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From: firstname.lastname@example.org (Bob Wilson)
Subject: Re: blocking signal to unpowered CMOS
Date: Wed, 20 Nov 2002 04:49:20 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article ,
>Is it good practice to use an NPN (3904) transistor to block a signal
>to an unpowered CMOS IC? I have a circuit that has a 5V signal on the
>collector all the time. However when I switch off the circuit, the
>base is left connected to other unpowered circuitry and the emitter of
>the 3904 is connected to the input of the unpowered CMOS IC via a 10k
>resistor. The ground for the circuit is always connected. When the
>circuit is powered up the IC gets power and the base of the 3904 also
>receives power thus allowing the signal to pass. (I know they're may
>be timing issues as to which powers up first)
>I've read about clamping diodes, but they only limit the signal's
>voltage range and not block it from an unpowered IC.
>Is this good? Bad?
An unpowered CMOS IC will do funny things when voltage is applied to its
input. What happens is that the voltage goes through the upper of the
protection diodes, and onto the interal Vcc line, and the IC springs to
There are a variety of ways to ensure the inputs are left "unpowered" choose
one that works for you.
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