Dear Veronica,


At 01.33 2012.06.12, you wrote:
>Hi all
>
>Has anyone worked on a system using PWM to create a SMPSU where the=20
>PIC/MCU supply is being generated from the PWM. I know this will need=20
>a bootstrap mechanism and I guess that is what I am most interested.

Yes, in my buck controller the PNP switch is driven by a NPN transistor,
whose base is pulled up. The Vreg of the MPU is after this PNP high switch.

When power arrives, the PNP high side switch is not fully saturated but
lets nice electrons pass, thanks to the pullup on the NPN transistor that
drives the PNP base, and power leaks into the VReg and consequently the MPU=
,
which is held in reset until power is good ( =3D voltage at VReg is within
98% or so of stable output voltage).

At this point the reset line is not held down anymore, the MPU gets out of
reset, and drives the NPN transistor base actively, 0 or 1, not tristate
anymore, being thus able to saturate the PNP transistor or switch it off
completely.

It works, and the oscilloscope shows that it even works very well.

I can give more details if you want, e.g. it's even a high tension power
input (400V). NPN collector into PNP base (through a resistor of course),
NPN base pulled up via high value resistor to PNP emitter. Not exactly,
as the pullup resistor doesn't go into NPN base but into MPU pin, and
the latter goes, through a low value resistor, into NPN base. The MPU pin
never seen high voltage, as it's clamped by the NPN base, through the low
value resistor. Currents into base, out of pin, etc.. are all much within
absolute max values, always, when MPU I/O pin is tristated, as well as when
it is actively driven to 0 or 1.

One difference with your design is that I don't use PWM, but I drive the
NPN base directly via an algorithm (my MPU is doing much more than the
buck part, it's a complex/articulated system), but it doesn't change the
substance, as after reset you can enable the PWM module instead of bit
banging the I/O pin like I do.


>Thanks

I'm really very glad if I had a chance to help you.

Cheers,
Mario


>
>Veronica
>
>
>--=20
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