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safe to drive inputs on an unpowered IC

I am trying to put together a battery-powered cicruit for our lab that does nothing more than show the state of logic lines between two pieces of equipment using LEDs. Since one of these is the output of an 82C55, I need to power the LEDs separately, e.g. using a hex inverter. I've put together this kind of thing before, but never in an application where parts of the circuit, in this case the buffer/inverter, LEDs, etc. would only be powered occasionally (we only need to see the state of the logic lines occasionally, and would want to conserve battery power in between).

So, my question is, is it safe to have the inputs of the inverter (e.g.

74HC04) tied to active logic lines when the IC itself is unpowered? If not, is there an easy workaround (e.g. another kind of IC)?

Thanks ahead of time.

Reply to
knxnst
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Depends what you mean by safe. The unpowered IC will probably power itself through its protection diodes leading to unexpected behavior. Use the LCX or VHC family of logic. eg 74vhc04 or 74lcx04

Reply to
a7yvm109gf5d1

Possibly only if the inputs are subjected to negative spikes, i.e., below ground. If, for some reason, power is applied to the IC while the input is negative, it may go into latch-up and let the smoke out.

Al

Reply to
Al

I believe a ULN2003 inverter can be safely driven whether powered or not.

Luhan

Reply to
Luhan

Correction: ULN2003 does not have any power feed. Its just a bunch of darlingtons with some diodes for the VCC connection.

Luhan

Reply to
Luhan

As others have written it'll try to supply the circuit through the inputs that are high. If you have tens of kohm in series it may be ok but you'd have to hold down VCC if you want to avoid unpredictable behavior of the other logic that is hanging on that rail.

The real way to do it is using transistors. With BJT the input current goes to ground and with FETs there is no input current. When using FETs make sure there are gate protection diode against excessive Vgs in either direction. Either version needs series resistors, of course. But so does your 74HC04 because if you exceed the abs max and then fire up the circuit it would latch up.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Look at the OnSemi NL17 series. The inputs tolerate up to 7 volts, powered up or not.

Or, A Joerg suggests, use some 2N7002's.

John

Reply to
John Larkin

What's wrong with dropping in a LCX or VHC part? No added parts needed!

Reply to
a7yvm109gf5d1

It just occurred to me that LM339 inputs can go above rail, powered or not.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Yes, by a lot, and very handy at times. Some data sheets make this very clear, and some hide it pretty well.

Reply to
John Popelish

Yep. Even National's data sheet is obscure about it :-(

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

Do they have esd diodes to Vcc?

John

Reply to
John Larkin

Many, many thanks for all the info/suggestions. Time to start looking over some datasheets.

-Dave

Reply to
knxnst

They're also obscure about going *below* the rail! That's a lot more interesting.

John

Reply to
John Larkin

Some of those parts (which I redesigned for ONSemi) have "magic floating" ESD structures ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Naaah! They say, "Don't", but there *are* ways ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

The datasheet says "input protection circuit" for power down operation. There is no diode shown on the data sheet. I'm using an LCX part in a project where I want to avoid the classic self powering problem and so far, it works, which leads me to think there is no diode to VCC.

Reply to
a7yvm109gf5d1

Abs max says -0.3V. So does note 6. So does the text on page 7. Ok, nothing in writing about performance but as we all know...

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

May be stupid response but .... how about keeping the 74HC04 powered all the time, and removing power to the LEDs? Might be ok if the switching speed isn't too great, so the 04 doesn't use too much power internally. hth Neil

Reply to
neil

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