Using Not Recommended Voltage Supply on IC

[mheruian]

Hi Folks,

Been a newbie in electronics for less than a year without a tutor, schooling or book, adviser etc. I am learning it via arduino approach, DIYs like that so not really technical and have strong foundation on fundamentals of electronics and electrical behavior.

I have an arduino interface on a 3 to 8 DMX SN74HCT138 where this DMX will activate some enable pins and chip selects, LEDs, etc. I was struck when this IC worked out when I accidentally supplied it with 3v3 supply. Though I know it should be 5v0 right? i even read the schematic recommending it needs 4v5 supply?? Why is that? Is it okay for me to proceed just using 3v3 supply? Its logic outs on the decoding outputs when high is about 3v0+ instead of 4v5+ if supplied with 5v which is good since i have SPI device that has 3v3 SPI device not 5v tolerant. Is there a drawback or side effect on the longer time implementation??

 

[FvM]

It has no guaranteed specifications (for speed, in- and output levels) with < 4.5 V supply, although it's probably working in most applications. You should preferably use 74HC138 type.

 

[d123]

Hi,

Why 4.5V? If you're curious and you can see an internal schematic of the IC (SN74HCT138), add up/guesstimate longest series string of devices voltage drops and that way you can guesstimate how much headroom may be left over at 5V. CD4xxx family and stuff like 555 can work below minimum 4.5V, down to 3V-ish allegedly but as with other devices, general rule of thumb is to bear in mind that the IC could perform erratically and not as desired - unexpectedly. Some block or other may not receive enough operating current with the headroom available, for example.

If it is working correctly as you are saying, then it's not recommended but for personal use may prove to be fine but only time will tell. Some are test parameters so if you want to abuse an IC in a positive way, only you can find out how far it can be pushed.

 

[dick_freebird]

Most CMOS "glue logic" ICs will work down to a couple
of volts - even old CD4000B 20V logic will work down
to 3V (if you like sllooowwww...). Drive strength and
delay will get much weaker and longer. But many cases
just don't care about that.

More complex logic with tight internal timing might
have more squirreliness at low supplies, where maybe
design sensitivities were never checked.

 

[KlausST]

Hi,

even old CD4000B 20V logic will work down
to 3V (if you like sllooowwww...).

Yes, CD4000 may be specified to operate down to 3V...
And here is the difference.
As long as a device is specified to operate at xx voltage you can rely on it's function.

But the OP asks to operate a device below it's specified voltage--> you can't rely on it's function.
And even if you test it ... you can't rely on it's function. It may refuse proper operation with time, with temperature....

The only reliable operation will be to operate the device within it's specified range.
Thus I fully agree with FvM's recommendation to just choose the correct locic family.

Why use an unreliable HCT type when there is a reliable HC type? And there are many other logic families that operate at 3V.

Klaus

 

[mheruian]

Hi Folks,

Thanks a lot for the help I learned a lot again. Last question though, I don't have HCT, should I use series resistors like 15 to 22 ohms on each outputs Y0-Y7 of the IC to get a 5v0 to 3v3 towards the SPI device data line inputs? or should I still use logic level shifter using mosfet? I lately know it's bad to use voltage divider since it's output is not stable right, due to paralleling the resistance of the load and the R2, something like that? Is that the same with series resistor of 22 ohms on SN74HCT138 outputs?

 

[BradtheRad]

By operating the IC at a non-spec supply voltage, it may create uncertain volt thresholds within it, as far as detecting lows and highs in its internal circuitry.

 

[mheruian]

@BradtheRad

ok, thanks a lot. I will now supply the DMX 5v then on its outputs Y0-Y7, each will have a 22 ohm resistor to have -/+3v3 voltage as its voltage logic out high

 

[KlausST]

Hi,

I neither know how you come to 22 Ohms...nor do I know how you want to connect it.
In my eyes it makes no sense.

Klaus

 

[mheruian]

Hi,

I neither know how you come to 22 Ohms...nor do I know how you want to connect it.
In my eyes it makes no sense.

Klaus

Hi Sir Klaus, i chose a worst case max current consumption of (or) input on the SPI lines of SPI device. Usually, other I saw only require 10mA and on the i based on this site calculator https://www.gtsparkplugs.com/Dropping_Resistor_Calc.html

and using ohm's law for me to see if calcu is correct:

V = IR
V/I = R
(5v0 - 3v3) / 0.100 = R
1.7 / 0.100 = R
17 ohm (i only have 22 ohms here since these are standard values on electronic shops.

Honestly, i don't know if I'm correct, did I? mess up on math? :bang:

 

[KlausST]

Hi,

some mistakes:

* 10mA is not 0.100A
* your worst case calculation calculates the maximum voltage drop, but you need a minimum voltage drop.
In other words: if there is reduced load, the voltage becomes too high.

Klaus

 

[mheruian]

Hi Klaus

Thanks a lot I want to learn electronics in terms of mathematics as much as possible.

uhm yeah, I use 100mA because other SPI devices use higher 10mA like 20mA, etc. My SPI device here is around 10mA only. So i did in order or if any case I change my SPI device into others that consumes higher, like I though 100mA is the max.

oh? ok, like I need to compute now the 100mA and 10mA as my max and minimum respectively, so now I have my max, going to compute the minimum and then in-between those two result, will be the resistor value i should choose? like averaging the two?.. so The website calculator I use also and my math approach is correct? only the variables are amiss??

 

[KlausST]

Hi,

I wonder where your "current values" come from. Honestly I've never heard about an SPI device that draws current in the range of 10mA at the signal lines.
They usually are orders of magnitude less.

No HCT device can sink current of 100mA. And the can source much less than sink.
Read the datasheet

Your resistor connection scheme is still unclear.
A single resistor in my eyes can't be a reliable solution.

I still recommend to use a suitable device familiy.
Usually: where "HCT" devices are available, there will "HC" devices be available, too. Look for it.

If you need more help you need to post your circuit - with supply voltages, all loads and every other information that is
involved with your DMX circuit.

Klaus

 

[mheruian]

Hi Klaus

Thanks for never ending helping me out

the SPI device I'm talking about is the CS5480 (here's the link), a power metering chip. I saw the 10mA on its page16 absolute rating.



my little DIY using arduino is the circuit below:



i just started using arduino, almost 2 weeks and learning spi through vids and diys only so i haven't gotten far on this little own project of mine :???: so pretty much its like that, i plan to integrate 5 to 10 CS5480 chips and use dmx as slave selecting. then after that, I would like to include relay module to act as breakers and rs-485 for long distance communication like up to 20 meters? but no knowledge yet on rs-485. Upon skimming, i saw i should do that for wired communication.

What should I consider? so i should do series resistor and go for logic level shifter??

 

[KlausST]

Hi,

Absolute max IO current rating.
This not the recommended/typical IO current.
This current will only flow when the IO input voltage is beyond the supply rails...and you have to take care not to cross this limit.

Typical input current is specified with +/-1uA.

5V (supply) signal to 3.3V (supply) signals:
* resistive voltage divider (two resistors)
* or AHC type logic (5V input tolerant while supplied with 3V3)

3V3 (supply) signal to 5V (supply) signals:
* read datasheets if the 3V3 supplied device output voltage is high enough for the 5V supplied input.
* use a 5V supplied HCT logic. (Is happy with 3V# input signals but generates 5V output signals)

All this has been discussed many times here in this forum and in the internet.

For long distance via RS485 you find 3V3 as well as 5V transceivers. --> no need for extra level converters.

Klaus

 

[mheruian]

Hi Klaus

i see thanks a lot again,

can i ask where did you see, what page was input current of 1uA? input leakage current?

so using voltage divider between arduino and SPI lines is the best pracice and there will be no change in Vout no matter what changed on load (cs5480), Voltage divider > logic level shifter?

so i guess i should buy HC dmx instead of HCT since my arduino is HC and cs5480 is HC right? for me to save component placement, cost and space on board?

for rs-485: oh thanks right now I'm reading something about it. I use ISL3260X of intersils since they said it was better since it has voltage suppressor/ESD protection said by the seller here. I want to ask some additions to this. I will create a separate thread for this right?

Thanks a lot again Mr. Klaus I appreciate everything. I'm not so genius though but been trying hard to understand things out as much as possible

 

[KlausST]

Hi,

can i ask where did you see, what page was input current of 1uA? input leakage current?

Yes, in this datasheet they call it "leakage" current. In other datasheets the just say "input current".

so i guess i should buy HC dmx instead of HCT since my arduino is HC and cs5480 is HC right? for me to save component placement, cost and space on board?

There is a confusion: in the first post there just was the supply_voltage problem of the HCT device.
Now we addtionally have the 5V (ARDUINO) to 3v3 (CS5480) voltage converson problem.
To solve both in one device you could use the recommended AHC family: --> 74AHC138

so i guess i should buy HC dmx instead of HCT since my arduino is HC and cs5480 is HC right?

not exactly.
ARDUINO is 5V CMOS
CS5480 is 3.3V CMOS or LVCMOS

If there is some time: I recommend to read through logic family comparison table. Focus on supply voltages, input voltages and output voltages.

In either case you need to read through the datasheets of the devices you use. (We professionals need to do this, too. Booring, but essential)

HC:
* Supply voltage 2V..6V,
* Input voltage: -0.3V ... VCC+0.3V.
* valid HIGH input level: about 0.7 x VCC
* output voltage: close to the supply rails.

HCT:
* Supply voltage 4.5V..5.5V,
* Input voltage: -0.3V ... VCC+0.3V.
* valid HIGH input level: about 2V
* output voltage: close to the supply rails. (Close to VCC is because of the CMOS stage. True TTL outputs are 3.5V HIGH only.)

AHC:
* Supply voltage 2V..5.5V,
* Input voltage: -0.5V ... 7V. Mind: Here it is independent of VCC! Thus thee are 5V input voltage tolerant while supplied with 3V3
* valid HIGH input level: about 0.7 x VCC
* output voltage: close to the supply rails.

ISL3260:
Check price and availability.
There are a lot of other RS485/RS422 converters. Most of them are rugged and reliable (industrial product).

Klaus

 

[mheruian]

Hi again Mr.Klaus!

Yes, in this datasheet they call it "leakage" current. In other datasheets the just say "input current".

Ok, i never thought it is since from the "leakage" term, from others i heard it was like even when a component is close or not active like ssr's, leakage current is some current that still passes through but small amount. Here's since it was said "leakage" as input not output means the different way around. Thanks for that.

There is a confusion: in the first post there just was the supply_voltage problem of the HCT device.
Now we addtionally have the 5V (ARDUINO) to 3v3 (CS5480) voltage converson problem.
To solve both in one device you could use the recommended AHC family: --> 74AHC138

Oh yeah? - :thumbsup: i mean if I use HC, i won't use any logic level shifters on its output towards SS of the CS5480 chips, only 3 (MOSI,MISO,SCK). So it's not best arduino's 5v outputs to dmx's inputs powered at 3v supply then dmx's 3v output to CS5480 slave select input.

Ok. now i get it right haha! HC and HCT input voltage depends on their VCC, even if these ICs are specified to be powered at 3v3 or 5v, ICs will be damage if I input voltage like IC is supplied by 3v3 and input is 5v (which will not happen if ic is supplied by 5v and input is 5v)

In either case you need to read through the datasheets of the devices you use. (We professionals need to do this, too. Booring, but essential)

yeah, i guess there really is a time where i should consume a lot of time for things like this.

thanks a lot Mr.Klaus!