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I've had a understanding that having a current sense resistor feeding into sense pins or a feedback pin meant that the inductor current was changing wrt the variations across sense resistor, hence outputting a constant current output.

In this controller though LM5123 (https://www.ti.com/lit/ds/symlink/lm5123-q1.pdf?ts=1713791277304&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fde-de%252FLM5123-Q1) , if you observe the typical application diagram below: enter image description here

You see a current sense resistor tied into the sense pins. Yet, there is not a single mention of calculating this sense resistor to program output current, and this controller is marketed as a output voltage tracking driver.

Even if you look at the internal block diagram, u see the sense pins eventually fed into an error amplifier - which is what I thought was the approach to design a constant current driver.

enter image description here

Does this mean that the inclusion of any current sensing does not mean a constant current output, but is just another way to output constant voltage? If so, the most important question here is, how does one design a constant current controller?

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Does this mean that the inclusion of any current sensing does not mean a constant current output, but is just another way to output constant voltage?

Having a sense resistor simply means the chip needs to sense current to do its job... and they all do, though there are many other methods.

The current information is used for overcurrent protection, and cycle-by-cycle current limiting.

Your chip is a current mode DC-DC so it uses current as part of the feedback loop. It is actually two nested feedback loops: the inner loop manages current in the inductor, and the outer loop adjusts the inner loop to achieve desired output voltage. Because inductor current responds much faster to changes in PWM duty cycle than output volatge (which is filtered by the output caps), current mode has better control than a voltage mode converter, which only uses output voltage as feedback.

If so, the most important question here is, how does one design a constant current controller?

Three ways:

  • Use a chip dedicated to constant current, like a switching LED driver or a battery charging chip.

  • Use a voltage output chip, but replace the voltage feedback with a current sense amplifier. All the feedback does is keep voltage on the feedback pin equal to a reference, if you hack the voltage on the feedback pin to reflect output current it will regulate current. Stability needs to be considered.

  • Roll your own, which is pretty easy with a hysteretic converter, not so much with other topologies.

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I've had a understanding that having a current sense resistor feeding into sense pins or a feedback pin meant that the inductor current was changing wrt the variations across sense resistor, hence outputting a constant current output.

This doesn't make sense - the purpose of the current-sense resistor is to capture a sample of the inductor current without introducing appreciable losses. It is there to tell the controller what the instantaneous current going through the inductor is at any given time. This resistor has nothing to do with "outputting a constant current output" - the current is governed by the inductor, duty cycle, Vsuppy, and to a (very tiny, negligible) degree, the current sense resistor). The current-sense resistor is just passive component not responsible for the circuit behavior.

You see a current sense resistor tied into the sense pins. Yet, there is not a single mention of calculating this sense resistor to program output current, and this controller is marketed as a output voltage tracking driver.

You are not understanding the purpose of this device - it is not a constant-current driver (like an LED boost convertor), rather, it is a programable voltage supply. The device is trying to maintain a voltage that "tracks" the TRC pin "setpoint".

The current-sense resistor that you keep referring to is used for things called "slope-compensation" and "cycle-by-cycle peak current limiting". I'm not going to explain those concepts here - they're beyond the scope of your question and a new question should be opened relating to those concepts. Basically, this resistor is for "managing" the inductor and has nothing to do with the voltage regulation on the output. BTW, there is no mention of "calculating this sense resistor to program output current" simply because that is not what this controller is intended to do; this chip is a voltage controller not a current regulator. However, the calculations for this resistor (as it's intended to be used) starts on page 15. The current-sense amplifier has a gain of 10.

Even if you look at the internal block diagram, u see the sense pins eventually fed into an error amplifier - which is what I thought was the approach to design a constant current driver.

As already mentioned, this is not a constant current driver. Even if it were, you would have the current-sense circuitry on the output (load) side, not input. You could make this chip behave like a constant-current controller, but you would need additional analog circuitry. And it wouldn't be worth the additional complexity considering there are already many controllers that are on the market and designed for CC operation.

Does this mean that the inclusion of any current sensing does not mean a constant current output, but is just another way to output constant voltage?

Yes. Including current-sensing elements does not imply the device is constant current output. It implies a control scheme known as current-mode operation. Flyback converters operate in the same way (mostly).

If so, the most important question here is, how does one design a constant current controller?

That's a big request without a single design criterion. I would first consider the application. Then, review some reference designs similar to your case. By then, you should have more insight on what you are trying to do with your life and choosing an appropriate controller will make more sense.

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