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Adding Current Limit Feature to a Buck Converter

ThomsCircuit

Active Member
Want to add an adjustable current limit to this project of mine.
its a 4.5-24 volt input buck converter using MP2315 Regulator
I came across this article where you can implement an adjustable current limit via the feedback (pin2) of LM2587S on the buck converter.
Ive got a few questions but first is will this work with the MP2315 on my project? Tapping Pin8 of course.

Ill post the limit feature schematic below mine.
-------------------------------------------------------------
The object is to tap the feedback pin on the regulator and connect it to the cathode of uf4007. Set the desired output voltage, then adjust the 10K trimmer to lower the current. I would be adding this to the circuit as an option so it could be bypassed if desired.
thank you
T.B.


Buck-S - Project-1.png

Limit Feature Circuit
currentLimit.png
 

ronsimpson

Well-Known Member
Most Helpful Member
The IC1A is watching the output current. What is the output voltage range? The OP-AMP can only see voltages less than it supply voltage. Probably the voltage at "Out-" is above 3.3V. There are some current sense IC that do what you want.
 

ronsimpson

Well-Known Member
Most Helpful Member
This IC and some others like it, measure the voltage across a shunt resistor and level shift it down to near ground. This one is good to 26 volts. It can measure voltages higher than its supply. Reference voltage =0V in your case.
1652667921802.png

INA214, NCS21674, NCS199.
 

ThomsCircuit

Active Member
What is the output voltage range?
Im may not be interpreting your question correctly but the input ranges from 4.5 to 24 and the output voltage would be dependent on what VR R2 is set at. In this example the author set the input voltage to 12 and the output to 5v. Then adjusted TM1 on the CLimit circuit to read somewhere around 10ma. Then as he adjusted the voltage output the current moved slightly but not much.
This IC and some others like it, measure the voltage across a shunt resistor and level shift it down to near ground. This one is good to 26 volts. It can measure voltages higher than its supply. Reference voltage =0V in your case.
OK i understand this. A good thing. Now let me explain what i think i understand.
INA214 has a gain of up-to 100ma. if i wanted 1A id use INA212
INA210--15 are all the same with exception of the output gain.
That gain is based on R1-4 (built into the IC)
And for my purpose Reference Voltage is ground. (don't know why)
INA210--15 need only the addition of a shunt. One like this one to complete the circuit. (and the bypass cap)
5W Cement Resistor Non-inductive Ceramic resistance 0.01
--------------------------------------------------

Questions
How would i make this adjustable?

Observation:
I see that TI.com has a tool to help calculate some values. I apologize but i do not know how to use it.
 

ronsimpson

Well-Known Member
Most Helpful Member
I am looking for the max output voltage when the adjust pot is set to max.

There is a voltage across the Rshunt. That voltage is moved to ground. (assuming "REF=ground)

So 1A across 0.02 ohms = 0.02 volts.
INA214 has a gain of 100. The 20mV across IN_ to IN+ now appears as 2V from GND.

This is about what you drew in post #1. The difference is that this part with only 5V on the supply can measure the voltage across the Rshunt as long as the output voltage is less than 26V. The internal resistors are very accurate! (very very!)
1652741676361.png
 

ThomsCircuit

Active Member
I am looking for the max output voltage when the adjust pot is set to max.
So sorry. The max output voltage when the pot is at max is 21volts.
If you could tell me what size pot i would need to adjust the current and if the shunt value needs to be adjusted.
I do appreciate the explanation. Im doing my best to understand this.
 

ThomsCircuit

Active Member
There is a voltage across the Rshunt. That voltage is moved to ground. (assuming "REF=ground)

So 1A across 0.02 ohms = 0.02 volts.
INA214 has a gain of 100. The 20mV across IN_ to IN+ now appears as 2V from GND.

This is about what you drew in post #1. The difference is that this part with only 5V on the supply can measure the voltage across the Rshunt as long as the output voltage is less than 26V. The internal resistors are very accurate! (very very!)
Forgive me for my lack of knowledge. My curiosity however is all ears. Im eager to learn and I take what i am given and try to make sense (pun intended) of it. Many times just when i think i have all i need I realize there is another way, method, or a different set of components. Im almost certain the answer to my question lies in your reply and i have tried to figure this out on my own but honestly, sometimes, I think id have a better chance of learning to becoming a Jedi.

So if I were to use this as an adjustable current limiter and connect it to my circuit I would do the following.
I would use INA211 so i could draw up to 500ma
Screenshot 2022-05-18 at 09-32-07 INA213 datasheet Pinout application circuits Voltage Output ...png

pin 1 GND
pin 2 GND
pin 3 (PIN 2 ON MY CIRCUIT)
pin 4 to Shunt | Other side of Shunt Pin 5
pin 5 Output -
pin 6 Output +
----------------------
I do apologize but this is all i understand. If you could help me complete this and integrate it into mine. I would like it to be adjustable so a Trimmer is needed. I also believe the feedback from pin 8 on the MP2315 must play a role in this circuit just not sure where. Perhaps it is the reference voltage (Pin 1)
Thank you for your help.
T.B.
 

ThomsCircuit

Active Member
If you could find some time to answer this i would be grateful.
If i need to provide any additional information ill do my best.
 

ronsimpson

Well-Known Member
Most Helpful Member
I used +5V from "VCC".
Out max is 500mA. Rshunt=0.05 ohms (or 0.047) Voltage across Rshunt at 500mA is 0.05V
Use INA213 with a gain of 50. The output voltage will range from 0V to 2.5V with current of 0 to 500mA.
1653003871068.png
 

ThomsCircuit

Active Member
Thank you!
Ive implemented it into my schematic. I am assuming the diode from pin 7 of the 602 goes to the feedback pin of the MP2315. Please let me know if i got this correct.
And supply / load is where I would connect an LED (for example)
Buck-S Plus Limiter - Project-1.png
 
Last edited:

ThomsCircuit

Active Member
I used +5V from "VCC".
Out max is 500mA. Rshunt=0.05 ohms (or 0.047) Voltage across Rshunt at 500mA is 0.05V
Use INA213 with a gain of 50. The output voltage will range from 0V to 2.5V with current of 0 to 500mA.
sorry for the delay. I reached out to a friend for help understanding this. He helped me understand gain and how the two IC's work together. Im still a long way off but ive revised my schematic. Please give me your opinions/feedback.
Thank you Ron.

Buck-S Plus Limiter - Project-1.png
 

ThomsCircuit

Active Member
If you could provide some feedback id appreciate it.

Ill tell you what i understand...
The MCP is an OpAmp. Its function is to amplify weak signals.
The 5V from pin7 is required to power the OpAmp. It will operate as low as 2.7v.
The INA21X is a current shunt monitor. it is powered directly from the circuits power supply 4-21volts.
The suggested INA214 has a gain of just 50ma but that will be amplified 100 times (across the shunt) producing upto 500ma
The output from INA214 may be weak so its passed to the OpAmps Inverting and Non-Inverting inputs (+ & -) for amplification.
The result voltage is available on the Analog Output. In this circuit that would be channel B of this 2 channel component.
----------
Im sure there is more to this but I want you to know that i am trying.
Thank you.
 

eTech

Well-Known Member
If you could provide some feedback id appreciate it.


The INA21X is a current shunt monitor. it is powered directly from the circuits power supply 4-21volts.
The suggested INA214 has a gain of just 50ma but that will be amplified 100 times (across the shunt) producing upto 500ma
Thank you.

The INA doesn't amplify current. It's inputs read the voltage drop across the current sense resistor and amplifies it by the gain factor of 100. If there is 1A flowing thru the 0.05 ohm current sense resistor, there will be a voltage drop of 50mv, which is then amplified 100x by the INA, producing an output of 5v. Its not apparent what you intend to do with the INA output.
 

ThomsCircuit

Active Member
Its not apparent what you intend to do with the INA output.
Im hoping that watching the associated video with the link above makes it clear what id like to make here. If i misunderstood your question i do apologize.
I was under the impression the INA output lead into the MCP then to the Analog Output. Thats what i thought you were asking.
In watching the video it appears the feature has a number of uses.
Testing LEDS. No resistor needed. Just set the voltage and current.
Set a constant current source
Now I admit i dont know what I would do with it but it seems interesting so i thought it would be educational for me to try to add it to my buck converter.
 

ThomsCircuit

Active Member
Did I say something wrong?
If I missed something please help me out. I'm doing my absolute best at this. I feel I've learned a great deal. Please tell me what you mean in regards to "what my intentions are with the output from INA214" I do appreciate all your help.
 

eTech

Well-Known Member
Did I say something wrong?
If I missed something please help me out. I'm doing my absolute best at this. I feel I've learned a great deal. Please tell me what you mean in regards to "what my intentions are with the output from INA214" I do appreciate all your help.

Hi

How is the INA214 output voltage supposed to be used?

I believe there is a reluctance to help because you are attempting to design features into a system that is really beyond its intended capabilities. SMPS chips designed for current limit control capability have internal circuits that alter the behaivor of the control loop to limit current. I'm afraid someone else will have to chime in here to help...
 

ThomsCircuit

Active Member
Hi

How is the INA214 output voltage supposed to be used?

I believe there is a reluctance to help because you are attempting to design features into a system that is really beyond its intended capabilities. SMPS chips designed for current limit control capability have internal circuits that alter the behaivor of the control loop to limit current. I'm afraid someone else will have to chime in here to help...
Which is ok. If its beyond the scope of the existing circuit then its fine. I appreciate everything from this group. The good and not so good. I'm the novice here. Now with that being said my first post in this thread was "will this work with my circuit." An alternate set of chips were suggested and I was all in to make it work.
E, you and the rest of the members here are considered friends. Im thankful for all you have done for me.
 

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