Re: Pls help me find extremely small 1:1 voltage-tracking regulator

On 25 May 2005 12:05:16 -0700, the renowned "Bo-Ming Tong"
<cyberstudio@xxxxxxx> wrote:

>Yes, thanks for the advice...
>
>I am rethinking this a bit due to the advice of you and Walter. The
>reason why I wanted my power supply to track the other power supply is
>that I am afraid of the I/O pin protection clamp diode conducting (on
>either side), blowing things up. But as I think about it, using a
>high-current rail-to-rail opamp leads me back to exactly the same
>original problem that I am trying to solve. I am just dealing with a
>protection diode in a different place. And on top of that I need to
>worry about output capacitance like you said.
>
>As you can see I have about 0.3V voltage difference between the 2 power
>supplies. I guess I can just add 10k resistors to each pin of the
>interface. If the power supply differences turns out to be great enough
>to trip the protection diodes (in the normal case this should not
>happen), the protection resistors should be able to limit the current
>and saving me from blowing things up.

Let me preface this by saying that I don't have a clear idea of what
you are trying to do. Is this correct?

i) You have system A with a power supply that's "about" 5V.

ii) You are designing system B that has logic level connections to
system A.

iii) You wish it to be reliable.

If that's correct, maybe you can just use level translation methods to
deal with any mismatch.

Simplest way is to put some series resistance in the signal lines. I
won't go into that, there are plusses (cost, mostly) and minuses (may
violate the abs max input voltage rating, may cause unforseen effects,
slows down signals etc.).

You could also use 74HC4049/50s (with dual Vcc connections- one Vcc
for inputs- tied to the internal Vcc, and one Vcc for outputs- tied to
the external Vcc) or other level translation chips to inexpensively do
this. That part will tolerate inputs as high as 16V and has a
reasonable input threshold for CMOS. There are also other specialized
level-translation devices:

http://focus.ti.com/pdfs/logic/dsprodclip.pdf

The 74LVC245 is one jellybean choice (accepts inputs as high as 5.5V)



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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