Op amp current draw

Thread Starter

MikeJacobs

Joined Dec 7, 2019
162
Can we talk a bit about the procedure to determine how much supply rail current an op amp will draw?

I understand this will depend on what it’s driving for sure

More curious about the proper procedure
So obviously the rail current will be the output current plus feedback current plus quiescent current right?

Sooooo
What should I be looking for in a data sheet to figure that out?

Thanks!
 

crutschow

Joined Mar 14, 2008
25,438
If it's not obvious, it's the Supply Current (quiescent) value in the above data sheet.
For worst-case purposes you use the Max values.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
162
I want to know how to calculate it for real
I am
Not interested in sizing things for worst case
If you are driving nothing with the op amp why would you size your supply rails for worst case. That is foolish
 

Analog Ground

Joined Apr 24, 2019
416
I want to know how to calculate it for real
I am
Not interested in sizing things for worst case
If you are driving nothing with the op amp why would you size your supply rails for worst case. That is foolish
In the LM358 example above, the worst case specification is with no load and over the entire temperature range. Using worst case would be a conservative design but not foolish. Sometimes the data sheet will have more information in graphs. Let me turn it around. How would you suggest the specification be stated?
 

Analog Ground

Joined Apr 24, 2019
416
So obviously the rail current will be the output current plus feedback current plus quiescent current right?

Sooooo
What should I be looking for in a data sheet to figure that out?

Thanks!
The data sheet cannot have information about load current and feedback current. Those are application dependent. So, only the quiescent current is specified.
 

crutschow

Joined Mar 14, 2008
25,438
If you are driving nothing with the op amp why would you size your supply rails for worst case.
Because if you size the supply for nominal current and you get a worst-case (Max current) device, then the supply will be drawing more current than you calculated.
 

OBW0549

Joined Mar 2, 2015
3,551
I want to know how to calculate it for real
I don't know what you mean by "for real." The op amp's operating current is given on the data sheet under "Supply Current" as mentioned above. Most of the time you'll see both a typical value and a maximum (i.e., worst-case) value, although sometimes a typical value won't be listed. As for load current and feedback current, those depend entirely on your circuit and you'll have to calculate them yourself based on the voltages and resistances involved. These add to the op amp's own current draw to form the total current requirement. Very simple.

I am Not interested in sizing things for worst case
Then use the typical values, if that's what you want; no one's forcing you to use the maximum values on the data sheet.

But be aware that some units WILL draw more current than the listed typical value, and a few will draw the maximum. And if your supply circuit cannot provide that much current you've got trouble.

If you are driving nothing with the op amp why would you size your supply rails for worst case.
So you can guarantee that adequate power will be available for all parts, even those whose current draw is at or near the maximum.

That is foolish
Ummm... no.

What's foolish is to think you can design a reliable circuit using only the typical values on a data sheet, while ignoring min and max values as if they will never happen. Competent designers simply don't do that; rather, they design with worst-case values in mind so a circuit won't just "probably" work, but will always work.

BTW, these principles apply to ALL op amp parameters on the data sheet, not just current draw. And they apply to ALL electronic parts, not just op amps.
 

schmitt trigger

Joined Jul 12, 2010
320
You talk about proper procedure.
The late Bob Pease, the fabled National Semiconductor design guru, in his seminars always admonished designing with the worst case scenario in mind.

Otherwise, he would continue, "you will be surprised by your circuit's behavior, and it will not be a pleasant surprise".
 

OBW0549

Joined Mar 2, 2015
3,551
A side note: the worst case for any op amp parameter isn't always the maximum value; sometimes it's the minimum.

Some examples would be slew rate; DC voltage gain; input common-mode range; CMRR; PSRR; minimum required supply voltage; output source and sink current; and output voltage swing. In each case you're concerned with "how much is enough?" whereas with worst-case maximum values (e.g., supply current; input offset voltage; offset voltage drift with temperature; input bias current; and input noise voltage) the concern is "how much is too much?"

It all depends on what your circuit needs to meet the design requirements.
 

Reloadron

Joined Jan 15, 2015
5,646
I want to know how to calculate it for real
I am
Not interested in sizing things for worst case
If you are driving nothing with the op amp why would you size your supply rails for worst case. That is foolish
Then measure it for real and size your power supply accordingly for real.

Ron
 
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