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power supply design help

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I'm trying to make a usb NiMH charger based on PIC 12F683 and a constant current source. (To charge 2*AAA NiMH batteries from laptop USB port)

12F683 is used to measure the battery voltage and battery temperature using its ADC pins.
I need to use the Vdd of 12F683 as the reference voltage of its AD converter. I cannot use separate Vref because I have use all other pins of the microcontroller.

Problem is When I change the current of constant current source voltage of USB port begins to drop. and when current is about 150mA it becomes about 4.1V. So using Vdd as reference to the ADC is not going to do any good unless I have fixed Vdd value.

Therefore I need to make some kind of regulator circuit that will deliver constant 4V or 3.5V output with about 40mA from varing input of 5V to 4.1V.

I tried zener regulator and TL431 but they didn't do much good, output of them tends to change as input voltage varies.

Can some one help me to make a voltage regulator that meets above requirements.

(I cant use any other PIC than 8 pin 12F683 because I need to finish it as small as possible. Only problem achieving this task is the voltage regulation problem.)

thank you

A 3.3 volt LDO (low dropout) linear regulator should do the trick.  A quick parametric search on one of the big semiconductor companies or digikey should find one with a suitable current range.  I just searched on national semiconductor website and found the LP2980 among many others that would fit the bill.

buying from digikey is going to be very very expensive.  I'm looking for something that i can find from local market.
I have LT1085 LDO but it does not meet my requirements

Does it work if you use a fixed 3.3 volt supply instead of trying to push it up to 3.5 or 4.0 volts?  Maybe even step it down to 3.0 volt.  That regulator has about a 1 volt dropout, so even with a 4.1 volt input it should be able to regulate at 3.0 volts with no problem, and you might get away with 3.3 volts given the low current you are drawing.  If you need a larger voltage range for the ADC you can just use a voltage divider.

4.1 volts is also out of spec for USB, so something is wrong.  Are you drawing 150+ milliamp from a low power port?  Don't do that.

i should reduce the current to 100mA. and find an available 3.3v regulator and give it a try.
thank you  :)


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