power supply design help

[aruna1]

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

 

[MrAl]

Hi,

You are saying that when you draw 150ma from the USB port that the voltage of the USB port drops to 4.1 volts? That doesnt sound typical.
There is a spec in USB that calls for the device to 'ask' the computer for 'permission' to use more than 100ma in increments of 100ma up to 500ma (USB 2.0) but usually the port puts out more current even if there is no negotiation. It could be that your particular port doesnt work like that though, so it thinks your device only needs 100ma.
Is there some way you can build this thing using only 100ma?

 

[aruna1]

Hi,

You are saying that when you draw 150ma from the USB port that the voltage of the USB port drops to 4.1 volts? That doesnt sound typical.
There is a spec in USB that calls for the device to 'ask' the computer for 'permission' to use more than 100ma in increments of 100ma up to 500ma (USB 2.0) but usually the port puts out more current even if there is no negotiation. It could be that your particular port doesnt work like that though, so it thinks your device only needs 100ma.
Is there some way you can build this thing using only 100ma?

Hi it does allows me draw that current, I measured it. but i guess regulation is not happening because there was no enumeration process and voltage begins to drop

 

[alec_t]

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.

If 40mA is all you need for charging then what is the rest of the 150mA being used for?

 

[aruna1]

rest current is for charging the battery

 

[ronsimpson]

FAN2500 or FAN2501.
Vin must be less than 7 volts.
Vout could be 3.3 volts depending on what version you use. (4.0V)
Vin-Vout = 0.1V at 100mA (low dropout).

There are about 50 different low current low dropout parts.

 

[alec_t]

Here's a circuit which should give up to ~ 200mA at 3.6V (using the components shown, but the voltage is adjustable with a pot). The dropout is about 0.1V.