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TLV8802: Is it possible use TLV8802 instead of INA138 for current sense?

Simon Poon
Prodigy 240 points
Part Number: TLV8802
Other Parts Discussed in Thread: INA138, , TINA-TI

Hi all, 

Since now we are developing a product which need to current sensing of the load (100mA to 7A full range) by 30V supply, then input to MCU A/D for current measurement, before I was use INA138 to achieve it, because change to battery supply, so now we need to takecare the current consumption by the INA138 also, I was find that the op amp TLV8802 current consumption is suitable for our usage, but I have some question:

1. Can I use TLV8802 instead of INA138? 

2. Is it suitable usage for High side detection or low side only?

3. Any other IC suggestion use current sense with low current consumption?

Thanks all for help!

  • 0
    TI__Intellectual 1095 points

    Hi Simon,

    Thanks for reaching out. I try to answer your questions but you might have give some more information so we can fully understand your application and support you.

    To 1.: You can use the TLV8802 

    • if you use an LDO before you supply the opamp with the 30V. It is a max. 5.5V part, therefore you have to attenuate the supply voltage accordingly. Our Webench Tool can guide you through this and offer you fitting parts. The question is what you are driving after the opamp, i.e. which output voltage is required and whether 5.5V are suitable? The TLV8802 is a rail-to-rail part which allows for an output voltage as close as 3.5mV from the positive rail, so it can reach the 5.5V very close.
    • if the common mode voltage stays within its range of 0-4.1V for the TLV8802. For a max current of 7V we recommend using a 500mOhm shunt resistor in a buffer configuration in order to satisfy this condition.

    To 2.: Since you are using a 30V battery, we are recommending a low side measurement if no fault detection is necessary. Since the TLV8802 has a common mode range including 0V, a low side measurement is possible. The INA138 was designed to be high-side only with its common mode range from 2.7V to 36V. Consider as well that the offset voltage of the TLV8802 is higher than of the INA138 influencing the accuracy of the measurement.But I guess you are more concerned about the quiescent current, right?

    At Electronic Design an article was published about this topic describing several thoughts that you might have to consider as well for a current measurement (battery supply, what is the output used for etc.).

    To 3.: You can choose many opamps in this application as long as the common mode range of the chosen part is not exceeded. Depending on the desired output voltage, you might have to choose another part. So please share the output voltage swing that is needed.


    Best,

    Miriam

  • 0 in reply to Miriam Mesek
    Prodigy 240 points

    Thanks Miriam to response my question quick and clearly, I have learn so much, thanks.

    Beside, after discussion with customer, we are changing it from current sense to comparator, because we just need to know the load (motor) have some operation then wake up the MCU is enough, below is our circuit block diagram for discharge part.

    However, we are face another problem, you can see we have a Q1 to control the battery on off by MCU, when MCU is cutoff the Q1, the Load is not cut because the path is change, current flow through the TLV8802 input to ground, when I cut the supply of TLV8802, the voltage flow to regulator from the battery through the load and TLV8802 input. Since we are operate very high current motor, change to P-FET to cut the power is not available (higher Rds)

    Is there any idea to fulfill this? cut the power and current detection when motor operate(no need to detect when cut the Q1 cause the Battery is slept)

    Thanks for the help!

  • 0 in reply to Simon Poon
    TI__Intellectual 1095 points

    Hi Simon,

    thanks for the update.

    Do you know the size of the load, shunt and the remaining resistors?

    Is there a specific reason why you don't consider using a comparator part for this application?

    In an article at out forum you can read some thoughts about why an op amp should be used carefully as a comparator:

    Do you use TINA-TI or another SPICE based program for simulations? TINA-TI is for free and can help you understand the behavior of your schematic.

    If you can give me more information about the schematic, I will try to simulate a solution for your application as well.

    Best,

    Miriam

  • 0 in reply to Miriam Mesek
    TI__Intellectual 1095 points
    Hi Simon,

    do you still have questions regarding the current sensing and battery control?
    Reach out to us if you need further support.

    Best,
    Miriam