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# current limit in buck converter

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#### yoosefheidari

##### Member level 5
hi guys
i saw in many commercial modules(buck-boost-etc...) for current limit section they use an op-amp and connect it to FB pin of controller.
but why they didn't use controller;s built-in current limit op-amp?just connect two sides of shunt resistor to current sense pins and then no need to extra op-amp or other circuits and even it costs less!
is there any disadvantage of using controller's current sesnse section? or any advantage of using external op-amp and connect it to FB pin of controller(actually using voltage feedback to limit current)?

Hi,

I haven't had a close look at the datasheet, but it mentions that the LM25116 [1] is most efficient for output currents of 2 A. Whereas in the lower right, a current range of 0 to 20 A is indicated. You would have to calculate the required variable resitance to cover 0 to 20 A, by the use of the current limiting function of the IC. Keep in mind, if you are using the current sensing pins, this variable resistor potentiometer, will also pass the 20 A. Would be interesting how much a such a potentiometer costs. Furter, by assuming A is 10 (section 7.3.7 in the DS) and you want to set the current limit to 20 A, you would need a potentiometer setting resistor of 6.1 mΩ. Which you will hardly achive with a potentiometer, due to its residual resitance [2].

BTW, is there an actual Vout-, it looks like the positive input of the LM321 is tied to GND by the 40 mΩ resistor.

[2] https://tech.alpsalpine.com/e/products/faq/potentiometer/features.html

BR

### yoosefheidari

Points: 2
Hi,

The current limit pin is for "low side cycle by cycle current limit", it is not the output current.

Klaus

### yoosefheidari

Points: 2
Yes, and where they connect the inner loop current sense is the "wrong" place....as it senses the current when the upper fet is off.......but this LM25116 module uses "emulated current mode control"....so it cleverly reconstructs the current ramp.....magically, as if it was actually sensing the current in the upper fet.............
This is good as it means you can have very very small on times and not have to worry about leading edge blanking time beiong longer than the on time.

But yes Klau sis correct..........that current sense in pin 13 is for inner current loop control.......look up "current mode control" and you will realise.

This course has a section on it...i think its in the "control modes folder" of the "small folders".

SMPS course_Big folders

SMPS course_little folders

Points: 2