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LM5141 DCR Current Sensing

Simon Zisler
Prodigy 20 points
Other Parts Discussed in Thread: LM5141

I will use the LM5141 for a power supply (8-60V -> 5.1V; max. 7A) and try to avoid a current shunt.

According to the datasheet I can use DCR sensing but if the LM should detect an overcurrent at 10A the sensing voltage has to be 75mV. The Rdcr of the inductor is 21mOhm => the voltage on the sensing capacitor is 210mV (at 10A).

1)      Can I insert a simple voltage divider on the high side of the sensing line (Vout) or is Vout used internally for additional things?

2)      Can I disable the OC detection by connecting CS to Vout?

3)      Are there additional parts necessary (I saw an application where they inserted a resistor between Vout and Ccs to compensate the offset).

4)      Does DCR sensing affect the loop stability?

 

I only need an over current protection which will trip between ~10A and ~15A. Can it be done as simple as shown in the datasheet with one resistor and one capacitor?

 

Thanks for your answers!

  • 0
    TI__Genius 16410 points
    Hello Simon,

    Let me move this to the non-isolated DCDC forum, they should be able to answer your question.

    Regards,
    Karl
  • 0
    TI__Expert 3545 points
    Simon,
    1)Use the circuit in Figure 25 in the data sheet. You can use a voltage divider across CCS to reduce the voltage to 75mV.
    2)The LM5141 is current mode control, do not short Vout to CS. If you do you will not have any current information to regulate the output voltage.
    3)See my answer "1".
    4)DCR sensing does not have an affect on the control loop. The thing you need to understand is that the resistance of the inductor will change by 20% over temperature, so the current signal into the LM5141 will also change. This normally means you need to calculate the worst case current sense signal, then pick an inductor with a higher saturation current.
    Terry
  • 0 in reply to Terry Allinder
    Prodigy 20 points
    @Karl I don't know why it was in the automotive forum, I posted it in the non-isolated DC/DC Forum, maybe there was an issue with multiple tabs. I also attached the Figure 25 of the datasheet.

    Terry, thanks for your detailed answer. After I created this thread I did some tests with the eval board and a voltage divider across Ccs. This worked fine but it always feels better to have a confirmation. Between maximum current and saturation current there are > 40%. I think this should be enough, a short circuit event is more realistic than an overload (due to selectivity). Thank you again for answering all questions.
    Simon