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INA849: Input protection resistors selection?

Tony Zhang
Prodigy 165 points
Part Number: INA849
Other Parts Discussed in Thread: TMUX6222, INA826S, TMUX6122, TMUX7222

Dear Friend

I have a low-power application requiring a power off in standby mode, I use INA849 to measure DC current of a main power line via a shunt resistor.  I have a booster to power the INA849 and make sure its Vs+ is always 3V higher than the main power line voltage. 

Since the main power line needs to be always powered on even during measurement circuits are in standby mode, I will have to use two resistors connected in series with INA849 inputs to protect the device's input clamping diode and also prevent voltage built-up at Vs+ (through the clamping diode) in standby mode when the Vs+ lost its power.

So the questions become:

1. Is this "resistor in series with inputs of INA849" a good solution to address the issue in my application? 

2. What resistance value and what power rating of the protection resistors are good enough to protect the internal clamping diodes if the answer to question one is YES? 

3. Is it necessary to prevent voltage build-up at Vs+ when INA849 is powered off? The concern is due to the requirement from the datasheet Vcm MAX = (Vs+) - 2.5V, if we allow voltage build-up at Vs+, Vcm will be (Vs+) - 0.7V.  Also, any output of INA849 is not good for other circuits which are powered off.  In the worst case, no matter what resistance value of protection resistors, Vcm at inputs of INA849 may be eventually much higher than Vs+, will this damage the device? 

Thank you in advance

Tony

  • 0
    Guru 281710 points

    Resistors can limit the diode current to less than the absolute maximum rating of 10 mA. For such small currents, the power rating does not matter.

    When you are not using the amplifier, Vcm does not matter. But the current into +VS will (partially) power up your circuit, which is probably not what you want. Consider an amplifier with shutdown like the INA826S, or disconnecting the inputs with a switch like the TMUX6122/TMUX6222/TMUX7222.

  • +1
    Prodigy 165 points

    Hi Clemens

    Thank you for the advice.

    I will consider TMUX switches.

    Best Regards

    Tony

  • 0 in reply to Clemens Ladisch
    TI__Guru 63890 points

    Just to add to Clemens' reply, the resistors limiting the ESD diode current to less than the absolute maximum rating of 10 mA prevent any damage ONLY if there is current flowing through these resistors that limit the supply voltage below maximum rated value (see below).  Thus, if +Vs is actually floating during power off condition, you must make sure that the INA849 supply pin is NOT pulled up thru ESD diode above its absolute maximum rated voltage of 40V with respect to its negative supply (GND?).  The same applies to amplifiers with shutdown option.  If you need further assistance, please provide circuit schematic of your application.

  • 0 in reply to Marek Lis
    Prodigy 165 points

    Hi Marek

    Thank you for the comment.

    The main line voltage is below 20V, we also have over-voltage protection (which is always on) to trip the main when it is over 20V.

    I think TMUX switches solution may be better than the limiting resistor, it removes concerns associated with the resistor.

    Thank you

    Tony

  • 0 in reply to Tony Zhang
    TI__Guru 63890 points

    The main line voltage below 20V and/or over-voltage protection (I assume TVS) eliminates the danger of damaging the part.