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TPS386596: The value of RSnL to ensure DC accuracy

high speed guy
Expert 3505 points
Part Number: TPS386596

Hello,

I have used this product previously and now I am working on a new design.

In section 9.2.2 of the data sheet, the text it refers to the use of 10k ohm resistors for RSnL to “ensure DC accuracy”. 

What is the purpose of this specific recommendation? 

I believe this recommendation was not always in the data sheet and I would like to know what particular problem it may have solved or how it improved performance; as I do not recall using specifically the 10Kohm resistor in past designs and believe I used something somewhat lower.

Thanks,

HSG

  • 0
    TI__Genius 12630 points
    Hello HSG,

    To clarify, RSnL stands for "the Lower Sense Resistor at channel #n" so RS1L is the lower sense resistor for channel 1 for example.

    When choosing your resistor values, there are infinite possibilities for the same monitored voltage. For example, if we are monitoring 3.3V at sense input 1 and we want the /RESET1 to trip when Vmon falls to 2.9V, one resistor combination that would work is RS1H = 32.4k and RS1L = 10k. That means the top resistor on SENSE1 is 32.4k and the bottom resistor is 10k. We could have also chosen Rtop = 32.4 and Rbot = 10. We could have also chosen Rtop = 324k and Rbot = 100k. As long as the ratio between Rtop and Rbot remains constant, any resistor combination will work to set the same threshold. The main trade off between the different combinations is Iq vs accuracy. With larger resistor values such as 324k and 100k, this minimizes the quiescent current but is less accurate because of the ratio of the current through the divider compared to the current into the SENSE pin becomes closer. Imagine if half of the available current is going through the resistor divider while the other half of the current is into the SENSE pin then your accuracy will be 50%. If you choose smaller resistor values while maintaining the same ratio, the Iq will be higher but the accuracy will improve as the current into the SENSE pin is much less than the current through the resistor divider so the SENSE current has very little impact on the accuracy.

    The trade off between resistor values vs accuracy vs Iq is detailed nicely in this app report: www.ti.com/.../slva450a.pdf

    To summarize, choosing a bottom sense resistor of 10kohm and then solving for the top sense resistor provides a good balance between accuracy and current consumption. This is not a mandatory design rule but just a guideline. You can choose the resistor values that best suit your application requirements.

    -Michael