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LMP92064

roger roisen1
Prodigy 50 points
Part Number: LMP92064

I was wondering if the LMP92064 is suitable for measure current draw in a 3.3 volt system.  My current sensing requirements are in the 700mA to 3 amp range.  The common mode range for low side will be in the 20 to 50mV range.  I need to communicate via SPI from a 3.3 volt uP.  I have a 5 volt supply available for the LMP92064.  I was planning on level shifting the SPI communication from 3.3 to 5 and visa versa.  

I ask because all the design notes are on much higher voltage loads.  My load is 3.3 volts.

thank you

  • 0
    TI__Mastermind 25945 points

    Hey Roger,

    Thanks for considering to use Texas Instruments.  I think the LMP92064 can be used for your design, so long as you use a suitably sized shunt.  From my calculations, the max shunt value you could use is 0.075V/3A=25mΩ. However, once you account for resistor tolerance, you should probably use a smaller value.  I see digikey has 24mΩ 1% resistors, which should keep your highest measurement within the maximum differential input voltage range. With that shunt value I would expect your minimum measurement be 24mΩ*700mA=16.8mV. That is significantly higher than the 20uV LSB resolution value and well above the input referred offset voltage.  Consequently, any error from offset should be quite low for your lowest measurement.  So long as you take a low-side measurement where Vsense is kept at or below 2V and your bus voltage is divided down to a voltage below 2.048V you should not have issues with this device.

  • 0 in reply to Patrick Simmons
    Prodigy 50 points

    Thank you so much.  I am correct on my plan to level shift from my uP SPI at 3.3V to the 5.0V SPI inputs on the LMP92064 and then from 5.0V SPI to 3.3 volt to my uP.  Also I assume my shunt resistor does not have to dissipate the power of 3.3 X 3 amps, correct?  i.e what is the wattage of the shunt?

    Looks like I can sample the current at many times per second which is more than enough for my application.

    thank you.

    RR

  • 0 in reply to roger roisen1
    TI__Mastermind 25945 points

    Hello Roger,

    Yes you will need to level shift as the datasheet indicates the digital supply should match the analog supply, and the analog supply needs to be between 4.5V and 5.5V.   As for your shunt, it will dissipate (Load^2)*Rshunt.  As this part does not appear to support common modes higher than the supply, the input bias currents should be negligible.