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INA181: Clarification

Menahem Hooper93
Prodigy 80 points
Part Number: INA181
Other Parts Discussed in Thread: DRV8814

hi

The doc titled "Low cost bi-directional current sensing using INA181"

states on page 5 equation 3 that:  -0.2 = 2.5 - (500/525)*(2.5 +Vin+)

1. where is the ratio of  500/525 is coming from?

2. what it the connection of the above calculation to the formula Vout = Vref + (Vsense * GAIN)?  

I want to use the INA181 for PWM driven PMDC motor.

Regards, M. Hooper

  • 0
    TI__Mastermind 24295 points

    Hello Menahem,

    1.Thanks for using our forum.  You found a typo and conceptual mistake in our document.  The gain ratio can be written as 500/25=20, where the impedances of the resistors may be 500k and 25k.  The formula appears to be mistakenly derived from Kirchhoff's current law with the assumption that the voltage at the node labeled below is -0.2V.  Turns out that even if the formula  did not have a typo, you cannot use it because the ESD diodes are at the input of the device and not at the node connected to the op-amp.  The device absolute max ratings must be observed.

    2.  There is not a direct relationship between these two formulas.  

  • 0 in reply to Patrick Simmons
    Prodigy 80 points
    Hi,
    let me be more specific
    I want to drive a mabuchi RS-385PH-10280 motor.
    The wending resistance is 49 Ohm. Stall current is 0.49A and the driving source is 24VDC.
    the motor will v drived by DRV8814 (PWM = 8KHz, decay mode is fast). since the decay mode is fast
    the voltage on the sensig resistor will be negative (copared the current mode of the DRV8814).
    my question is: Do you think I meet the common mode restrictions of the IC?.
    Please recall that the sensing resistor of the DRV8814 will be also the current sensing resistor and thus it is a lower side current sensing.
  • 0 in reply to Menahem Hooper93
    TI__Mastermind 24295 points

    Hello Menahem,

    I presume your setup is like the figure below with a low-side implementation.  If that is the case, I think the common mode will be fine.  However, you will need to be careful with your board layout.