Sharp LC-40LE550U no backlight

[budm]

Re: Sharp LC-40LE550U no backlight

Yes its actually a little more each string has eleven led's so the voltage required is 66volts...

So it has 55 LEDs to total then? If that is the case, then the 340VDC (329V) for the LED string is too low, 55 LEDs will need 330V + Vdrops on the MOSFET + V drops on the current sensing resistor.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

Sooo...further testing pin#13 (Back light On) I'm getting .224 volts...basically minimal perhaps even ghost read...the guy in the first post had 3 volts here...

[budm]

Re: Sharp LC-40LE550U no backlight

Post1, and 2, plan 14 is the BLON which must be working otherwise you will not see the LED supply Voltage jump up from 89V to 240Vdc.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

Post1, and 2, plan 14 is the BLON which must be working otherwise you will not see the LED supply Voltage jump up from 89V to 240Vdc.

Thanks, budm, so now that I think about it, he had 3 volts there PRIOR to finding his short and his was off? I have <3v which means it is on...?

Or am I backwards?

I get what you mean about the supply voltage but is it possible that the ricoh chip (UD101) drops the current when it find a led fault/short/open to below "strike" threshold for the lamps? (I know thats not what happens here but its the basic idea)

"On most sets, BL_ON (backlight on) going high (> 3) means turn on. On some sets, it might be the opposite.
"

[budm]

Re: Sharp LC-40LE550U no backlight

Right now the BLON is high enough to turn on the BL power supply to put out the boosted Voltage but at this point that 340VDC may be too low.
I need to study the function of UD101 to see how it works.

[budm]

Re: Sharp LC-40LE550U no backlight

So what is the P/N of the UD101?

[Markpixley]

Re: Sharp LC-40LE550U no backlight

So what is the P/N of the UD101?

P/N for UD101 is RT8300

http://www.richtek.com/en/Products/L...0Driver/RT8300

[Markpixley]

Re: Sharp LC-40LE550U no backlight

So further testing...

By jumping pins 13>>>10 and 12>>>9 o the UD101 I was able to get the backlight to light up...

I have 13.58 volt on 13/12 and none on 9/10

9/10 are not connected to anything except ground, so basically forcing 13/12 to ground turns on the back light...

[Markpixley]

Re: Sharp LC-40LE550U no backlight

So this is where I am at...the FB pin (1) was getting 12.28 volts...waaay to much as far as I can tell...by jumping pins 13/12 to ground it dropped the voltage on FB to 1.9 which is below the high tolerance set (3.3) basically I forceed the FB to low and everything works...now I need to figure out if it is the UD101 or the circuit from the FB pin ...

[budm]

Re: Sharp LC-40LE550U no backlight

OK, that IC is the 4-Channel LED driver.
Pin 13 is LED string 1 Cathode return
Pin 12 is LED string 2 Cathode return
Pin 9 is LED string 3 Cathode return
Pin 10 is LED string 4 Cathode return
The Cathodes return currents flow through 4 internal MOSFETs inside the IC and then to GND pin of the IC. See app notes AN006 PDF page 2 attached.
The circuit in this board does not have the Cathode return connected to pin 12 (LED2), it has the Cathode return of the LED string connected to the Drain pin of the Power MOSFET UD103, then Source pin is then connected to pin 12 (LED 2) and then the LED current will then flow through the internal MOSFET IC pin 12 and goes out to GND pin.
So if you connect pin 12 (LED 2) to pin 13 (LED 1) and you get backlights, then it means that the current sink circuit for LED 2 is bad and you are using pin 13 (LED 1) as the current sink path for the LED string. To proof that you will have to take out pin 12 and use pin 13 only.
You have 4 current sink for 4 LED strings, the board is using only one (pin 12 LED 2) of the four, so if pin 12 LED 2 bad (bad internal MOSFET?)and you shift the current circuit to pin 13 LED 1 instead and it works then you are in business.
I also would like to know what the Boosted Voltage is now since the LEDs light up now.
It looks me that they use external power MOSFET in series with the internal MOSFET to reduce heat dissipation of the IC.
Bottom line, try shifting the circuit to pin 13 see what happen, pin 12 has to be removed from the circuit, leave pin 9, 10 alone.., then check the DCV feeding the LED to see if it is >340VDC, also the DCV for FB pin, need to find out where the FB pin is connected to in the circuit, if it is still way above 3V then you need to check that Voltage divider on that output of the 390V to see if the resistor to GND of the Voltage divider may have gone up in resistance. See page 6 of the APP NOTE AN006 PDF page 6. It can be that the FB is so high so the output Voltage is only 340VDC instead of higher Voltage. If you can find that Voltage divider resistors then you can calculate what the output Voltage for the LED should be, the formula is also on that page 6.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

Well my quickest reply is the back light does NOT come on when shorting 13>>>12

ONLY when shorting 12>>>Ground or 13>>>ground...makes no difference which one...

9/10 are soldered to the ground plane they have no other connection they are all connected to the ground pin (8) on the same plane...I am pretty sure after inspecting this board it is only using outputs 13/12...

My voltage readings on the Cn202/203...

340>>>drops to 339
287>>>>drops to 271
243>>>>>drops to 203
181>>>>>>drops to 135
129>>>>>>>drops to 67
78>>>>>>>>>drops to .04

the only other difference I read is pin 7 (slp) sits at .99 until I short 12/13>>>grnd then it goes to 1.3v

[Markpixley]

Re: Sharp LC-40LE550U no backlight

Pins 8,9,10,11, and 14 all connect and join up to the (-) of the large 450v47uF capacitor...

[budm]

Re: Sharp LC-40LE550U no backlight

The LEDs light up because return pin for the LED return is grounded so the ineternal MOSFET and the internal current sensing are bypassed.
I still believe that 340vdc feeding the LED string is too low. You need to find that FB pin 1 divider resistors to find out what the actual Voltage for the LEDs should be.

[budm]

Re: Sharp LC-40LE550U no backlight

Pins 8,9,10,11, and 14 all connect and join up to the (-) of the large 450v47uF capacitor...

Pin 8 is N.C, No Connection, not used.
Pin 9 LED 3, not being used so it is Grounded.
Pin 10 LED 4, not being used so it is Grounded.
Pin 4 & 11 are GND pins.
Pin 14 IS GND pin.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

The LEDs light up because return pin for the LED return is grounded so the ineternal MOSFET and the internal current sensing are bypassed.
I still believe that 340vdc feeding the LED string is too low. You need to find that FB pin 1 divider resistors to find out what the actual Voltage for the LEDs should be.

The first resistor FB hits is a 4703 smd that measures 470k out of circuit...

[budm]

Re: Sharp LC-40LE550U no backlight

Looking at the App notes, there should be at least 3 resistors.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

ok the second possible path is through three resistors all marked "O" smd...measuring .3 Ohms each then into a smd capacitor with no marking but measuring .182 nF

[budm]

Re: Sharp LC-40LE550U no backlight

'0' is just a jumper. You can also trace the + leg of the 450V cap for the LED supply to see where it goes.

[Markpixley]

Re: Sharp LC-40LE550U no backlight

'0' is just a jumper. You can also trace the + leg of the 450V cap for the LED supply to see where it goes.

Thanks budm, I will try to follow the (+) side and see how it lands, it goes through a string of resistors that seem to be in parallel with the outputs CN202/203...the RL series (about 5 immediate) on the far bottom left side of img.3...they all seem to head to UL103 a transistor...I'll see if I can get values tommorow...

[budm]

Re: Sharp LC-40LE550U no backlight

I try using the pictures from shopjimmy to trace out the circuit but it is not clear enough for me to see where the FB of the IC or the + cap really goes to.