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LM20343: Using COMP to set a current limit

Paul Shepherd17
Intellectual 595 points
Part Number: LM20343
Other Parts Discussed in Thread: LM5117

I am working on a design where I want a buck converter to operate primarily as a current source, and I am trying to decide the best way to do it. This link was helpful to get me thinking about more elegant ways to do it than by brute-force use of a current-sense resistor. https://e2e.ti.com/blogs_/b/powerhouse/archive/2017/11/24/how-to-design-a-simple-constant-current-constant-voltage-buck-converter

I'd like to use an integrated synchronous regulator, such as the LM20343, and place a voltage limiter on the COMP pin to reduce the maximum current the controller will allow. Does TI have any recommendations for this? I've been through the datasheet a few times, and have not been able to figure out the sensitivity of the current sensing signal (or the min/max values of the slope compensation). 

Thanks,

Paul

  • 0
    TI__Genius 11040 points
    Hello Paul,

    In the TPS43350 controller datasheet is a description of using the Comp pin to set the current limit, and I believe this is similar to your question above.

    I will check with the product expert if a similar configuration can be used for the LM20343.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Intellectual 595 points
    Thanks for looking into this, Katelyn.

    The topology is hard to know for sure, but it appears that the current sensing is a bit different in the TPS43350, because it is relying on the external sense resistor and an amplifier with both terminals exposed to the outside world. If the current/voltage gain of the internal current sense on the LM20343 (or similar part) is known, then the calculation of what the maximum value for COMP can be is pretty straight forward.

    Looking at the LM20343 Eval board, it might be easy enough to debug an adjustable voltage clamp on the COMP pin, but it would be good to know if I need to clamp it above or below a given voltage.

    Thanks,
    Paul
  • 0 in reply to Paul Shepherd17
    TI__Genius 11040 points
    Hi Paul,

    I have talked with the product expert on this configuration. The recommendation is to use an external comparator/current mirror with the LM202343.

    The COMP will vary over temperature, and it is not guaranteed to have a linear relationship with temperature.

    For current limit control internal to the device, as you previously noted the LM5117 is designed with this feature.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Intellectual 595 points
    Hi Katelyn, can you be more specific about the current mirror and comparator? I can see using the current mirror and a load resistor to convert the sensed output current into a voltage for comparison to a known reference voltage, but how would the comparator output be useful in the circuit? I can't see any pin on the package that looks like it could be driven by a comparator.

    Eventually, I'd like to be able to adapt this design approach to a hi-Rel/Space part, so using the LM5117, which has a pretty unique feature set, isn't the best solution for me.

    Thanks,
    Paul
  • 0 in reply to Paul Shepherd17
    TI__Genius 11040 points
    Hi Paul,

    I apologize for the delay with this response.

    The idea is to connect the comparator output to the FB pin. The FB voltage is the reference for the device, and by controlling the voltage at the FB pin, you are able to control the output voltage/output current.

    Best Regards,
    Katelyn Wiggenhorn