[SOLVED] TPS61196 LED Driver chip for high Voltage Backlight applications: What to do for prop

[Alper özel]

Using TPS61196 LED Driver chip for high Voltage Backlight applications

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 **broken link removed**.

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.

 

[betwixt]

The first diagnostic check is to check the voltage at TP3, if it is less than ~84V the LEDs will not light anyway.

I suspect you are not producing enough voltage at the output (TP3) point for 28 series LEDs. The schematic shows "Vo < 60V" so your only option would be to use fewer LEDs in series to drop their voltage requirement or use an alternative power supply for them.

Note that if you increase the output voltage with the values in the schematic, the over-voltage protection will shut it down.

Brian.

 

[Alper özel]

Thanks for the reply. However, my design is for 118 V, I assume you confused that I use reference design. You can see it **broken link removed**.

 

[betwixt]

Please post images directly here rather than using file sharing services - especially ones that send adverts to the downloader!

The rectifier D4 is shown backwards but I guess that's a drawing mistake. What voltage do you actually measure to the LEDs?

Brian.

 

[Alper özel]

Yeah sorry for that. I choose to share it with tinyupload because it was a PDF file. Will never do that again.

I measured the LED string voltage 24V which is the supply voltage.

 

[super-1943]

Re: Using TPS61196 LED Driver chip for high Voltage Backlight applications

I have encounted the same problem... with 12V supply and result in 12V output on leds
the weird thing is the voltage on ifbv is 5V, I have not tryed to improve supply voltage,
have you got any clue ?

 

[Alper özel]

Re: Using TPS61196 LED Driver chip for high Voltage Backlight applications

I have encounted the same problem... with 12V supply and result in 12V output on leds
the weird thing is the voltage on ifbv is 5V, I have not tryed to improve supply voltage,
have you got any clue ?

Yes, regarding to my experience you should do two things:

1- Make sure that you soldered the chip right. Do conductivity tests for each pin.

2- Make sure your switching inductance Self Resonance frequency is at least 4 times more than your switching frequency. To be safe, choose an inductor which has at least 4 MHz SR frequency.

I did those two and now it is working very good. I hope I could help.