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PWM controlled current sink for LED backlight.

Hello,

I have a monitor with a LED backlight that uses PWM to directly switch on/off the LEDs to vary the brightness.

I can't stand the flicker this produces and want to modify the LED driver to keep the LEDs on all the time, setting the brightness by limiting the drive current, depending on a smoothed version of the PWM signal.

I've kind of managed this already, but can't get the full output brightness range because i am using a LM324 op amp with a single 5v supply and it can't pull it's output down below 0.6V while sinking current. (Only existing easy to get at supply is 5V). Here's some details of what i've found / tried so far:

The monitor is a ViewSonic VLED221wm, which contains a CHI MEI A220Z2-H01 panel and Asian Power devices FM-60L12-AAB power supply.

The LEDs are driven by 3 MBI1816GT drivers (separate R, G, B). These drivers have an active low output enable (!OE) pin and a pin where you are supposed to put a resistor (Rext) to ground to control the current output. (R,G=2K9, B=3K6 ohms). The data sheet doesn't have any internal details or suggest anything other than just using a resistor, it gives the current for each LED as 59*1.24/Rext and there is 1.24V on this pin and by varying the voltage of the "grounded" end of the resistors (driven from an op-amp) i can change the output led brightness. I'm pretty sure the output current is just 59xRref current.

There is a 7way connector coming from some chip i can't get to on the side of the lcd panel. The pins seem to be a i2c bus, power, gnd and R, G, B PWM signals. The PWM signals go via transistors to the !OE pins. This chip must have colour sensors as it adjusts the duty cycle of the PWM to try to get the backlight to be white as well as at the correct overall brightness. (found by manually changing the value of Rref for one colour and watching it try to compensate with the others).

So, what i've done so far is: tie low !OE inputs, for each R,G,B, take the !PWM signal that used to go to !OE, pullup with 1K, low pass filter it with R=100K, C=1uF, divide this by 4 with a 1M pot, buffer with op-amp follower and connect this to what used to be the ground end of the Rref resistors. Here's a diagram: (need fixed-width font)

------+-------------------------------------------------- 5V | +++ | |Rpullup(1K) | | +++ (want 0-1.24V) | +----+ 0.2-4.8V +--------+

------+---+ +----+-------+ | | !PWM +----+ | | | |`. | Rref=2K9/3K6 R=100k | +++ +-+- `. | +----+ --+-- | | | >-+--+ +----- Rref --+-- | |

Reply to
Sean Peters
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Why not (only if the led has a series resistor, or a current source), put a cap of about 100 microf accross it? Or is that to simple :)

Reply to
Sjouke Burry

"Sean Peters" wrote in message news: snipped-for-privacy@eddie.wibble.net...

There are quite a few opamps with near rail-to-rail output swings. NatSem has several. Then there are the venerable CA3130/3140. These operate at 5V and have outputs of less than

100mV sinking the current levels you mentioned.
Reply to
pimpom

According to the user manual, you can disable this feature by holding down the "Up triangle button" for 3 seconds.

Cheers, Nicholas Sherlock

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Reply to
Nicholas Sherlock

Thanks for the suggestion, you're right, it would work, but the 3 LED drivers each have 16 outputs. I don't think there'd be space for the caps even if my soldering skills were up to it.

Sean

Reply to
Sean Peters

Thanks for the pointers, i went with the CA3140 in the end as it was easiest to find.

Sean

Reply to
Sean Peters

Yes, That stops the backlight adapting to the brightness of what's on the screen. What i'm trying to stop is the high frequency flicker from the PWM modulated backlight (Most people wont even notice this).

Sean

Reply to
Sean Peters

Sean, you are super man. what's the frequency of the flicker that you can sense? PWM dimming method is widely used nowadays, that's bad news to you.

Reply to
canitch

The frequency of the PWM in this monitor is 6.2kHz.

This LED backlit monitor was probably the best i'd used yet, but the flicker was still a big problem for me, now i've filtered the PWM, it's brilliant!

That's a shame, i thought that now LED's were beginning to take over from other light sources i'd be better off.

In case you're wondering: I've got a problem where i end up with lots of noise in my vision. The intensity of the noise is dependant on the speed of changes in what i'm looking at. The noise can build up to a level where i can't see. Flicker causes me a huge problem. Low frequency strip lights are the absolute worst, and old crt tv's. higher frequency tv's and monitors are slightly better, then compact fluorescent, ccfl, high frequency strip lights ... etc. Incandecent bulbs seem to be the only lights i don't have a problem with.

Sean

Reply to
Sean Peters

I am working for a ccfl driver producer, some of the ccfl monitors in the present market use analog dimming. i don't know if you still have problem with this kind of monitor. or maybe led backlight is your best choice.

By the way, can you watch film in a cinema? I think a film has a really low frequency flicker.

Reply to
Matthew

I've not found a flourescent light of any kind that is not a problem to me. I've been assuming that it's physically not possible to drive a flourescent light in a way that prevents it from flickering. Is it possible? Do you have any spectrum analysis results of fluorescent light flicker?

Actually i've only been to the cinema a couple of times since i've had this problem (about 5 years ago, it started after i got Glandular Fever). But from what i can remember, the flicker wasn't really a big problem. Not as bad for me as a monitor.

Sean

Reply to
Sean Peters

Any such light source will have some level of flicker component in its output; the question is whether or not the flicker will be perceptible. I seriously question whether it is actually flicker that gives you your trouble with fluorescent lights, at least in the case of LCD backlighting.

Which again makes me think that it's not flicker that's causing you a problem with monitors. Movies are FAR worse in terms of flicker than an LCD.

Bob M.

Reply to
Bob Myers

it's possible - you can drive them with DC, but then you get bright and dark bands, it looks kind of neat as the band move as you adjust the voltage, but I don't think it'svery good for the tube.

using a higher AC frequency should reduce the visible flicker

48 or 72 hz depends how many lobes on the shutter.

Bye. Jasen

Reply to
Jasen Betts

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