My question really wasn't about overloading a PIC input (I even specified i=
t
won't happen, at least from the shunt normal operation) but if there could =
be
some "parasitic" or anyhow difficult to understand phenomena that will make
the shunt go negative. I'm thinking about overshoot, etc.. although theoret=
ically
this should not happen.

Anybody that can elaborate on it please?



>Hello,
>I'm playing with my SMPS step up (boost) design, where the controller is a=
 PIC.
>
>I thus have +12V power input, an inductor, a MOSFET, and a diode that char=
ges a
>high voltage capacitor, and a voltage divider before the diode, to=20
>monitor voltage
>(via a first PIC ADC input) of the cap without actually discharging it=20
>continuously.
>
>The gate of the MOSFET is connected to a +5V rail through a 1k=20
>resistor and I use
>a PIC output in open drain configuration to ~quickly stop the MOSFET=20
>from conducting
>(the opposite doesn't need to be ultrafast, as the SMPS will be always use=
d in
>discontinuos mode, thus current is zero whenever the MOSFET starts to cond=
uct).
>Switching frequency is pretty low.
>
>While it's certainly not the most performant boost circuit, I like its=20
>simplicity
>and low components count.
>
>Now, I also need to monitor current (for which I have devoted a second=20
>ADC input),
>and thus I have placed a shunt resistor between the source of the=20
>MOSFET and the
>0V rail. I can be 100% sure that the voltage out of it will never=20
>exceed the PIC
>ADC max input voltage, also due to insufficient input current from the=20
>power supply.
>The shunt has also pretty low resistance so the voltage never reachs=20
>0.5V (not much
>resolution from the ADC, but enough to not want to add an OpAmp, as=20
>space on board
>is very very limited).
>
>Question is: should I put a series resistor between the PIC ADC input and =
the
>shunt? Theoretically the voltage will never go below 0V and will never go =
above
>0.5V (as explained above), but I fear some insidious aspects that=20
>maybe I haven't
>been able to consider, maybe for some hard to grasp parasitic=20
>capacitance, etc..
>could the shunt voltage go below 0V and thus fry my PIC ADC input pin=20
>unless I put
>a series resistor? Is this series resistor necessary?
>
>If it's totally useless, I don't want to put it, it's a matter of principl=
e. :D
>
>Thank you very much for augmenting my design knowledge and experience.
>
>Kind regards,
>Mario

--=20
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