Understand the Current Consumption of an IC in its Datasheet

[mheruian]

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Hi, I'm new here and maybe this is the best section where I could need help :) I've been interested in electronics lately (not schooling anymore, more on self-study, but I'm not that smart, just a dedicated one and i can't understand everything right through). I don't have any equipment yet like osci, etc. Then I thought of doing mathematics on the physics side of electronic components like integrated circuits (IC) since even though ideal, math in some percentage of little marginal error, result should reflect close to reality (simple math is what I rely on).

Here's where I am at, I'm trying to know the current consumption (minimum and max) of this IC (http://www.ti.com/lit/ds/symlink/sn74hct138.pdf). Upon reading the documentation, I am at first thought it was easy since 80uA was brought up in the description but skimming on the electrical characteristics (image link below) looks like this is the max current consumption when there is no output current (Io = 0) on its decoding output lines.

https://drive.google.com/file/d/1b4nBmQ8Yd2g5oYXemnUTcV-VD0B0YhtM/view

That means the IC is in idle state right consuming current just for powering it self up and not for output/decode operation? Then i found the continuous current output and continuous current through VCC or GND? I got confused but still proceeded.

Here's what I computed for getting current consumption for output operation:

I_consume = I_cc + I_OL
I_consume = 80uA + 4mA
I_consume = 80uA + 4mA
I_consume = 4.08mA

I_min = 1000nA or 1uA (this is idle state also?)

I_max = 50mA (Maybe an addition how did they get this?)

If I can determine these currents in the 3 states of the IC, I can now know the power consumption of the IC. I hope someone could verify this or correct me.

 

[Tom.G]

That means the IC is in idle state right consuming current just for powering it self up and not for output/decode operation?

Correct.

Here's what I computed for getting current consumption for output operation:

Iconsume = Icc + IOL
Iconsume = 80uA + 4mA
Iconsume = 80uA + 4mA
Iconsume = 4.08mA

Note that the 4mA is for each output pin.

Imax = 50mA

That is the current thru the V_CC or GND pins that, if exceeded, will cause internal damage to the device. That limitation is indicated by the table heading on the Data Sheet of "absolute maximum ratings..."

Imin = 1000nA or 1uA (this is idle state also?)

That is the most current that will flow into or out of any Input when that input is at either V_CC or GND.

NOTE: The ∆I_CC (in electrical characteristics... table on pg4) shows that if an input is at 0.5V or 2.4V then the supply current will increase by 2.9mA for each input at that voltage. This is due to the internal construction of the IC. For each input, there are two transistors in series that connect to V_CC and GND. When an input is at either 0.5V or 2.4V both of the transistors ar turned ON, supplying a direct path between V_CC and GND.

Cheers, and welcome to the world of Electronics,
Tom

 

[mheruian]

Thanks a lot Tom.G ! :D just got home and did some familiarization on technical terms. I hope I'm using the right technical terms.

That means the IC is in idle state right consuming current just for powering it self up and not for output/decode operation?

Correct.

Ok? so that means its current consumption while no operation would be 8uA (25°C ambient temperature) up to 80uA (if SN74 IC was used), this 80uA will be achieved if temperature is either more than OR less than 25°C? Sorry if I can't pretty much perceive it since I'm new to this, datasheets are quite not specific for my understanding haha!

Note that the 4mA is for each output pin.

Oh I see, right now, I'm looking again at the datasheet at the Function Table pg.2 - So since there can only be 1 I_OL (1 Low output pin) on every input combination, I_consume while on operation is always 4.08mA? but this consumption will be decrease since the 7 remaining output pin would be High - I_OH, that'll be:

Total I_OH = -4mA x 7
Total I_OH = -28mA
Total I_consume = 4.08mA - 28mA
Total I_consume = -24mA (this'll be like no current was consume or power used?)

Do i get that right?

That is the current thru the V_CC or GND pins that, if exceeded, will cause internal damage to the device. That limitation is indicated by the table heading on the Data Sheet of "absolute maximum ratings..."

Ok? so like power surge? just this day, I had a talk on a colleague of mine at school on old days. Well, he is smart that's why I ask him some questions too though he is not into electronic stuff (he has the highest grade on physics on our class that time). I show him the datasheet and ask a lot of stuffs then he said that "load will only consume a current that it needs with respect to its total resistance...". That made me think, how come such surge could happen if the load (circuit, appliances, etc said by my colleague) will only use current that it needs? some resistors on the load worn out so current it takes increases??

That is the most current that will flow into or out of any Input when that input is at either V_CC or GND.

Got it! so that's the maximum I_input that applies on all the ICs pins? I'm seeing this Test Condition "VI = VCC or 0", so even you input 0 volts on any pins, it will still consume current? why is that? isn't 0 volts = 0 charge / current flow?

NOTE: The ∆I_CC (in electrical characteristics... table on pg4) shows that if an input is at 0.5V or 2.4V then the supply current will increase by 2.9mA for each input at that voltage. This is due to the internal construction of the IC. For each input, there are two transistors in series that connect to V_CC and GND. When an input is at either 0.5V or 2.4V both of the transistors ar turned ON, supplying a direct path between V_CC and GND.

Ok, i got this one as clear as day. Thanks for helping me understand it :) Quite having some goose bump since I'm excited to know a lot at the same time, my mind hurts on some confusions i don't get. BTW, thanks for the warm welcome on the electronics world!


Mheruian

 

[Tom.G]

Ok? so that means its current consumption while no operation would be 8uA (25°C ambient temperature) up to 80uA (if SN74 IC was used), this 80uA will be achieved if temperature is either more than OR less than 25°C? Sorry if I can't pretty much perceive it since I'm new to this, datasheets are quite not specific for my understanding haha!

Generally (almost always), current will increase with increased temperature and decrease with decreased temperature Also note that the SN54 series has 160μA maximum current.

Oh I see, right now, I'm looking again at the datasheet at the Function Table pg.2 - So since there can only be 1 IOL (1 Low output pin) on every input combination, Iconsume while on operation is always 4.08mA? but this consumption will be decrease since the 7 remaining output pin would be High - IOH, that'll be:

Total IOH = -4mA x 7
Total IOH = -28mA
Total Iconsume = 4.08mA - 28mA
Total Iconsume = -24mA (this'll be like no current was consume or power used?)

Do i get that right?

Not quite. No subtraction of the supply current occurs. The current that the chip itself uses remains 8μA. (at 25°C)

electrical characteristics... table pg4:
Lets look at V_OH first. What the data sheet indicates is that any output that is High will be able to supply 4mA from V_CC to a load connected to Ground, and will keep the output voltage to at least 3.98V.

Now the next datasheet line, V_OL, says: Any output that is Low will be able to sink 4mA to GND from a load connected to V_CC and will keep the output voltage at or below 0.26V.

This table shows the "normal" operating range of the IC. You notice that the signal pins are specified for both voltage and current values. This is to ensure that they will operate correctly when connected to other chips from the same logic family (74HCT in this case).

That made me think, how come such surge could happen if the load (circuit, appliances, etc said by my colleague) will only use current that it needs?

The absolute maximum ratings...table pg3, show what the IC is physically capable of without damage. Perhaps instead of driving another IC, you want to drive some LEDs. The table shows that each output pin can supply ±25mA, which would be plenty to drive an LED. You could connect an LED to each output.
But, the maximum current thru either V_CC or GND is 50mA. so you couldn't have more than 2 LEDs on at once.

Also notice that in this table that there is no statement of what the voltage will be at higher load currents. So if you draw higher currents there is no guarantee the IC output voltage is sufficient to drive another IC.

Got it! so that's the maximum Iinput that applies on all the ICs pins? I'm seeing this Test Condition "VI = VCC or 0", so even you input 0 volts on any pins, it will still consume current? why is that? isn't 0 volts = 0 charge / current flow?

Internal components of the IC connect to both V_CC and GND so there is a path to both. As an example consider two resistors in series connected between V_CC and GND. The common point of the resistors will be at some voltage and will draw current thru an external connection to either Power or Ground.

Whew! You brain exploding yet?
Tom

 

[mheruian]

Hi Mr.Tom!

Thanks a lot for always keeping this newbie lad being informed. I appreciate all the efforts and i learn a lot :)

Whew! You brain exploding yet?

it already did ! hahaha! I'm trying to imagine and even draw sketches of your every explanation so i can get this right.

electrical characteristics... table pg4:
Lets look at V_OH first. What the data sheet indicates is that any output that is High will be able to supply 4mA from V_CC to a load connected to Ground, and will keep the output voltage to at least 3.98V.

(After searching and asking others what load really means, it could be represented by resistor because load means something that has resistance since it consumes current they say )

Did I get that right? (Do i look desperate with this one? haha)

Now the next datasheet line, V_OL, says: Any output that is Low will be able to sink 4mA to GND from a load connected to V_CC and will keep the output voltage at or below 0.26V.

Based from the previous sketch of mine and judging the datasheet's value and direction/flow of current?? why i feel -4mA value is +4mA? do i compute them right? Should I consider that 4mA an addition and not subtract to the current consumption of the IC since current flow is sinking inside IC?

This table shows the "normal" operating range of the IC. You notice that the signal pins are specified for both voltage and current values. This is to ensure that they will operate correctly when connected to other chips from the same logic family (74HCT in this case).

Well, this one is a good thing! glad you had explained it to me :D since this day, i saw some electronic students passing by and did some quick question interview (desperate one ) with them about this computation thing specially when it was about IC to IC stuffs. They told me that their school only teaches voltage, current, power, etc computation on resistors, caps and inductors but not on ICs so they are not pretty sure about my question (maybe not yet covered). So based on what I learn here, below are some sketches.

Is the above sketch is correct?

how about this one?? got confused on sinking stuffs

The absolute maximum ratings...table pg3, show what the IC is physically capable of without damage. Perhaps instead of driving another IC, you want to drive some LEDs. The table shows that each output pin can supply ±25mA, which would be plenty to drive an LED. You could connect an LED to each output.
But, the maximum current thru either V_CC or GND is 50mA. so you couldn't have more than 2 LEDs on at once.

Also notice that in this table that there is no statement of what the voltage will be at higher load currents. So if you draw higher currents there is no guarantee the IC output voltage is sufficient to drive another IC.

Oh ok, :D if loads are not LED but other ICs, i should base these specs like IC1 output current is able to fit on IC2's input current rating?

Internal components of the IC connect to both V_CC and GND so there is a path to both. As an example consider two resistors in series connected between V_CC and GND. The common point of the resistors will be at some voltage and will draw current thru an external connection to either Power or Ground.

Ok :D like this two sketches below??

I'm learning a lot right? Well, seriously, I'm grateful thank you very much for being supportive on my study.

 

[Tom.G]

I'm learning a lot right?

Yup! Enough that I can start taking shortcuts in answering your questions without much likelyhood of confusing you.

These sketches pertain to the first two and last two sketches of your post. Note that the output voltages are a little different than what you had. The 80μA I_CC (I_consume in the sketches) is the maximum over the temperature range, so I used the worst case Output voltages that apply over the temperature range.

Edit: RATS, those images didn't show up. I am trying again.
...Ahhh... Success!
end Edit:

Your 3rd and 4th sketches, the ones with two ICs, need to show that the current flow (signals) between the ICs won't exceed the minimum of the current either needed by the input(s) or available from the output(s). For instance if an Iinput only draws (needs) 1000nA (1uA) there will not be 4mA flowing thru the Output that feeds it. Since the Output can reliably supply up to 4mA, you could connect it to 4000 inputs! Well, not really, you run into several different problems trying to do that. I'm just using that 4000 number as a worstcase possibility.