Oops!

I forgot the reference to the enhanced midrange parts. I think that the
12F1822 or 12F1840 can do this in an 8 pin part with the possibility of
using the 16F1825 if you were willing to use the larger 14 pin platform.

BAJ

On Thu, Jan 17, 2013 at 10:11:05AM -0500, Byron Jeff wrote:
> On Thu, Jan 17, 2013 at 02:18:23PM +1000, David Duffy (AVD) wrote:
> > I have the need to regulate the voltage going out to some LED strips. =
=20
> > These strips are simple 12V DC types that have 3 LEDs and a resistor pe=
r=20
> > section, repeating throughout the strip.
> >=20
> > The voltage available is nominally 12V, but can vary between 12V and=20
> > 14V.  The ability to handle up to 18V input is also desired by the=20
> > client.  The current draw of the LED load is approx 3 Amps.
> >=20
> > I was thinking of a simple 12V LDO, but at 2V differential that's 6=20
> > Watts in heat - ouch!  Ideally this unit needs to run with minimal heat=
=20
> > dissipation.  I also thought about a SEPIC converter, but that would=20
> > take too long to design and debug and be overkill for this application.
> >=20
> > My next thought was to use a PIC12 to PWM a logic level MOSEFT and use=
=20
> > the A2D in the PIC to sample the output voltage via an R/C network.  In=
=20
> > theory this will allow me to regulate the output and even allow for a=20
> > dimming function to be offered as an option.
> >=20
> > There are only 100 units to build initially and maybe some more after=20
> > that, but not thousands.  A PIC based solution would be low parts count=
=20
> > and fairly easily tweaked to my mind.  Does this sound like a reasonabl=
e=20
> > solution?
> > David...
>=20
> David,
>=20
> Interestingly enough this is an item that's on my immediate to do list as
> I'm looking to develop a switcher based sealed lead acid battery charger.
>=20
> The most promising route points to the use of an enhanced PWM module. I
> happen to be partial to 16F1XXX parts so I will reference them for this
> discussion.
>=20
> Enhanced PWM has the ability to do auto shutdown using a comparitor. So i=
t
> should be possible to build a simple automatic buck regulator using the
> ePWM output to drive the buck switch, and feeding the output via a
> resistive divider into the comparitor. When the auto shutdown feature is
> turned on, each PWM cycle is halted when the comparitor changes state. Wi=
th
> auto restart, the ePWM module will started PWM at the start of the next
> cycle. So if you set the PWM duty cycle to the maximum voltage you wish t=
o
> allow (say 13.5V in your example) and set the comparitor to trigger at 12=
V
> (using a combination of the voltage divider plus the internal resistive
> DAC for the other end of the comparitor) then once the ePWM is turned on,
> it will dynamically cycle by cycle drive the buck switch until the
> comparitor cuts it off, regulating the output voltage.
>=20
> I'd be interested in anyone pointing out any holes in the idea.
>=20
> BAJ
>=20
> >=20
> > --=20
> > ___________________________________________
> > David Duffy        Audio Visual Devices P/L
> > Unit 8, 10 Hook St, Capalaba 4157 Australia
> > Ph: +61 7 38235717      Fax: +61 7 38234717
> > Our Web Site: www.audiovisualdevices.com.au
> > ___________________________________________
> >=20
> > --=20
> > http://www.piclist.com PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
>=20
> --=20
> Byron A. Jeff
> Chair: Department of Computer Science and Information Technology
> College of Information and Mathematical Sciences
> Clayton State University
> http://faculty.clayton.edu/bjeff
> --=20
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist

--=20
Byron A. Jeff
Chair: Department of Computer Science and Information Technology
College of Information and Mathematical Sciences
Clayton State University
http://faculty.clayton.edu/bjeff
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.