TPS61196 LED Driver chip for high Voltage Backlight applications: What to do for proper backlight driving?

Question

I need to design a Backlight Driver which has to have high brightness.

These are the equipments I have in the design:

• My Backlight String combined of 2 parallel of 28 serial LEDs(2 * 28 = 56 LEDs)
• My LED Driver Chip is TPS61196
• My supply is 24V 15A 350W SWİTCH MODE ADAPTOR
• My switching N-Channel Transistor is IRL640A

These are the features I need to cover:

• I need to draw at least 350 mA from one channel(28 LEDs)
• I need to adjust the Duty cycle 20% @ 120 Hz
• When I draw 350 mA from 28 LEDs, output voltage will rise to about 28 * 3V = 84V + ~1V(Headroom) = 85V
• I use 2 channels currently(each channel drives 28 LEDs in series) so 350 mA * 2 = 700 mA current needed to be boost
• I will apply synchronised PWM dimmnig to 2 channels @ 120 Hz with 20% duty cycle

So designed this circuit (I took my reference design as TPS61196 EVM module). I followed the design guide carefully. Double checked the calculations. Yet, the circuit does not work. No strobe at all! Here is the schematic:You can download the SmartPDF file of both the schematic and the PCB here.

I tested the circuit in anyway I thought. Short circuits, wrong soldering, misplacement of components etc. checked for anything might be wrong, but everything was as in my design. So, I do not know what to do next. Maybe somebody here might have used the same chip and encountered the same problem. I am desperate here.

Only clue I have is this. I thought maybe, boost regulator is not working properly so I used another LED string which works with estimated 12V supply(When I apply 12V it lights up) and set the supply voltage to 12V. Because the initial IFB pin was 3.5V. Then I increased the supply voltage to 24V and I saw the initial IFB pin is increased as well to 16V. I could not see anything about this issue in the datasheet. I hope someone can help.

Answer 1

This was going to be a comment, but there's too many of them to fit:

Just to start:

1. The ground pad of your chip has no vias to provide cooling or low inductance returns.
2. Your MOSFET is not a great replacement, but might work okay enough below 10A peak currents.
3. Your inductor is drawn as a capacitor on the board, that's bad design form to start and downright confusing if you want actual help. Same with the block you drew as your MOSFET in the Schematic to then add insult to injury by using a Bipolar board package for it (B-C-E!!!). 3.b. Same goes for the schematic component for the diode.
4. You're using a 25miliOhm current sense resistor with no Kelvin Connection (seemingly designed on in a tiny 0805 package and replaced later).
5. With 25mOhm and 400mV over-current protection comparator level, that's 16A. Why? Your inductor and diode will not like that.
6. I see a solder wire go under the board in a picture and no preventative measures to protect from things like that shorting other things out.
7. The type of inductor you're using isn't particularly great at High Frequencies, which I'd expect the Chip to use for the boost circuitry. Note how the original specifically states how it's great at 1MHz and even 2MHz energy storage: That won't work that well with a huge I-Core choke.
8. There's no pictures of the bottom side of the finished PCB.
9. It doesn't even really look like the Chip's Thermal PAD is actually soldered to the board, but I'll give you the benefit of the doubt there.

Otherwise, your PDF contains only a compound print of the PCB, so who knows if there's mistakes I can't see on layers that are not the Top layer.

Oh, and (EDIT:)

10. Your BOM is not very usable. "Typical" this and "Sim Cap" that with only sometimes a part number in the footprint, and sometimes nothing usable at all makes any form of review nigh impossible.