> Has anyone worked on a system using PWM to create a SMPSU where the
> PIC/MCU supply is being generated from the PWM. I know this will need a
> bootstrap mechanism and I guess that is what I am most interested.

You need to be more specific about

 - available supply voltage and

 - PIC minimum startup voltage. Part number would help muchly.

- acceptable startup conditions / triggers.

For example, if startup is a user switch action you can drive a
voltage doubler with a single switch action. The excessively enthused
can manage double-double, with diode losses with a push and release
two part action with an eg DPDT switch.

If have 2 cell supply it is much easier than 1 cell supply.
etc

eg With a simple voltage double Cpump will charge to Vbat - Vd where
Vd =3D a diode drop.
Switch action produces Vbat + (Vbat-Vd) - Vd =3D 2Vbat -2 Vd.
You can remove 1 x Vd loss by resistor charging the cap originally and
assuming that back flow during pumping is small.
You can also "get synchronous" and eliminate Vd drops.

Fron 2 (Vb-Vd) and one cell and say Vd =3D 0.4 then if Vbat =3D low =3D 1V
you get 1.2V with 2 diodes, 1.6V with one diode and 2V with no diodes.
Not too marvellous except for 1.8V parts.

If Vbat is 2V min you get 3.2V, 3.6V, 4V respectively =3D good/better/best.

If you ring an inductor yiou can get almost any voltage you want.
Extra points for ringing inductor which is subsequently used by PIC.

You can get 1 cell to X Volt smps IC's for < 10 cents in Asia. These
can make the task MUCH simpler (but less fun)

More details please.


  Russell McMahon





             Russell
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