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INA381: Sensing current on 5V VBUS

Antony Lin
Genius 13480 points
Part Number: INA381
Other Parts Discussed in Thread: INA303, INA302, INA190, INA191

Hi,

I need a current sense solution to send an ALERT when the current is above a specific threshold which could be between 200mA to 500mA (undecided yet.)  The purpose is to sense current on the 5V VBUS power path, and we would like to minimize the power loss on the Rsense.  Is INA381 a good option?  (Any better proposal?) Where should we start to calculate related component values?  Should I define the maximum power dissipation on Rsense firstly?

Thanks!

Antony

  • 0
    TI__Mastermind 27715 points

    Hello Antony,

    If the current to be sensed is unidirectional, and you only need to detect one threshold, then INA381 is a great choice. If you need bidirectional sensing and multiple threshold detection, then the INA302 or INA303 are a good option.

    My only concern about INA381 is the input offset voltage (150uV max at Vcm = 0V, and 500uV max at Vcm = 12V), which will make accurate current sensing at lower currents a bit difficult without calibration. For reference, 500mA through a 1mΩ shunt generates 500uV, equal to the INA381 offset at Vcm = 12V. You may want to consider a higher precision device such as the INA190 or INA191 (15uV max offset) with a discrete comparator, or even a precision amplifier + comparator solution.

    As for selecting component values, section 7.3.6 and 7.3.7 of the INA381 datasheet show how to set the current-limit threshold and select a suitable Rsense. There is also an error calculator tool on the right side of the INA381 product folder and an overcurrent event detection reference circuit that can help get you started.

    If this answered your question, please click the button "This resolved my issue." Thanks!

    Best regards,

    Ian Williams
    Applications Manager
    Current & Magnetic Sensing

  • 0 in reply to Ian Williams
    TI__Genius 13480 points
    HI Ian,

    Thanks for your comments. Do you mean we shall need calibration for each single INA381 unit to get a better acuuracy for current like 500mA for this case? If we don't do calibration, what's the MAX error of current we could have?

    In addition to the proposal you mentioned of adding a comparator, do we have any better proposal with comparator embedded?

    THanks!

    Antony
  • 0 in reply to Antony Lin
    TI__Mastermind 27715 points

    Hi Antony,

    Whether or not you would require calibration depends on your error requirements for the system. For lowest error the highest Rsense must be used, this creates more input signal from the load current and helps counteract errors from input offset voltage, input bias current, etc. 

    For example, with INA381, if you used Rsense = 100mΩ with Vcm = 5V you would expect 1.5% error when Isense = 500mA. This is RSS (root sum of squares) error, a more realistic error condition than worst-case. At max Isense, you would burn 25mW across Rsense.

    The other, more accurate option with an integrated comparator is INA302 or INA303. The offset voltage on these parts is much lower (30uV to 80V max), so your total error will be much lower as well. With the same Rsense, your error drops to 0.35%. 

    Best regards,

    Ian Williams