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Original question:

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

INA181: INA181A4QDBVRQ1

beyondeva
Intellectual 360 points

Part Number: INA181
Other Parts Discussed in Thread: INA186

Hi  peter ,TI members 

    Due to we consider that we want to use the low side to detect the current

    so that we don't need to  worry about overvoltage for this chip when power interruption over 26v.

    I think the accuracy of  detection may be worse when have small current consumption.

    There is a circuit diagram here, can you please confirm it?

TI_current monitor for power supply.pdfThanks

B,R

hejun

  • 0
    TI__Mastermind 32395 points

    Hey Hejun,

    Error for any shunt-based, current sensing circuit will become worse as sense current becomes small due to amplifier offset. We would need to know what is the minimum desired sense current and with what desired accuracy over what range of temperature.

    The shunt resistance seems to be 4mΩ. The INA181A4 has an input offset (Vos) of ±150µV worst-case. You would at worst get 5% error from the input Vos when shunt voltage is 3mV (150uV/3mV *100 = 5%). Vshunt=3mV when load is 750mA = 3mV/4mΩ. Gain error can be ±1%, so total error possible when load = 750mA is ±6%.

    I would recommend these equations when doing an error analysis.

    https://www.ti.com/lit/an/sboa336/sboa336.pdf?ts=1604085399871&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FINA190

    Best,

    Peter

  • 0 in reply to Peter Iliya
    Intellectual 360 points

    Hello peter 

          Thanks for your feedback.

          ok, let me explain it .

          Firstly: in this case , we haven't the desired accuracy ... 

          the minimum desired sense current is about 0.1A, and the maximum current is about 3.8A, general, the working current is about 2A... 

          i know, when the current is small likes 0.1A, accuracy rate is not so good... 

          we can use the one Rsense then is 8mohm... 

          due to have no the desired accuracy so we design specification of the range is 10% , i mean if the current is 0.1A 

          we can monitor the current value maximum is about 0.11A ... ..so how can i modify the Rsense?

    B,R

    hejun

          

  • 0 in reply to beyondeva
    TI__Mastermind 32395 points

    Hey Hejun,

    In order to achieve ±10% error at 100mA, Rshnut needs to be > 16.67mΩ. I quickly calculate this by first accounting for potential ±1% gain error. So error from offset now needs to be < ±9%. The max input offset for INA181A4 for VCM=0V is ±150µV. So the minimum Vshunt is 150µV/0.09 = 1.67mV. Thus, minimum Rshunt is 1.67mV/100mA = 16.67 mΩ.

    This is not a final recommendation because it does not account for device offset drift, device gain drift, shunt tolerance, and shunt drift, but it shows that at very least Rshunt need to be greater than 16.67mΩ. However, this cause you to run out of dynamic range because at 3.8A, Vout = 3.8A*16.67mΩ*200V/V = 12.66V, which is clearly not possible because Vout is limited by 5V Vs rail.

    Thus, in order to achieve the 10% error at low current and measure all the way to 3.8A, you will need to use the 50V/V variant (A2) or lower, or choose a device with lower offset voltage such as the similar INA185A4 or INA186.

    Sincerely,

    Peter