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LM358- Significant voltage drop when sinking current.
Harshadb13:
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.
tom66:
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.
magic:
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.
Harshadb13:
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. :(
magic:
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 ;)
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