Why is the NAND gate so hot?

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tony_lth

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When I debug one design of 2-Input Positive-Nand-Gate, I found it was very hot sometimes and oscilated, but when I power-off it for several minutes, then it works well again.
What is the issue?
1. Should I put some resistors as load for each output?
2. Or it should not be connected as such way?
Thanks.
 

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  • Positive-Nand-Gate.JPG
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the layout of the ics physically may lead to the reason why it oscillates.
 
Hi, Srizbf,
What is "ics"?
Do you think the SCH I pasted is right?
Best,
Tony Liu
 

tony_lth said:
Hi, Srizbf,
What is "ics"?
Do you think the SCH I pasted is right?
Best,
Tony Liu
Click to expand...

integrated circuits -ics.

in the schematic , where is to pin 7 and pin 14 of U1D connected?

pin 7 of U1C is to AGND.why ? there is GND in power.
are the AGND and GND connected somewhere?
 
Hi,

Test if supply voltage is correct.
Don't leave the pwm input open = floating.
The IC may have other NAND gates inside. Don't leave any of their inputs floating.

Is there any load connected to the NAND outputs?

Klaus
 
Yes, Pin7 AGND is linked to GND somewhere.
Pin14 linked to +3.3V.

- - - Updated - - -

The 2nd layer is GND plane, all GND and AGND linked to the plane.
But the dielectric is polymide soft materials and only be 0.05mm thickness maximally, is this the root cause?

- - - Updated - - -

Hi, KlausST，
Nice to receive your reply. I had gotten your help before.
1. When working, PWM_In is a clock, but when non-work, no signal input. So I should put a 1MR connected to all the input, right?
2. One of the output of the NAND gate is floating, is this OK or not? Should I put a 1MR to its output?
Best,
 

What is the difference of the load resistors of 20K or 100K or 1M respectively?
Thanks.
 

Hi,

This is the most basic question for any electrician or electronic technician.

Refer to ohm's law.

I recommend to read some basic electronic tutorials.

Klaus
 
Thanks.
I am confused about one NAND-gate, it's SN74ALVC00PWR, I supposed its input resistance is about 100K.
But when I connected 4 inputs of the IC together, I found the port resistance is about 1K.
Am I wrong to do so?
Best,
 

Hi,

a complete circuit schematic could clarify.

A CMOS input definitely is very high impedance.

either there is a problem with the power supply, or you use wrong pins, or the IC is damged (ESD?)

How do you calculate the 1k?

Klaus
 
How about the gates not shown in the schematic? Open output is no problem but you shouldn't have open CMOS inputs.
 
Hi, All,
Thanks for your replies.
The SCH is as following.
In is +3.3V 2MHz square wave, from Pin3 and Pin6 output I got 3.3V Vpp waves, but at Pin12, which is linked to Pin9/10/13 together, I only got 1V Vpp waves, so that Pin8 and Pin11 don't have normal output. (+3.3V).
If I decreased R56 from 698R less, the voltage of Pin12 increased, so I estimate the pin12 input resistance is less than 1K.
Somewhere wrong?
Best,

- - - Updated - - -

PS: AGND and GND is linked together somewhere else in the circuits.
 

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  • 捕获3.JPG
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Hi,

If your schematic is true, there is only one IC and there is no soldering or other probkem...

Then I'd say the IC is damaged.
Maybe caused by ESD.
CMOS inputs are sensitive. Therefore your working place should be equipped with ESD protection.

For sure there may be other errors, like accidentally shorting two outputs... check on this.

If you don't find any wiring error, then I recommend to replace the IC.
It must not become hot, and the levels should be clean.

Klaus
 
The IC is weird, when just power on, it works well. But after some time and with some environment interference, the NAND gate output is always High level, not working anymore, unless power-off and waiting a long time.
Is this the damaged performance?
Thanks.
 

Hi,

you are busy with this cheap IC for some days now.

Try to replace it.


Klaus
 
And you mean damage because the pin12 Vpp is varied along of the 698R or 500R?

- - - Updated - - -

Could you recommend one to replace it? Thanks.

- - - Updated - - -

Actually I like one IC with bad performance, especially with big overshooting, could you suggest one?

- - - Updated - - -

Hi,
Do you think 54F/74F00 of NS is a good replacement?
And, one ouptut (Pin3) to drive 6 input(Pin4/5/9/10/12/13), do you think the current is OK for this config?
I really doubt that.
Best,
 

Hi,

And you mean damage because the pin12 Vpp is varied along of the 698R or 500R?
Click to expand...
no, ESD.

Actually I like one IC with bad performance, especially with big overshooting, could you suggest one?
Click to expand...
???

Do you think 54F/74F00 of NS is a good replacement?
Click to expand...
usually one replaces a damaged part with the same type..

And, one ouptut (Pin3) to drive 6 input(Pin4/5/9/10/12/13), do you think the current is OK for this config?
I really doubt that.
Click to expand...
Either you or i have to read datasheet(s) to answer this. I suggest you read the datasheet.

Klaus
 
Hi, Klaus,
I checked the datasheet for several times, its output driver is 24mA, but one input is 5uA, so it should be no problem.
But I found Pin8 (the 3rd gate) and pin11 (the 4th gate) can't work at the same time.
It seems that driver capability of pin3 is not enough.
Pls check the attachment.
 

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  • sn74alvc00.pdf
    870.9 KB · Views: 125

Why do you have R56? It should be a piece of wire.
The datasheet says that the recommended maximum current from any output is 24mA so with your 3.3V supply the minimum load resistance is 3.3V/24ma= 138 ohms. What is your load resistance on each output?
The datasheet says the maximum allowed total output current is 100mA so that is up to 4 outputs at the same time, each with a load of 24mA.
Edited
 
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