Hi
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
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