I looked at the nokia 6100 lcd board with number LCD-08600.
Is the current draw of the backlight led that high at 7Volts ?
I read numbers from other measurements of 70mA at 6Volts.
I noticed the inductor of 220uH has a rather small physical size.
The mc34063 is not really a fast switcher and i think that the core of the inductor can get magnetically saturated in some cases. I think if such a case would occur, the current consumption goes up (only limited by the current sense resistor)and efficiency is lost. Because the stepup converter should reach around 75 % efficiency easy. But at 3,3 Volts it seems, the switcher might stay in the on position to long and the inductor gets saturated.
Maybe i am wrong though…
Because i do not know the drawn current of the nokia 6100 backlight led at 7Volts.
I am building my own step up converter. I thought of using the LT1317. But i found a better one. It is cheaper and runs at 1MHz. I am going to order a few of these to play around with. CAT4238 or CAT4240. Because these step up converters run at very high frequencies, the L and C components can be smaller as well.
I think the led drivers can be dimmed. I have to see if i can directly pwm the shutdown pin with a 300 Hz pwm or that i have to pwm the feedbackpin by adding a voltage through a resistor divider to make the CAT chip think that the led current is too high and it will shutdown as long as the FB pin is 0,3 Volt or higher. I think that is possible.
I found a supplier for the little DF-23 connectors for the lcd too.
Yep, i just finished the datasheet. The CAT4240 it willl be.
