Author Topic: LM358- Significant voltage drop when sinking current.  (Read 1613 times)

0 Members and 1 Guest are viewing this topic.

Offline Harshadb13Topic starter

  • Contributor
  • Posts: 28
  • Country: in
  • A DIY electronics enthusiast
    • My Instructables
LM358- Significant voltage drop when sinking current.
« on: August 17, 2023, 01:51:42 pm »
Hello members!

I am trying to make a CC-CV change circuit. While I know that my circuit might not be the best, I am facing a new kind of challenge. The output of the two op amps are connected through a large resistor. In the snap, one op amp is under high output state while the other one is low. But the op amp which is sinking current is not at 0V. With increasing current sink, the voltage keeps on increasing.

The data sheet of the op amp clearly shows the op amp can attain minimum of 0V. I have attached a screen shot of LTSPICE simulation. The real circuit also has the same issue.

Can someone please explain why this is happening. I tried to understand the internal circuit of the op amp as well, but to no avail.

Any leads are appreciated.

 
 

Offline tom66

  • Super Contributor
  • ***
  • Posts: 7336
  • Country: gb
  • Electronics Hobbyist & FPGA/Embedded Systems EE
Re: LM358- Significant voltage drop when sinking current.
« Reply #1 on: August 17, 2023, 01:57:53 pm »
All single-supply amplifiers will raise the output voltage above ground when sinking current. This is simply because the op-amp's transistors, die bond attach and silicon have resistance that cannot be overcome.  If you need the supply voltage to go to perfect ground, you would need to connect the op-amp to a slight negative supply.
 
The following users thanked this post: Harshadb13

Offline magic

  • Super Contributor
  • ***
  • Posts: 7454
  • Country: pl
Re: LM358- Significant voltage drop when sinking current.
« Reply #2 on: August 17, 2023, 02:18:10 pm »
Q10 is the transistor which pulls almost all the way to ground, but it only sinks a small constant current (less than 0.1mA) because this chip isn't smart enough to increase Q10 current when necessary.

Anything more than 0.1mA and Q11 becomes involved, at which point output voltage increases to 0.7V or so. Then it may keep increasing slightly more due to resistances inside the chip and finally it hits a wall at 10~30mA, which is the hard current limit.

There is a plot  attainable output voltage versus sink current in most LM358 datasheets.
 
The following users thanked this post: Harshadb13

Offline Harshadb13Topic starter

  • Contributor
  • Posts: 28
  • Country: in
  • A DIY electronics enthusiast
    • My Instructables
Re: LM358- Significant voltage drop when sinking current.
« Reply #3 on: August 17, 2023, 03:06:47 pm »
Okay.. Understood. But why does this not happen when sourcing current. When sourcing, the op amp maintains the voltage till 20mA is reached, only after which the voltage starts collapsing. :(
 

Offline magic

  • Super Contributor
  • ***
  • Posts: 7454
  • Country: pl
Re: LM358- Significant voltage drop when sinking current.
« Reply #4 on: August 17, 2023, 03:13:22 pm »
Does it maintain output equal to full supply voltage until 20mA is reached?

I think it only goes up to maybe VCC-1.5V or so.
At GND+1.5V, the sinking also works perfectly fine until 20mA is reached ;)
 
The following users thanked this post: Harshadb13

Offline Harshadb13Topic starter

  • Contributor
  • Posts: 28
  • Country: in
  • A DIY electronics enthusiast
    • My Instructables
Re: LM358- Significant voltage drop when sinking current.
« Reply #5 on: August 17, 2023, 03:21:39 pm »
Ohhh! great point. Thanks a lot
 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 17428
  • Country: us
  • DavidH
Re: LM358- Significant voltage drop when sinking current.
« Reply #6 on: August 17, 2023, 06:12:46 pm »
The 358/324 only actively pull down to one transistor Vbe (Q11) above their negative supply or about 0.6 volts.  Below that a pull down current of about 50 microamps exists (Q10).

Linear Technology has the LT1006/LT1013/LT1014 if you want to actively sink current all the way down to the negative supply.  Otherwise some other rail-to-rail output operational amplifier is required.
 
The following users thanked this post: Harshadb13

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: LM358- Significant voltage drop when sinking current.
« Reply #7 on: August 17, 2023, 06:36:30 pm »
It's on the LM358/324 data sheet.


Try a current sink between the output and negative supply. A BC547, collector to output, emitter to negative and the base to +10V via a 1M resistor.
EDIT: Here's the schmatic.

I'll probably get some criticism for posting this, because it relies on the transistor's hFE which isn't well controlled, but that doesn't matter. It just needs to pass enough current, to keep the op-amp's output near the negative rail.
« Last Edit: August 18, 2023, 01:03:51 pm by Zero999 »
 

Offline Harshadb13Topic starter

  • Contributor
  • Posts: 28
  • Country: in
  • A DIY electronics enthusiast
    • My Instructables
Re: LM358- Significant voltage drop when sinking current.
« Reply #8 on: August 18, 2023, 07:39:23 am »
Thankyou zero999.

Can you please share the pdf or the link for the attached image?

I would appreciate it
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: LM358- Significant voltage drop when sinking current.
« Reply #9 on: August 18, 2023, 12:58:09 pm »
I forgot to post the link. I got it from the Texas Instruments' data sheet on their website.
https://www.ti.com/lit/ds/symlink/lm358.pdf

They also have a very useful application note about the LM358.
https://www.ti.com/lit/an/sloa277b/sloa277b.pdf
 

Offline Harshadb13Topic starter

  • Contributor
  • Posts: 28
  • Country: in
  • A DIY electronics enthusiast
    • My Instructables
Re: LM358- Significant voltage drop when sinking current.
« Reply #10 on: August 18, 2023, 02:37:02 pm »
Thank you!
 

Offline Harshadb13Topic starter

  • Contributor
  • Posts: 28
  • Country: in
  • A DIY electronics enthusiast
    • My Instructables
Re: LM358- Significant voltage drop when sinking current.
« Reply #11 on: August 18, 2023, 03:24:42 pm »
yes Kim, you are right. That is what I am hoping for as well
 

Offline Kim Christensen

  • Super Contributor
  • ***
  • Posts: 1819
  • Country: ca
Re: LM358- Significant voltage drop when sinking current.
« Reply #12 on: August 18, 2023, 03:29:01 pm »
I removed my post since I made an error. I thought I caught it before anyone saw it.  :palm:
Quote
If you fix your circuit, the fact that the output doesn't go all the way to zero doesn't really matter. As long as it goes low enough so the transistors can fully turn off. Still not perfect (transient response and stability may still be an issue) but...
Ok. Swapped the +- on the OpAmps so that the feedback is correct now:


UGH! Major brain farts this morning. Ignore me. Your original circuit is OK.

« Last Edit: August 18, 2023, 04:40:48 pm by Kim Christensen »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf