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Thread: Good Design versus what (usually) works - Tutorial / was ...
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picon face BY : Russell McMahon email (remove spam text)



>I think its all a matter of what you expect your circuit to see.  Adding
>external SOT23 Schottky diodes may be no better than relying on the
>internal "diodes", the current still must be limited by some means.  The
>SOT23 blowing up is no better than the PIC blowing up.  The SOT23 has
>current limit X, the internal diodes have limit Y.   The internal "diodes"
>are sufficient if I can guarentee a current limit significantly less than Y
>(keeping in mind that the Abs Max Voltage spec. must also be met).

Yes, a series input current limiting resistor is needed,
BUT
No, the situations are quite different.
When you use internal protection diodes they cannot start "protecting"
anything until they are injecting current into the IC itself and causing
changes in the voltages at points internal to the IC. Disaster is
threatening if you want it to also to chew gum while it is doing this. The
series resistor will limit the current for a given applied voltage but any
fault current drawn will go where it may do harm.

If you want the circuit to run properly when the Vin >> Vcc is applied then
you must limit Vinmax at the PIC pin to ~= Vcc
(and Vinlow to ~= 0v

If you use external diodes you can design the circuit to start clamping the
input voltages before the IC's internal diodes START to pass current into
the IC.
The reason to use Schottky diodes is that they have a lower "on" (forward
conduction) voltage than the internal silicm diodes so they will conduct
vefore the internal diodes do and shunt the "fault" currents to Vcc or
ground or where-ever the design allows them to.

In the first case the IC (PIC) is definitely violating the manufacturers
"normal operating conditions" specification.
In the latter case it may not be (it's up to the designer).

In practice if Schottky diodes are taken from input to Vcc and Ground
(appropriate polarity to prevent conduction during normal operation then
technically the input spec will be violated by the time the Schottky's are
conducting (about +/- 0.3 volts ouitside rails) but in practice this will
work with utterly minimal current injected into the protection diodes.


The following simple cct nominally allows the formal input Vinhi spec to be
met.
ZZZ is a zener at say 4.4 volt (hard to buy :-))(4v3 and 4v7 are standard
values)
Diode D conducts when Vin is > Vzzz + 0.6
A single zener can be used for many inputs but one diode is needed per
input.
the soft knee of a zener diode makes this cct less sharp than ideal.
It has the advantage of NOT pumping up the Vcc for large Vins.
A mirror image circuit referenced to Vcc would protect against low
Vin-low's.

.

                   D             |    PIC
         |-------|<|---- .---- | In
        Z                  |      |
        Z                  |
        Z                  |---------------RRR----- Vin
        |
Gnd-.-------------------------



     Russell McMahon
_____________________________

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(* - or woman, child or internet enabled intelligent entity :-))


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