Microcontroller turns off charger before it gets proper Vcc voltage.

[treez]

Hello,

We implemented a SAM3N00B microcontroller, and one of its pins switchs on an NPN which turns a battery charger current source OFF.

Unfortunately, when the microcontroller has just 1V on its Vcc pin, it turns this BJT ON and this unfortunately turns OFF the charger current source.
-this behaviour is not wanted.
-when the microcontroller's Vcc voltage is below the start up voltage of the microcontroller we do not want the micro to turn ON the NPN.

Anyway, its happening, so we need some component or circuit which is like a switch, and this switch must only apply volts to the Vcc of the microcontroller when the voltage is 3V. -then the micro won't be able to switch the NPN ON when the micro's Vcc is just 1V, because the micro will never have just 1V on it.

Does such a component exist? (it would probably be composed of an internal reference and a high side FET which turns ON when the rail voltage gets to 3V.

The SAM3N00B is not supposed to start up until its Vcc is 1.62V, so when the Vcc is just 1V, it must be some strange behaviour thats being exhibited....it couldnt be anything to do with the code, because the micro is programmed to start up and keep the NPN switched OFF at first

I thought a microcontroller's pins were "high impedance" until the full Vcc voltage was present on the Vcc pin?

SAM3N00B datasheet
http://www.atmel.com/Images/doc11011.pdf

Anyway, do you know of the above described "switch" component? (i could make one from a FET, a reference and a comparator, but really need a small integrated solution.)

 

[BradtheRad]

Right now the NPN base turns on above 0.6V, correct?

Consider adding a diode at the NPN base. This will raise the bias threshold an additional 0.6V.

Add a second diode if necessary.

If this causes too much reduction of the bias current, which in turn could reduce battery charge current...
then you might want to substitute a darlington arrangement so you'll have more gain.

 

[treez]

Thanks, but that just raises the voltage at which the charger gets turned off.

The behaviour we need is for the charger to be off whenever the micro voltage is less than the micro's nominal vcc.....ie if the micro doesnt have enough vcc to allow it to run its code, then we dont want the bjt to be turned on.

i think we need to stop voltage from getting to the micro vcc pin until the voltage is 3v.

 

[BradtheRad]

It might be possible to arrange a PNP transistor in a way that does what you want. Its behavior is the opposite of an NPN, in some ways.

 

[jayanth.devarayanadurga]

Does you MCU have an ADC? If yes, use the adc to read the Vcc of MCU and make a condition in code so that NPN is driven only when adc reading is >= 3V.

 

[treez]

Does you MCU have an ADC? If yes, use the adc to read the Vcc of MCU and make a condition in code so that NPN is driven only when adc reading is >= 3V

sorry no can do, the npn is being turned on before the micro starts running its code.