Maybe it is useful to remember that any protection circuit, like these SP72x
beauties, demands some complementary circuit to absorb the excess of current in
the power rail. Conventional regulators would not shunt the energy coming from
protection circuits to the VCC, allowing the VCC rail to jump well above limits.
The VCC rail should not be inductive in the path to the shunt, and the shunt must
be fast enough to deal with the energy surge.
-Augusto
On Qua 19/08/09 13:04 , OneStone o...@bigpond.net.au sent:
> Hi Michael. I specifically selected the SP724 because it has an SCR
> as
> its core. I have designed many ECU's in the past most using the
> SP724's
> big brother, the SP720, which became discontinued when Harris (the
> original manufacturer) shut down, and passed the baton to Intersil,
> who
> later passed it again to Littlefuse. I still have a couple of
> hundred
> SP720s in boxes somewhere. I never lost an input or an output line
> to
> one, despite some really nasty stuff, like fires, major load dumps,
> 42V
> connects to signal lines, even some idiot connecting a coil to one.
> In
> my opinion they are a better option for heavy duty automotive work
> than
> anything else I've found.
> Al
> tintronic wrote:
> > Al,
> > When I saw the word SCR, my first thought was "that's an
> interesting idea, that would allow the protection to absorb more
> current than the TVS solution I use, but then it would stay clamped
> unitll power is recycled". I finished reading the post and searched
> for the datasheet. In fact it is till open an the 4th tab prior to
> this one on my web browser. The way I saw it, the SP724 is a TVS.
> That is its function and purpose. The fact that it internally uses
> SCRs as opposed to zener or shotkky diodes or other techniques
> doesn't correctly classify it, in my opinion, as an SCR, but rather a
> TVS. Or am I wrong and the SP724 is an automotive solution that took
> the previously used arrays of single SCRs into a single chip, turning
> it into a TVS?
> >
> > That was my opinion and why I asked you if you meant TVS or in
> fact you meant SCR. I guess that depends on what you were looking for
> when you desided for the SP72x. Were you looking for an SCR array or
> were you looking for a high current TVS?
> >
> > My intention in writing the post was more to guide the OP, having
> him searching on the TVS concept and solutions instead of SCRs, if
> that was what you in fact meant, which is why I wrote it as a
> question. I also quoted what I understood by SCR, because it might
> just as well have standed for a different concept than Silicon
> Controlled Rectifier, althought on a quick search throught google and
> wikipedia I hadn't found any other electonic related concept
> abreviated by SCR.
> >>> Al,
> >>>> When I used to design for automotive I generally used a series
> >>>> resistor and an SCR clamp like an SP720 (...)
> >>> Surely you meant a TVS (Transient Voltage Supressor) instead of
> >>> SCR (Silicon Controlled Rectifier), right?
> >
> > Your quoting of the datasheet doesn't clarify this for me,so I
> would very much appreciate to know (if you remember, as it might well
> be that this is part of your backround knowledge), if you were looking
> for an SCR array or a high current TVS (or something else) when you
> came across the SP72x.
> >
> > Best Regards,
> > Michael K.
> >
> > --- In m...@yahoogroups.com, OneStone wrote:
> >> Read the spec for the SP724, you will find that each circuit is a
>
> >> combination of SCR's resistors and a diode. Whereas a transient
> voltage
> >> suppressor is actually a diode, or diode pair, and may be
> >> uni-directional or bidirectional.
> >>
> >> "The SP724 is a quad array of transient voltage clamping circuits
>
> >> designed to suppress ESD and other transient over-voltage events.
> The
> >> SP724 is used to help protect sensitive digital or analog input
> circuits
> >> on data, signal, or control lines operating on power supplies up
> to 20VDC.
> >>
> >> The SP724 is comprised of bipolar SCR/diode structures to protect
> up to
> >> four independent lines by clamping transients of either polarity
> to the
> >> power supply rails. The SP724 offers very low leakage (1nA
> Typical) and
> >> low input capacitance (3pF Typical). Additionally, the SP724 is
> rated to
> >> withstand the IEC 61000-4-2 ESD specification for both contact
> and air
> >> discharge methods to level 4.
> >>
> >> The SP724 is connected to the sensitive input line and its
> associated
> >> power supply lines. Clamping action occurs during the transient
> pulse,
> >> turning on the diode and fast triggering SCR structures when the
> voltage
> >> on the input line exceeds one VBE threshold above the V+ supply
> (or one
> >> VBE threshold below the V- supply). Therefore, the SP724
> operation is
> >> unaffected by poor power supply regulation or voltage
> fluctuations
> >> within its operating range."
> >>
> >> Cheers
> >>
> >> Al
> >>
> >>
> >> tintronic wrote:
> >>> Al,
> >>>> When I used to design for automotive I generally used a series
> >>>> resistor and an SCR clamp like an SP720 (...)
> >>> Surely you meant a TVS (Transient Voltage Supressor) instead of
> SCR
> >>> (Silicon Controlled Rectifier), right?
> >>>
> >>> merapcb,
> >>>>> Also, by the way, what is the best way of protecting the
> external
> >>>>> RS232 lines from unwanted (accidental) voltages getting
> >>>>> connected?
> >>> I guess, like assumed by Al, you don't mean to protect the
> device against a wrong connection but against voltage spikes.
> >>>
> >>> I agree that series resistor and TVS bypass should be an
> adequate solution. I'd add ferrite beads in series too.
> >>>
> >>> Regards,
> >>> Michael K.
> >>>
> >>> --- In m...@yahoogroups.com, OneStone wrote:
> >>>> You're welcome, the link doesn't work for me. When I used to
> design for
> >>>> automotive I generally used a series resistor and an SCR clamp
> like an
> >>>> SP720 (which I think is now discontinued) or the SP724. These
> could
> >>>> handle typical automotive load dumps, unlike some other
> possible
> >>>> solutions which were not designed to deal with such high energy
> loads.
> >>>> They are available from Littlefuse. The SP724 is a quad array.
> >>>>
> >>>> Al
> >>>>
> >>>> merapcb wrote:
> >>>>> Folks,
> >>>>>
> >>>>> Thanks to all for the various ideas and insights about this
> issue.
> >>>>> After deliberation it was decided to use UART0 (by re
> configuring from I2C when needed and then back to I2C). MUX can be
> done with 4052 as Al suggested. Thank you Al.
> >>>>>
> >>>>> I have posted the schematic of the proposed solution here:
> >>>>> http://tinyurl.com/lmn8k5
> >>>>>
> >>>>> Comments are most welcome, I have not used this before so I
> hope I got it right.
> >>>>>
> >>>>> Also, by the way, what is the best way of protecting the
> external RS232 lines from unwanted (accidental) voltages getting
> connected? (automotive environment). Diodes? Series resistors?
> >>>>>
> >>>>> Thanks
> >>>>>
> >>>>> PS - Pay attention to he nifty way of being able to connect
> more than one device to the same port (not at the same time) and
> having it sensed (well, nifty I think so)
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> ------------------------------------
> >>>>>
> >>>>>
> >>>>>
> >>>>>



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