Hi.
In the datasheet of the L78XX regulator from this address:
https://www.st.com/resource/en/datasheet/l78.pdf
There is a design consideration in 6.1 section.
Among other things,it says:

"...In many low current applications, compensation capacitors are not required. However, it is recommended that the regulator input be bypassed with capacitor if the regulator is connected to the power supply filter with long lengths, or if the output load capacitance is large..."

What current value is considered to be low current applications and what value is
considered as large capacitance?

 

To be on the safe side, I would use the compensation capacitors unless this design is critically cost sensitive.

 

I think it's important to understand the -why- the capacitors are important. Your voltage and current are never smooth steady lines. There is too much noise, even through the ground-plane. What a bypass capacitor actually does (among a couple of other things), is it acts as a sponge. It tries to soak up excesses, and fill in gaps in both voltage and current so that anything downwind of it has smoother power. I recommend always using them as a means to help stabilize the power environment in your circuit.

 

Okay,i was planning,anyway,to add compensation capacitors for the reasons that
you mentioned.But the main purpose of my question(and this is the reason that i emphasized it with red color)is to know:What is the current value that is considered to be low current applications and what capacitance value is considered as large capacitance?

 

hi,
For 78xx voltage regulators.
I would consider a current of 100mA or less low current and capacitor over 220uF as large.

E

 

Okay,i was planning,anyway,to add compensation capacitors for the reasons that
you mentioned.But the main purpose of my question(and this is the reason that i emphasized it with red color)is to know:What is the current value that is considered to be low current applications and what capacitance value is considered as large capacitance?

Its a bit hard to explain. You have to consider the things mentioned in your first post, calculate the current that will flow and the voltage drop do to long lengths, after all the traces or wires have resistance.

 

hi,
For 78xx voltage regulators.
I would consider a current of 100mA or less low current and capacitor over 220uF as large.

E

Thanks,ericgibbs,this is what i was looking for.
Those values help me to understand better the design consideration details.
Thanks for all replies.

 

Hi.
In the datasheet of the L78XX regulator from this address:
https://www.st.com/resource/en/datasheet/l78.pdf
There is a design consideration in 6.1 section.
Among other things,it says:

"...In many low current applications, compensation capacitors are not required. However, it is recommended that the regulator input be bypassed with capacitor if the regulator is connected to the power supply filter with long lengths, or if the output load capacitance is large..."

What current value is considered to be low current applications and what value is
considered as large capacitance?

Most manufacturers publish application notes that specifically address possible instability - apparently; uninformed use of ultra low ESR MLCCs can do more harm than good, and LDO regs seem to be more at risk.

Generally speaking; the input side is the reservoir and the upper limit is what the transformer & rectifier can charge from a cold start - the output side is more about noise filtering/damping. About 22uF is OK for quite a lot of stuff, this is where ultra low ESR can turn out not as well as you'd hoped.

Don't forget an anti backflow diode so the output cap is brought down by the input side instead of discharging back into the regulator.