 47.5k for 2 MHz switching frequency

LBoost >= 4.23 uH so I chose 6.8 uH

Cout 4.7 uF 50 V 1206

Inductor Part No BWVS005050406R8M00, PMOS Part No SI7309DN-T1-E3, Diode Part No PMEG6020ER_115

My concern is if the required voltage is around 21 V and 1 A through the string and I calculate the discrete value of the components based on the datasheet; however, based on the TI App note, if the switching current is higher than the internal FETs switching current limit of 2.4 A then the LT3966 IC will be not relevant even though I chose an appropriately rated inductor, a diode which are external to the IC. Is my concern valid?

Image of the Excel sheet calculation based on TI's app note attached, the ILIM(min) = minimum value of the current limit of the integrated switch (given in the datasheet) was 1.6 A for LT3966 integrated switch, which can be found on page 4.

TI's App Note on Boost Converters Design Calculation

Useful Post

• Please ask more direct questions. At the moment your words are just fishing for opinions and opinions are not good answers as per this site guidance: This question is likely to be answered with opinions rather than facts and citations. It should be updated so it will lead to fact-based answers.. Also, why don't you check the unconnected pins for the data sheet and figure out which pin in particular might be the most problematic. Dec 15, 2021 at 8:47
• @Andy aka I'll modify the question, I did check the complete datasheet regarding the unconnected pins but just wanted to make sure it is right from an experienced person's topology standpoint. The Datasheet explicitly mentions about few Pins like EXT1 and EXT2, CTRL but other pins are ambiguous. Dec 15, 2021 at 8:57

Yes your concern is valid.

If your input voltage is at its lowest value of 5 V, and your LEDs are all at their highest requirement so you need 21 V, you have a ratio of 4.2.

That means even a perfectly efficient boost converter would draw 4.2 A on the input side (and through the switch) to get the voltage and current of 21 V / 1 A on the output.

With that you are out of specification of your chip and it won't work. Note that you should consider the min value of 1.6 A if you are looking at the worst possible outcome.

The chip has a 4 channel design, so you could split the series in two parts of 3 LEDs each. But your schematic looks like the LEDs are integrated into one housing.

Or make your design a fixed 12 V input design, that will reduce the current down to 1.75 A - with the LEDs unlikely being all at 3.5 V it's probably good enough.

Or switch to a different device. At these current levels you might have better luck with drivers with external MOSFET.

• Thanks for the insight, do check out the rectified excel calculation image Dec 15, 2021 at 13:10