AVR-Chat

You need to limit the current going into the clamping diode IN the AVR.
So, under over-volt conditions, one end of the resistor is at the AVR's
Vcc and the other is at the transient protector's "clamping voltage".  I
simply chose 5V as an example of a system Vcc value.

Hmm, I slipped a decimal point in my example. I would choose 7.5V as the
clamping voltage and 3.3V as your system Vcc if that is how it operates.
That means a drop 4.2V and a minimum resistor value of 42K (I'd choose
47K).

Even with this larger resistor value, 50KHz will be just fine.

Jim


> Got it.  Thanks.   But just to make sure I truly get it, in your equation
> "(7.5V-5V)/0.1ma", "7.5V" is the breakdown voltage and "5V" is the working
> voltage?   In my case, with 3.3 V Transguards and Vcc = 3.3 V, the
> equation
> would be (5.0-3.3)/0.1 = 17K?
>
>
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> From a practical point of view, what is the significance of the "clamping
> voltage"?
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> Clearly the Transguards are just fine for 50 KHz signals.
>
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>
> Steve
>
>
>
> From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
> Of wagnerj@proaxis.com
> Sent: Thursday, January 06, 2011 10:08 AM
> To: AVR-Chat@yahoogroups.com
> Subject: RE: [AVR-Chat] AVX Transguard
>
>
>
>
>
> If data rates will allow, simply add a resistor between the AVR port and
> the transient protector. I think (but may be wrong) that communication
> with Atmel has established a limit of 0.1ma forward current in the
> internal over-voltage protection diodes inside an AVR. Thus, if the
> transient protector will practically limit the input voltage to 7.5V, then
> a series resistor of (7.5V-5V)/0.1ma = 25K would be appropriate; I would
> use 27K or 33K.
>
> The thing to watch out for is rise/fall time at the port pin. A typical
> CMOS input has 5-7pf. Lets assume 5pf for the small geometry recent
> generation chips. So, in combination with 27K, the time constant would be
> around 140ns and the rise/fall time would be about 300ns. So, as long as
> the data rate on any protected input is less than 1bit/300ns, you would be
> OK.
>
> The transient protector is still important in your environment, especially
> with the possibility of lightning. It will limit the input voltage to
> something under 12V instead of 100s of volts.
>
> Jim
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