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LM25149: Control of EMI countermeasure function

Kazuto Odajima
Prodigy 235 points
Part Number: LM25149

Tool/software:

When designing a circuit using the LM25149, I would like to switch the AEF and DRSS functions ON/OFF with a switch to evaluate the EMI countermeasure function.

I would like to know the optimal circuit configuration.

When R_CNFG=29.9k or 41.2k

・Will AEF be turned OFF if I connect it to GND through R_CNFG?

・What will happen to DRSS if I leave it connected to GND through R_CNFG and branch it off from the CNFG pin and connect it to VDDA? Also, will AEF be turned ON?

Is there a circuit configuration you would recommend?

  • 0
    TI__Genius 9460 points

    HI,

    29.4k resistor will turn DRSS off, 41.2k will turn it on. We do not recommend grounding this

    AEFVDDA pin will control AEF at start up.

    You can use webench or our design calculator to help with configuration based on what conditions and settings you want - https://www.ti.com/tool/download/LM5149-LM25149DESIGN-CALC

    Regards,

    Rahil

  • 0 in reply to Rahil Ajani
    Prodigy 235 points

    Hi Rahil, 

    Specifically, if I use a circuit like the one attached, can I switch between AEF and DRSS properly?
    If this is not OK, how can I improve it?

    Regarda, 

    Kazuto

  • 0 in reply to Kazuto Odajima
    TI__Genius 9460 points

    Hi Kazuto,

    It is not that simple. 

    DRSS cannot be changed on the fly so you will have to have it start with DRSS on or off so S2 can only be used before start up

    After start-up, CNFG pin will be latched to DRSS on/off. Then you can have a strong pull up/down to turn AEF on/off. I would put a switch to have CNFG connected to VDDA and GND or use a microcontroller. 

    AEF is on/off by default based on the load current but can be manipulated with the CNFG pin as mentioned above

    Hope this explanation helps.

    Regards,

    Rahil

  • 0 in reply to Rahil Ajani
    Prodigy 235 points

    Hi Rahil,

    Thank you for your answer.

    I understand that DRSS cannot be changed after start up.

    If I set the switches as shown in the table in the previous diagram and then turn on the power, will AEF and DRSS be as shown in the table?

    I'm worried about the behavior of DRSS when S1 is ON.
    If the CNFG pin and VDDA are connected when DRSS is latched (Vcc = 3.3V), will R_CNFG be recognized correctly?

    Regards,

    Kazuto Odajima

  • 0 in reply to Kazuto Odajima
    TI__Genius 9460 points

    CNFG needs to be connected to VDDA to turn AEF on and to GND to turn AEF off. S1 above will do this.

    However, S1 will have to be open at start up to allow for DRSS to be on/off

    Regards,

    Rahil

  • 0 in reply to Rahil Ajani
    Prodigy 235 points

    I understand that DRSS cannot be set properly when S1 is closed.
    Is there a way to set both AEF and DRSS as desired without switching S1 and S2 after power-on?
    For example, with the current circuit, to turn AEF on, I must always turn S1 off before booting, and then switch S1 on after booting.
    What is the optimal circuit configuration?

    Regards, 

    Kazuto Odajima

  • 0 in reply to Kazuto Odajima
    TI__Genius 9460 points

    Hi Kazuto,

    DRSS cannot be changed after power on. AEF can only be toggled by CNFG pin after power is turned on to not disturb DRSS at startup. So yes, what you said is correct about turning S1 on after booting. 

    Please use webench or our design calculator to help with finding the configuration you want. 

    Regards,

    Rahil