LM358- Significant voltage drop when sinking current. - Page 1
Forum section: electronics / general

author: Harshadb13
author_rank: Contributor
timestamp: August 17, 2023, 01:51:42 pm
content: 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.

author: tom66
author_rank: Super Contributor
timestamp: August 17, 2023, 01:57:53 pm
content: 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.

author: magic
author_rank: Super Contributor
timestamp: August 17, 2023, 02:18:10 pm
content: 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.

author: Harshadb13
author_rank: Contributor
timestamp: August 17, 2023, 03:06:47 pm
content: 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.

author: magic
author_rank: Super Contributor
timestamp: August 17, 2023, 03:13:22 pm
content: 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

author: Harshadb13
author_rank: Contributor
timestamp: August 17, 2023, 03:21:39 pm
content: Ohhh! great point. Thanks a lot

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2023, 06:12:46 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2023, 06:36:30 pm
content: 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 hFEwhich 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.

author: Harshadb13
author_rank: Contributor
timestamp: August 18, 2023, 07:39:23 am
content: Thankyou zero999.Can you please share the pdf or the link for the attached image?I would appreciate it

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2023, 12:58:09 pm
content: 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.pdfThey also have a very useful application note about the LM358.https://www.ti.com/lit/an/sloa277b/sloa277b.pdf

author: Harshadb13
author_rank: Contributor
timestamp: August 18, 2023, 02:37:02 pm
content: Thank you!

author: Harshadb13
author_rank: Contributor
timestamp: August 18, 2023, 03:24:42 pm
content: yes Kim, you are right. That is what I am hoping for as well

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 18, 2023, 03:29:01 pm
content: I removed my post since I made an error. I thought I caught it before anyone saw it.QuoteOk. 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.