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Clarification on half powered circuit board

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Hi,

I have this clarification, I have a hardware circuit board and if I am partially powering on the board that is some IC's are powered on and some IC's are not powered ON. what will be the behaviour of these powered off devices, like is there a restriction that on current flow through them? If current flows through some pins without providing the VCC, does the IC get damaged? Just an extension for the same if I have two seperate boards which are connected together and if I power ON one board and power off the other board. What precautions I need to take?

Thanks in advance,
Regards,
Satya
 
If current flows through some pins without providing the VCC, does the IC get damaged?
Possibly. Why don't you power-up all the ICs to prevent that happening?
 
Certainly it would affect some of the ICs.But there are some of the ICs in which the Vcc pins and the input pins were in the same voltage nodes.

precaution is better to short the supply pins of boards.i.e (1) Positive terminal of Board1 and Positive terminal of Board2. (2) Ground terminal of Board1 and Ground terminal of Board2.
 
Most IC´s have protection diodes on inputs, so if you feed 5V into an input of unpowered device without any series resitor, the IC will get powered through the upper protection diode and will have ~4.3V on it´s power rail, thus powering all other ICs connected to the same bus.
Also note that linear regulators have another protection diode going from ouptut to input, so this way you might turn on even devices on the 3.3V rail if there is one, also the load on the total load might bee too high and then something will give way - either the first output or the protection diode. If the protection diode fails open due to overcurrent, you will probably destroy the IC via SCR latchup.
 
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