I have some designs that require several 4017 ICs to be switched in and out of the circuit intermittently, for counting circuits that require more than 10 outputs.
The way I am currently designing this is to have a master counter, with outputs connected to Vcc of the slave 4017s, to basically remove the chip from the circuit and activate the next one when the count advances.
Will this approach work, or will the chips suck power from another pin?
Also, is there a counter chip similar to the 4017s but with more than 10 outputs?
Thanks!
Best Answer
The quick answer is, no. It won't work. Most IC's have ESD protection diodes on the inputs and sometimes the outputs. If the chip is powered down, it will be like having a diode to 0 volts on the signal. Best case, that signal will see a large load on it. Worst case, the signal+diode will actually cause VCC to go up, causing the chip to actually power up.
Here's a schematic that shows some protection diodes. The schematic isn't great, but it does the job. Imagine everything to the right of the dashed line to be inside the chip:
The two diodes from ground and to VDD are there to prevent the input signal from going too much above or below the power rails. Everything to the right of the diodes can safely be ignored, as they are chip specific.
As you can see, if the input goes above VDD (which is 0 volts when powered down) then there is a current path from the pin to VDD.