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INA181-Q1: Common-Mode Transients Greater Than 26 V

Part Number: INA181-Q1
Other Parts Discussed in Thread: INA181

Hello.

       I used the INA181-Q1 for the current monitor of the battery in automotive production.(Max current :3.87A )

       

As the datasheet description :The input circuitry of the INAx181-Q1 accurately measures beyond the power-supply voltage, VS. For example,
VS can be 5 V, whereas the bus supply voltage at IN+ and IN– can be as high as 26 V. However, the output
voltage range of the OUT pin is limited by the voltages on the VS pin. The INAx181-Q1 also withstand the full
differential input signal range up to 26 V at the IN+ and IN– input pins, regardless of whether or not the device
has power applied at the VS pin.

if we want to protect the INA181 when have transients more than 26v ,

1.so we how to modify this Sch?

2.According the datasheet(page 31),  if we will follow this transient protection using dual zener diodes,

could you propose the part number of the zener diodes for me?

3.According our design the Sch, If we want to change the R342/R343 to bigger avoid the bigger current to damage the resistors,

   but change the resistors to bigger can impact on the measurement of the current accuracy, right?

 so could you give me some suggestions ?

B,R

hejun 

      

  • Hey Hejun,

    This is one base schematic for device protection you can work off of from this here: The one you attached is also valid of course.

    We have done testing using this circuit for certain types of transients in this document although your transient situation may be different.

    Overall, you ideally want the Zener diode to begin clamping at or before the 26-V Abs Max rating and for the resistance to be high enough so that the diode does not get hot if overvoltage event is held as long as it possible could.

    You can calculate the error due to the input resistors using material from this video/pdf along with the equation 5 of the INA181-Q1 datasheet. Note that the 1250Ω term is actually the half of the typical differential input bias stage resistance (Rbias=2.5kΩ, ±20%). Simulation is not required to calculate the new gain and offset error. Simply use equation 5 to get the gain error factors for typical, max and minimum gains. For the max/min gains enter the worst-case ±20% values according to the video/pdf.  Multiply typical gain by the (1-gain error factor) to get the max or min possible gain from filter. Then multiply this value by the inherent max device gain error, which for the INA181 is ±1% as seen in device specification table.

    There is also an equation on how to calculate the new high and low offset error range for any given input common-mode voltage (VCM).

    I attached a spreadsheet that implements the equations as described. With 10-Ω/1% input resistors, total worst case error goes from ±1% to -1.302% to +1.21%. Assuming the input VCM is 12-V the worst-case offset generated solely from the input 10-Ω's is +16 µV and -14.76µV.

    INA181A4 filter error.xlsx

    Hope this helps.

    Best,

    Peter

  • Hi,  thanks for your explanation...

    i checked the "TIDU473.pdf', maybe we can use the ESD protection and resistors to avoid the high voltage destroy the INA181.....

    we want to use this Config-4...

    so could you share the part number of these components in detail?

    thanks 

    HEJUN

  • Hey Hejun,

    All of the information available is in the document itself. We do not have the specific part numbers for the resistors nor capacitors, but the part numbers for the ferrite beads and diode clamps are shown there in the aforementioned table 1 of the document.

    Best,

    Peter

  • Hi peter

       Thanks for your support...

       I understand,  sorry could you explain how many size of the resistors(7Ω/3Ω), Is 1/16w enough? 

       we should prepare these components to verify the feasibility...

    thanks 

    B,R

    hejun

  • Hey Hejun,

    Based upon the image of the PCB, it looks like 0603 resistors were used. However, the power ratings chosen for R1 and R2 were not enough according to Tables 2 and 3 of the document. For some of the configurations, the failure mode was the destruction of R1/R2 meaning they did not have enough power rating for the high voltages even though they only last hundreds of nanoseconds.

    In order to know the required power rating, you will first need to know what type of voltage surges you want to protect against (amplitude and duration).

    After this there can be some iteration because you need to find the right clamping diode and determine what the max conducting current should be. Some TVS diodes are specifically designed for certain voltage surges, but others may require current limiting especially if the voltage surge in reality differs from the theoretical surge it's designed to protect against. If the real voltage surge is something that lasts substantially longer, then current limiting is almost always required.

    Next is to determine what the current-limiting resistance should be. The smaller the resistance is, the higher power rating it will need. But increasing the resistance will increase the gain error and offset errors of the total end current measurement. See this video/pdf on how input resistors increase error for current sense amplifiers. https://training.ti.com/ti-precision-labs-current-sense-amplifiers-input-filter-error

    In addition to the input resistance error, leakage currents from the clamping diodes will also add offset error that will be proportional to the value of the input resistors and their manufactured tolerances.

    Hope this helps.

    Best,

    Peter