At 07:25 PM 2/4/2003 -0800, you wrote:
>Spehro Pefhany wrote:
>
>>You could consider 3 74HC4053s. No isolation, but < 80 cents total.
>
>These come really close.  10V Vcc-Vee is the absolute max, though, I'd
>hate to push this.

http://www.fairchildsemi.com/ds/MM/MM74HC4053.pdf

+/- 7.5 seems to be abs. max.

>Also, the ON resistance is fairly high (typical 70, max 160).

The on resistance is highest well inside the Vcc- Vee voltage range, and
is lowest at the rails, right where you'd use it. Typical is more like
20-30 ohms, you can probably scale that safely by the typ:max ratio.
See the curve on page 7.

>The
>signal must drive a line that ultimate terminates in two diode drops to
>ground, so a total of about 200 ohms needs to be used in series to drop
>from 5V Vcc to 1.2V nominal at 20 ma.  The output FET would then be
>about a third to two thirds of this--wouldn't this make for a lot of
>spread in current values between parts?

I'll leave those calculations to you, as I don't know your
application.

Best regards,

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

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