Gameboy Pocket, Flash Carts, Backlights.
So you just installed a backlight to your GB Pocket and didn't destroy the LCD in the process? Congratulations! Your gameboy now has very short battery life, poor to no compatibility with flash carts, wild contrast variations in game, random losses of saved games.
And the Everdrive/EMS/Derp/Smartboy/Elcheapo cart you just spent good money on doesn't work on your modded pocket? But *RandomUser* from reddit said his works perfectly!
Here's what you need to know, needed to know, and how to fix what could have been a very expensive learning experience!
Gameboys, Flash Carts and backlights have all been designed to operate at 5volts. The Pocket operates from 2 AAA batteries supplying anywhere from 3.2volts to 1.5v at the end of the batteries life.
A small circuit called a flyback converter is responsible for boosting the constantly varying battery voltage to a rock steady 5v for the CPU and cart, and -19volts for the LCD to make the pixels do their thing.
When you design a circuit like this, you must know the expected maximum and minimum current of the load. You then choose the inductor, the switching components, diodes and capacitance to give a satisfactory output with little ripple for the specified load.
15yrs later and along comes backlights and Flash carts. A typical flash cart will consume as much as the entire GB, more during flashing from an SD card. Backlights on average draw 15-50ma, this too is about 50% of the GB's consumption, and in conjunction will exceed the design specifications of the original converter.
So what do you do? You install a supplementary converter to generate a steady 5v from the battery supply. Is this hard? No. Here's how:
Above is an unmodified pocket running my V1.9 ElCheapoSD flash cart (not the low power mod version) These batteries are about 50% used, the 5V system voltage is now at 5.0volts, contrast is stable. During Flashing the contrast will flicker because of the high current draw. As the batteries get more depleted the contrast will vary more, the CPU will reset when you attempt to flash a game. Now lets add a backlight:
Excuse the 3" LCD, it is just being used as a backlight load on the system. The 5v has now dropped to 3.8volts. The contrast is adjusted to maximum for the image to be visible. The power supply is getting warm. Flashing drops the voltage to 3.7volts and after a few seconds the CPU resets and the image is no longer visible. It is important to note here that at 3.7volts, the cartridge is now using it's internal save battery to power the SRAM, at this rate the cell will last around 90minutes before it is completely flat, you save is lost forever and the battery will need replacing.
How do we fix this? We install the new converter.
This is a modified 101 backlight driver module from my store. The cutoff voltage for this module is 1.8volts which means it'll put out a steady 5volts all the way down to 1.8volts battery voltage. This is not ideal as NiMh cells will still have quite a bit of charge in them even beyond this point. THIS is what I'd recommend. I haven't tried this but according to the specs, it is what you want! It'll run until your batteries are completely discharged.
Depending on its output voltage, you may or may not need to cut the original 5v pin from the regulator to keep the LCD voltage alive. The first person to mod their pocket with the pololu module I'm sure will let me know.
With the module fitted, the system is at a steady 5v. There is zero contrast fluctuation with almost any flash cart. No random resets, no save game loss. You'll still only get 60-90minutes out of a fresh set of batteries but this can't be helped. There just isn't that much energy in a set of AAA batteries.
If you found this useful, head over to my FB page and leave a comment. I'll be making a few more of these about various GB subjects including backligting a DMG earlier than Rev 07, Backlighting a GB Color etc...