I have a TV here that when powered up, the backlight blinks between dim and normal brightness over and over.
I started troubleshooting and found that the main processor on the board was sending a signal to turn the LED driver OB3350 on over and over, like it was trying to start it up but failing. On the schematic that's coming from ASIC_PINx.
I bypassed this signal and forced the LED driver IC high which still resulted in the backlight flashing brighter. I spent some time probing things and found that there was a constant 3.2V being applied to the PWM pin of the LED driver, and traced that back to the main processor. Now the OB3350 datasheet mentions that:
"OB3350 is enabled by applying a voltage of greater than approximately 2.5V to PWM pin."
So 3.2V appearing on this pin sounds correct. But to test it further I decided to feed a 0-5V PWM signal into this pin and found that the backlight's brightness corresponded perfectly to the duty cycle. It seemed to work fine.
My theory is that the main processor on the board is perhaps looking for an active High/Low signal on one of its pins when the LED driver is activated but never sees it. It just keeps trying to turn it on, and the PWM signal is locked to ~3.2V as if its stuck in an initialization mode.
At this point I could just set the brightness to a fixed value with some resistors and leave it but now it's starting to bug me.
Here's an approximate schematic of the LED driver:
I started troubleshooting and found that the main processor on the board was sending a signal to turn the LED driver OB3350 on over and over, like it was trying to start it up but failing. On the schematic that's coming from ASIC_PINx.
I bypassed this signal and forced the LED driver IC high which still resulted in the backlight flashing brighter. I spent some time probing things and found that there was a constant 3.2V being applied to the PWM pin of the LED driver, and traced that back to the main processor. Now the OB3350 datasheet mentions that:
"OB3350 is enabled by applying a voltage of greater than approximately 2.5V to PWM pin."
So 3.2V appearing on this pin sounds correct. But to test it further I decided to feed a 0-5V PWM signal into this pin and found that the backlight's brightness corresponded perfectly to the duty cycle. It seemed to work fine.
My theory is that the main processor on the board is perhaps looking for an active High/Low signal on one of its pins when the LED driver is activated but never sees it. It just keeps trying to turn it on, and the PWM signal is locked to ~3.2V as if its stuck in an initialization mode.
At this point I could just set the brightness to a fixed value with some resistors and leave it but now it's starting to bug me.
Here's an approximate schematic of the LED driver:
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