ADC negative VREF on 18F4550 question

[comwarrior]

ok, the task is simple, to create a precise 3.4V output from a standard li-ion battery and be stable under varying loads...
Li-ion batteries output upto ~4.2V and down to less than 3.4V over their full capacity so a voltage regulator such as the LM317 can't be used due to it's drop out...

so, that leaves me with a PIC and a FET...

Connection the FET to switch the positive line of the output load can not be doen due to the FET's own requirement of 0.6V 'on' voltage at the base compaired to it's emitter. The emitter is ofcourse connected to the output load and therefor i'd require 4V input minimum...

So, this leaves switching the load via it's negative voltage rail and it's positive rail is connected straight to the battery positive...
However... as soon as i connect AN2 to the load negertive and enable -vref the negative current flows through the chip and thus the output is always on.

Is their something i'm missing?

Thanks

[rmteo]

Firstly, a FET does not have a base (or emitter for that matter) and it requires more than 0.6V (typically 3-10V relative to the source) on its gate to turn on.

Best way to do what you want is to use a SEPIC DC-DC Converter (http://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter) switching converter. It's output voltage can be lower or higher than its input voltage.

[Acetronics2]

Hi,

For such a simple task, I would use a simple capacitive voltage doubler, driven by the µP clock output, i.e. and then use a linear regulator ( or a voltage ref ... or TL431 ).
using some MAX 630 ( a max 232 or ICL 7660 could also do it ... ) as a step-up converter is there somewhat overkill.
Max 761 ANs are to look at too ...

a convenient one had been the MAX 879 ... but production stopped !

Alain

[comwarrior]

looked at the max660... It's expensive and does not give the required output...
I'm looking for 1Amp @ 3.4V
And cost is an issue...

For testing purposes i'm using the TIP35(C), gate to source threshold 2V but i generally find that it operates as low as 1V just not in the 'fully on' setting...

If the -Vref is tied to the voltage rails by those diodes... then what is the point of having an external -VREF?