Buck converter as constant current source

Thread Starter

sageras

Joined Aug 16, 2021
5
Greeting everyone,
iam trying to use conventional buck converter (lm5117) as constant current source for variable resistive load (8-14 ohms). I managed to simulate constant voltage, but resistance of load can almost double so my current drops a lot. When I tried typical application scheme from datasheet (https://www.ti.com/lit/ds/symlink/l...l=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLM5117) figure 35 analysis shows that nothing happens. It is very probable that i made novice mistake somewhere.

Attached:
Image 1 - Scheme
Image 2 - Transient analysis

Thanks in advance for any useful answers.
Mike
 

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LowQCab

Joined Nov 6, 2012
1,189
I think it would be easier to just use a Hall-Effect-Current-Sensor if
You are so concerned with Efficiency.
But, with the relatively light-Loads You mentioned I can't see it as being
worthwhile to eliminate a standard Current-Sense-Resistor.
.
.
.
 

Thread Starter

sageras

Joined Aug 16, 2021
5
Thank you! Pcb will have to be very small (probably less than 2 cm squared and 4*6mm is size of converter) so every unnecessary part had to be eliminated. Less parts also increases reliability and reduces cost. At last but not least circuit will be operated in between 70 and 90 deg Celsius, so any additional unnecessary heating is unwanted.

edit: yes I can use op-amp and smaller resistor for better efficency but ..... more parts :/
 
Last edited:

Ian0

Joined Aug 7, 2020
3,525
Why don’t you find a buck regulator that is designed to be a constant current source? There are hundreds of them available for driving LEDs?
 

Thread Starter

sageras

Joined Aug 16, 2021
5
This one is also designed with interal analog current monitor. Iam just not sure if transient model accurately describes behavior of CM pin. In datasheet (link above) is equation which describes the relationship between R_sense current and output of CM pin but in analysis behaviour doesn't follow equation (img4,5).
 

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Ian0

Joined Aug 7, 2020
3,525
This one is also designed with interal analog current monitor. Iam just not sure if transient model accurately describes behavior of CM pin. In datasheet (link above) is equation which describes the relationship between R_sense current and output of CM pin but in analysis behaviour doesn't follow equation (img4,5).
The correct circuit is shown in figure 35 of the datasheet. That’s not the circuit you have. You have the FB and CM connections to the voltage divider reversed.
 

Thread Starter

sageras

Joined Aug 16, 2021
5
Yes, thanks :) But that divider only reduces voltage and does not change rising tendency of voltage, it would also rise if divided only it would be smaller value, my problem is that CM is rising while voltage across R_sense is gettig lower. I am working on stability, set up wanted current should be quide easy afterwards.

At fist I tried excatly follow datasheet (img1) but it didnt work so i moved R_sense (img3) because I am not sure how I should measure current through energy dissipation from coil (through R_sesnse to ground) because it is on low side.

ps: img6 Reaction iam aiming for but this is with use of big R_sense like in old times.
 

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