LCD backlight is inconsistent with small VCC fluctuation...

[alank2]

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?

[gilbenl]

Do you have a decoupling cap?

Something like this, maybe?

[LukeW]

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.

[alank2]

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 difference
4.5V = 42.2mA, 5V = 68mA (6 ohm built in + 10 ohm external) 10% voltage variation, 37.9% current difference
8.1V = 40.7mA, 9V = 48.6mA (6 ohm built in + 100 ohm external) 10% voltage variation, 16% current difference

I only have 5V to work with, but this shows the issue.  At 5V a 10% variation causes a larger current difference...

[alank2]

[amyk]

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.

[alank2]

Can anyone suggest the simplest constant current circuit that you can control the brightness using PWM from a microcontroller pin...

[View[+]Finder]

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

[amyk]

It has the dreaded 'image retention' problem

Check the VCOM voltage.

http://www.badcaps.net/forum/showthread.php?t=35331