AVR-Chat

Hello guys.. I have an application where in the output pin has a 
greater posibility being shorted to VCC. Now when this pin is set low, 
it causes high current. To avoid this, i am thinking of placing a 
buffer after the output pin (i dunno if this is the best idea, i 
appreciate feedbacks). I am searching the farnell and i saw some 
LVC07A from TI. Can anyone can recomend a similar device with only a 
single (or two) buffer in it? running from 1.7 to 3.3 VCC..

regards,
mago

magzky02 wrote:
> Hello guys.. I have an application where in the output pin has a 
> greater posibility being shorted to VCC. Now when this pin is set low, 
> it causes high current. To avoid this, i am thinking of placing a 
> buffer after the output pin (i dunno if this is the best idea, i 
> appreciate feedbacks). I am searching the farnell and i saw some 
> LVC07A from TI. Can anyone can recomend a similar device with only a 
> single (or two) buffer in it? running from 1.7 to 3.3 VCC..
>   
What are you driving? How fast? How big a load? The LVC07A is open 
drain, right? Do you need to source current too?

There are high-current "low-side FET" or relay drivers available in 1- 
and 2-driver packages. Some are overload protected.

Then there's the automotive P-FETs with current limiting, but these are 
probably much higher current than you need.

If you're looking at using an LVC07A, you should consider the 
single-gate "tiny logic" drivers.

There's a lot of unsaid things here, even a series resistor might
work, depending on what you're doing with the pin.

On Wed, 20 Jun 2007 09:43:47 -0000 you wrote:

What does the output pin normally drive? How does it get shorted to VCC?

>
>Hello guys.. I have an application where in the output pin has a 
>greater posibility being shorted to VCC. Now when this pin is set low, 
>it causes high current. To avoid this, i am thinking of placing a 
>buffer after the output pin (i dunno if this is the best idea, i 
>appreciate feedbacks). I am searching the farnell and i saw some 
>LVC07A from TI. Can anyone can recomend a similar device with only a 
>single (or two) buffer in it? running from 1.7 to 3.3 VCC..
>
>regards,
>mago

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A series resistor was my first solution but it didnt work because of the the 49 Ohm pull up resistor at the other end of the line. when the uC pin set the line low, the output voltage will become a voltage divider and is quite high unless i will use a very low series resistance (compared to 49) which will defeat my purpose. I can not change the 49 Ohm pullup resistor at the other end because it is another system i just want to control.

regards
mago

David VanHorn <microbrix@gmail.com> wrote:                                  There's a lot of unsaid things here, even a series resistor might
 work, depending on what you're doing with the pin.
 
     
                       

       
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On 6/20/07, mago Umandam <magzky02@yahoo.com> wrote:
> A series resistor was my first solution but it didnt work because of the the 49 Ohm pull up resistor at the other end of the line. when the uC pin set the line low, the output voltage will become a voltage divider and is quite high unless i will use a very low series resistance (compared to 49) which will defeat my purpose. I can not change the 49 Ohm pullup resistor at the other end because it is another system i just want to control.

Knowing this, I'm wondering why you don't use a 2N7000 or similar
mosfet as a buffer.
It will crank that line to ground, and draw essentially no current
from the AVR while doing it.

You might consider leaving it as is. Most of the AVRs can take
a short to VCC without killing the device. Is the short likely
to happen during development, or in use? Short or long time?
With or without a buffer, a short will cause loss of functionality
during the short; you may find the risk of permanent damage is higher
with the buffer - the current and heat will be higher.
Ken

>
> Hello guys.. I have an application where in the output pin has a
> greater posibility being shorted to VCC. Now when this pin is set low,
> it causes high current. To avoid this, i am thinking of placing a
> buffer after the output pin (i dunno if this is the best idea, i
> appreciate feedbacks). I am searching the farnell and i saw some
> LVC07A from TI. Can anyone can recomend a similar device with only a
> single (or two) buffer in it? running from 1.7 to 3.3 VCC..
>
> regards,
> mago
>
>

At 12:25 AM 21/06/2007, you wrote:
>the 49 Ohm pull up resistor at the other end of the line.

The first thing is to try and remove the possibility of shorts
to VCC.

Is the resistor tied up to 5V at the other side? That would be
a 100mA sink you are trying to get from a pin!! Even at 3.3V it is
still about 70mA. Then you have to consider the possibility of having
you board powered off and the other powered on (maybe it can't happen??).

As a minimum I would fit an open connector (drain) driver and perhaps
even a small fuse.


Regards

John Samperi

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