XOR gate IC supply current

1. XOR gate IC supply current

Hi,

I have a circuit that I need to use XOR gate IC using battery power. The chip in question is 74HC86.

As per data sheet the supply current should be between 20 to 40 micro amp, but , if i just power up the chip using vcc and ground, it is using 3 to 7ma of power. tested different chip but same result , the supply voltage is 3.3v, at 5v, it is taking 23ma. am not sure what mistake am doing. the circuit ultimately would use only one gate, although it has 4.

datasheet https://www.diodes.com/assets/Datasheets/74HC86.pdf

Thank you

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2. Re: XOR gate IC supply current

Hi,

The Icc of 20-40 uA is static current. You'd notice that it is specified for Io=0A. You'd also notice that the absolute maximum Icc current is specified as 50mA.

You are not doing anything wrong. The 7mA and 23mA that you get are dominated by dynamic current.

If you want to measure the static Icc, then you'd have to ensure that Io=0A.

3. Re: XOR gate IC supply current

Akanimo,

Thanks for the reply, I have connected only vcc and ground to test,, rest of pins are floating. so Io should be zero.

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4. Re: XOR gate IC supply current

you might want to see what you get with the input pins all tied low
floating input pins can float high, making an output high, without current
which changes internals and can change supply current

5. Re: XOR gate IC supply current

Another thing to consider, what instrument are you measuring with?

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6. Re: XOR gate IC supply current

Originally Posted by wwfeldman
you might want to see what you get with the input pins all tied low
floating input pins can float high, making an output high, without current
which changes internals and can change supply current

got it, let me check that

7. Re: XOR gate IC supply current

Hi,

General rule:
Never leave (unused) inputs floating. This is true for digital circuits as well as analog circuits.

This has been discussed many times here in this forum and more often in the internet.

Also in the datasheet there should be the test conditions for the static current.
Usually: "All inputs are GND or VCC".

Klaus

8. Re: XOR gate IC supply current

If CMOS inputs are left floating, the input voltage is not defined and it can end up at the midpoint voltage where both the N and P MOSFETs can turn on (in normal operation one is fully on and one fully off so only leakage current flows).
That's likely what's causing the observed current.

I once was using a CD4000 series logic gate operating from 12V where I didn't tie down the inputs.
It drew so much current that the IC got hot.

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9. Re: XOR gate IC supply current

Thank you everybody, so after making the input pins tied to ground, all is normal now, the current draw is at 5ua. the first pcb I made out of general pcb did not show high power usage but the second one did, so thought I made a mistake or bad chips.

10. Re: XOR gate IC supply current

Originally Posted by krazy707
.... the first pcb I made out of general pcb did not show high power usage but the second one did, so thought I made a mistake or bad chips.
Each chip will respond differently to floating inputs, due to slight fabrication differences between units.

11. Re: XOR gate IC supply current

Originally Posted by krazy707
Akanimo,

Thanks for the reply, I have connected only vcc and ground to test,, rest of pins are floating. so Io should be zero.
Absolutely not. You can get static current that is beyond all specs if a pin is left floating. Have seen it time and time again.

12. Re: XOR gate IC supply current

Originally Posted by ThisIsNotSam
Absolutely not. You can get static current that is beyond all specs if a pin is left floating. Have seen it time and time again.
You're a little late to the party.