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TLV7021: feed the input of Window Comparator

? ?
Prodigy 120 points
Part Number: TLV7021
Other Parts Discussed in Thread: TLV4021

Hi ti team,

   I configure two TLV7021s as fig 40 in  datasheet to implement over volt and under volt protection to the PCBA's main power. TLV7021 is supplied with 3.3V. Later I will adjust R1,2,3 for the proper Vth+ and Vth-.

These is a issue: the main power is normal +12V and the circuit should be able to detect up to +17V for OVP. Both 12V and 17V excess the VCM of 7021. 

  Can volt divider to be used to decrease the main power to the range in VCM? That means the  input of Window Comparator will be divided by two resisters through the main power. I am not sure the the impedance of the IN+ and IN- high enough to the volt divide resisters.

  Thanks to your reply.  

  • 0
    Guru 258280 points

    If you can guarantee an upper limit for the input voltage, then you can indeed use a voltage divider to move the highest voltage into the VCM range.

    CMOS comparators (like this device) have an extremely high input impedance; it is easy to design a voltage divider whose current is much higher than the input bias current. (Devices like the TLV4021‍Sxx just use an internal voltage divider; the specified input bias current of 1.65 µA for the S model is that current.)

  • +1
    Guru 140200 points

    Hi,

    ? ? said:
     I configure two TLV7021s as fig 40 in  datasheet to implement over volt and under volt protection

    Figure 40? Are you sure?

    Yes, you can very easily use the TLV7021 in combination with an input voltage divider. To keep the error below 1%, set a current through the voltage divider resistors which is 100 times higher than the maximum input bias current of TLV7021. If you want to run the TLV7021 at elevated temperatures, see figure 13.

    An example: The typical input bias current of TLV7021 at 40°C is 10pA. For a very conservative estimation the maximum input bias current may be 100 times 10pA = 1nA. 100 times this 1nA gives 100nA. For an input voltage of 20V keep the voltage divider resistance below 200MOhm. 

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 23970 points

    Thanks everyone for the excellent support even over the holiday break. Both great suggestions.  If further support is needed, we will respond on Jan 3rd.

    Chuck

  • 0 in reply to kai klaas69
    Prodigy 120 points

    I just checked in English version of datasheet, it is fig 35. 

  • 0 in reply to ? ?
    Guru 140200 points

    Hi,

    ? ? said:
    it is fig 35.

    Ok.

    What is your maximum ambient temperature of application?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 120 points

    This is a module level thing can be integrated in upper level system. 

    In general, the max ambient temperature does not excess 40º. 

  • 0 in reply to ? ?
    TI__Mastermind 23970 points
    • Thanks for sharing this additional information. This means the input bias current will remain quite low (pA range) as shown by the input bias current vs temp curve. So keeping the current through the resistor divider more than 100x is easily achieved without burning much power. 
      Chuck