LCD backlight is inconsistent with small VCC fluctuation... - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: July 07, 2015, 08:23:00 pm
content: I've got a LCD module that has an LED based backlight. The cathode has 6 ohms of resistance in the form of a pair of 12 ohm resistors in parallel with each other. I am powering this by using a low side driver PZT2222 NPN transistor with 1K base drive from a 5V microcontroller pin.The smallest fluctuation in VCC is visible. If I power another load that changes VCC by only 36mV, you can see the display brightness fluctuate. I looked at the cathode and it is very near ground, but not ground as expected, and it fluctuates 4mV when the VCC fluctuates 36mV.I am driving it with a PWM channel though my tests are at 100% so PWM really isn't a factor in this test.Any thoughts on what I could change/try to reduce its sensitivity to vcc fluctuating?

author: gilbenl
author_rank: Regular Contributor
timestamp: July 07, 2015, 08:44:44 pm
content: Do you have a decoupling cap?Something like this, maybe?

author: LukeW
author_rank: Frequent Contributor
timestamp: July 07, 2015, 09:33:51 pm
content: Can you show a schematic for your circuit?What is the anode supply voltage, is it 5 volts?What is the designed forward voltage and current for the LED backlight array?Try bringing up the resistance a little bit - say 22 ohms and see how it appears.

author: alank2
author_rank: Super Contributor
timestamp: July 07, 2015, 09:44:20 pm
content: I do have a LPF on the contrast pin with a cap.The R1 in your picture is built into the display (between Q1 and LED-) which is 6 ohms (two 12 ohms in parallel).I think part of the issue maybe the way it is designed, I think it must have LED's in series. If I turn down the voltage to it, no current flows below 3.6V...Here are some measurements:4.5V = 81.7mA, 5V = 134.6mA (just the 6 ohm built in resistance) 10% voltage variation, 39.3% current difference4.5V = 42.2mA, 5V = 68mA (6 ohm built in + 10 ohm external) 10% voltage variation, 37.9% current difference8.1V = 40.7mA, 9V = 48.6mA (6 ohm built in + 100 ohm external) 10% voltage variation, 16% current differenceI only have 5V to work with, but this shows the issue. At 5V a 10% variation causes a larger current difference...

author: amyk
author_rank: Super Contributor
timestamp: July 07, 2015, 10:00:04 pm
content: That's normal... it's an LED, i.e. a diode, so I-V curve is exponential. Put in a current regulator if you want to maintain constant brightness.

author: alank2
author_rank: Super Contributor
timestamp: July 08, 2015, 01:01:26 pm
content: Can anyone suggest the simplest constant current circuit that you can control the brightness using PWM from a microcontroller pin...

author: View[+]Finder
author_rank: Unknown
timestamp: September 08, 2015, 07:09:46 pm
content: General VCC question: Is 11.87v 'good enough' for 12v VCC?I have a 52" Samsung LCD TV that I'm helping a friend repair. It has the dreaded 'image retention' problem and I've replaced a couple of boards and tried the 'scotch-tape' ribbon cable fix w/o success.On measuring the VCC (12v) to the side of the display that has the problem, I get 11.87v. Could this be a problem?Any help appreciated,Thanks Donal

author: amyk
author_rank: Super Contributor
timestamp: September 09, 2015, 12:20:56 am
content: Quote from: View[+]Finder on September 08, 2015, 07:09:46 pmCheck the VCOM voltage.http://www.badcaps.net/forum/showthread.php?t=35331