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INA240: The output wave of INA240A3 is not correct when I measure the inductor current in BUCK topology.

anxing wang
Prodigy 20 points
Part Number: INA240


Hi TI,

I use the INA240A3 to measure the inductor current in CCM BUCK topology which switch frequnce is 300KHz.The INA240A3 is placed on the high side.Then I found the output wave of the INA240A3 is very strange.

I know the BW of INA240 is 400KHz(-3dB).Theoretically the output should be a triangle wave even its amplitude is attenuate. However its output wave surprised me, yes it did surprised me. 

This is the surprising wave.

Here is the condition:

vin = 30V

vout = 9V

load current = 5A

Here follows the schematic

So,could you tell me why the output is so strange? 

  • 0
    TI__Mastermind 32410 points

    Hello Anxing,

    The problem is with the VCM change.  Can you place the Rshunt on the other side of the resistor?  You can see the Common-Mode transient response.  The INA240 has some circuitry to limit this but as you can see in figure 7-21 and 7-22 there is still some interaction from a VCM change to the output.  So you are seeing a combination of the current along with a VCM transition.  Placing on the other side of the inductor limits the VCM change.  

    We have another device that is better for these transients and a better bandwidth but it is in preview mode, but I would still recommend placing the Rshunt on the other side of the inductor if possible.

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier，

    Thanks for your reply. I knew the Rshunt should be placed with the capacitance in most instance.However, my real topology is a Bi-directional 2-phase parrallel interleaved 4-switch Buck/Boost.That means there is not exist a static voltage point.I just test the topology works in CCM Buck firstly in the above post.

    So, what shoul I do in this situation?

  • 0 in reply to anxing wang
    TI__Mastermind 32410 points

    Hello Anxing,

    Can you provide a diagram?  From the version I saw you cold still place Rshunt after inductor and before the capacitor you are charging.

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier,

    Below is the block diagram of my application. There is no static point because the topology is Bidirectional and it can work at Buck, Boost and Buck-Boost mode.

  • 0
    Guru 140200 points

    Hi Anxing,

    a 300kHz triangle wave as input signal and a 300kHz square wave as common mode noise is way too much for the INA240. Remember the extremely far reaching harmonics of a triangle wave and of a square wave and look at the spedicifations of INA240 at >300kHz. You don't allow the INA240 even the least amount of linearizing gain reserve. Would you run an 1MHz OPAmp at 1MHz? No you wouldn't. The common mode rejection at 300KHz is less than 75dB (132dB at DC!). The distortion at 300kHz is in the 10% range. The settling time is 10µs.

    What do you expect? You extremely dynamically overload the INA240.

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 32410 points

    Anxing,

    You may want to see if the INA241 or INA296 could work for you.  They are in preview mode.

  • 0 in reply to kai klaas69
    Prodigy 20 points

    Hi kai,

    I get your idea.But I believe the reason is the INA240 do not work at a static voltage level point not harmonics.Just like you said the 300KHz square wave is the reason. And I had decided replace INA240 with hall.

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier,

    I thought that INA240 may not be suitable for working at a dynamic common voltage status.I decide replace it with hall.Thank you.