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yoq · 05-05-2013, 01:47 PM

After hijacking the thread with the inversion mode, lets get back to actually lowering the brightness

I played around with the PWM frequency and the duty-cycle but I couldn't get lower than brightness 1, even with lowering the frequency. (this creates more steps and Ton/Tcycle decreases)
So I dug into the hardware and this is what I found:
The PWM is generated by a MSP430 microcontroller (U6 on the board, output on pin 20). This signal feeds into a step-up converter that powers the 5 LEDs with a constant current, depending an the duty-cycle. There is also the signal line FL_R_EN which effectively doubles the current for a given PWM duty-cycle. The switch happens at a brightness of 50. (details in the driver code)
I drew up the schematic for the LED driver part:
https://www.mobileread.com/forums/att...1&d=1368175933

But I couldn't identify the specific converter chip, so I gave up on a software solution. But maybe someone has a brilliant idea, who knows?

As for a hardware solution, with a current source the easiest way to lower the brightness is to divert some of the current with a parallel resistor. Because LEDs keep the voltage almost constant, the diverted current doesn't change over the brightness range. An because LEDs are driven logarihmic the change affects only the lower steps and the change at the maximum brightness is not perceptible.

If I lost you along these paragraphs, continue here!

So here is the hardware hack to get a higher dynamic range for your frontlight with a material cost of about 2ct:
You will need the following:

1x 33k Resistor in size 0805 (0603 should do as well)
- Yep, that is about 2x1mm, so better get more than one
- Look at Farnell PartNo 1099810 for an example
A soldering iron with a ~1-2mm tip + some solder (the one with lead is highly recommened)
Tweezers

Ready?

Open you Kobo, refer to:
https://www.mobileread.com/forums/sho...d.php?t=209122
Unplug the battery, simply pull the white part up
Locate this part:
Now solder the resistor between the bottom terminal of RA4 and the spot marked TP62:
Replug the battery and you're done!

05-05-2013, 01:47 PM	#71
yoq Developer Posts: 77 Karma: 206176 Join Date: Apr 2013 Location: Europe Device: Kobo Glo	After hijacking the thread with the inversion mode, lets get back to actually lowering the brightness I played around with the PWM frequency and the duty-cycle but I couldn't get lower than brightness 1, even with lowering the frequency. (this creates more steps and Ton/Tcycle decreases) So I dug into the hardware and this is what I found: The PWM is generated by a MSP430 microcontroller (U6 on the board, output on pin 20). This signal feeds into a step-up converter that powers the 5 LEDs with a constant current, depending an the duty-cycle. There is also the signal line FL_R_EN which effectively doubles the current for a given PWM duty-cycle. The switch happens at a brightness of 50. (details in the driver code) I drew up the schematic for the LED driver part: https://www.mobileread.com/forums/att...1&d=1368175933 But I couldn't identify the specific converter chip, so I gave up on a software solution. But maybe someone has a brilliant idea, who knows? As for a hardware solution, with a current source the easiest way to lower the brightness is to divert some of the current with a parallel resistor. Because LEDs keep the voltage almost constant, the diverted current doesn't change over the brightness range. An because LEDs are driven logarihmic the change affects only the lower steps and the change at the maximum brightness is not perceptible. If I lost you along these paragraphs, continue here! So here is the hardware hack to get a higher dynamic range for your frontlight with a material cost of about 2ct: You will need the following: 1x 33k Resistor in size 0805 (0603 should do as well) Yep, that is about 2x1mm, so better get more than one Look at Farnell PartNo 1099810 for an example A soldering iron with a ~1-2mm tip + some solder (the one with lead is highly recommened) Tweezers Ready? Open you Kobo, refer to: https://www.mobileread.com/forums/sho...d.php?t=209122 Unplug the battery, simply pull the white part up Locate this part: Now solder the resistor between the bottom terminal of RA4 and the spot marked TP62: Replug the battery and you're done! Attached Thumbnails Last edited by yoq; 05-10-2013 at 04:52 AM.