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LM5175: Is it no problem to to use 51kohm(+/-5%) resistor for mode pin setting?

Kazuya Nakai59
Guru 20130 points
Part Number: LM5175

 Hello guys,

 One of my customers is about to start LM5175 evaluation on their own board.

 They have a question as the follow. Could you please give me  your reply for it?

 Q. The device datasheet says that 49.9kohm or 93.1kohm should be used between MODE pin and GND for mode setting.

     But they want to use E24 series. Can 51kohm and 91kohm be used instead of 49.9kohm and 93.1kohm.

 Best regards,

 Kazuya.

  • 0
    TI__Guru 58225 points

    Hello Kazuya,

    The source current and the voltage levels for the different values are given in the datasheet on page 7.

    With the given source current between 17 uA and 23 uA and the inaccuracy of the used resistors, they need to check if they are in the worst cases within the given voltage levels.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Guru 20130 points

     Hello Brigitte,

     Thank you very much for your reply.

     According to the electrical characteristics of MODE pin on page 7, if they want to set "DCM with hiccup" mode,

    they need to set 0.60V~0.76V to MODE pin. The typical voltage is 0.70V.

     Imode (Source current out of MODE pin) current range is 17uA ~23uA. In case of typical value, Rmode = Vmode/Imode

    = 0.70V/20uA = 35kohm.

    In case of the minimum Vmode and the maximum Imode, Rmode becomes the minimum value.

    It is 0.60V/23uA = 26kohm. 

    In case of the maximum Vmode and the minimum Imode, Rmode becomes the maximum value.

    It is 0.76V/17uA = 44.7kohm.

    So from these calculation, Rmode range is 26kohm~44.7kohm. 

     Why does TI recommend to use 49.9kohm? 

     Is the threshold voltage not correct? Is the correct value is 1.0V as typical? 

     Could you please give me your reply?

     Thank you again and best regards,

     Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Guru 58225 points

    Hello Kazuya,

    Please excuse that the datasheet seems to be misleading as it speaks about the MODE pin thresholds and not the levels it needs to be in.

    If the voltage is below the lowest threshold (0.60V), the device reads DCM no hiccup.

    If the voltage on MODE is above the DCM with hiccup threshold, but below the CCM with hiccup threshold (>0.76V and <1.18V), the device reads DCM with hiccup.

    And so on.

    If the voltage is between 0.60V and 0.76V, the device can read DCM with or without hiccup mode. So it is not save to be in this area.

    So the lowest resistance is with 0.77V/23uA = 33.5 kohm

    The highest resistance is 1.17V/17uA = 68.8 kohm

    This means that the used resistor with the full inaccuracy needs to be between 33.5 kohm and 68.8 kohm.

    (I used 0.77V and 1.17V to be for sure outside the threshold level).

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Guru 20130 points

     Hello Brigitte,

     Thank you very much for your reply.

     Could I ask about your calculation?

     I understood that MODE pin voltage must be in 0.77V~1.17V to set LM5175 operation mode to DCM with hiccup.

     I think the lowest resistance should be 45.3kohm with 0.77V/17uA.

     Also I think the highest resistance should be 50.8kohm with 1.17V/23uA.

      And the MODE pin voltage must be in 1.39V~2.21V to set LM5175 operation mode to CCM with hiccup.

    I think the lowest resistance should be 81.8kohm with 1.39V/17uA.

     Also I think the highest resistance should be 96.0kohm with 2.21V/23uA.

      Is my understanding not correct?

     Thank you again and best regards,

     Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Guru 58225 points

    Hello Kazuya,

    You are right, sorry for the wrong calculation. You are right, with the smallest current we need to get above 0.77V and with the highest current we need to stay below 1.17V.

    And yes, for CCM with hiccup your values are correct.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Guru 20130 points

     Hello Brigitte,

     Thank you for the prompt reply.

     Your reply is very helpful for the customer and me.

     Thank you again and best regards,

     Kazuya.