LT3966 LED driver design

Question

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

Answer 1

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.