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INA237-Q1: several questions about the calculations and alert

DW Lee
Prodigy 40 points
Part Number: INA237-Q1
Other Parts Discussed in Thread: INA237

Hello,

We have some questions about calculations and alert

[Design parameters example]

Bus supply rail                           48V

Maximum Expected Current      10A

Power over limit                         480W

Q1. If we want to put 480W to PWR_LIMIT register, please check below calculations are right.

Max_Current = 10A

Current_LSB = Max_Current / 2^15 = 10 / 2^15 = 0.0003051758

Power_LSB = Current_LSB*0.2 = 0.0003051758 * 0.2 = 6.10352e-5

Power Over Limit register LSB = 256 * PowerLSB = 0.015625

So, to have a limit of 480W, set the register to 480 / 0.015625 = 30720  ( = 0x7800)

_

________________________________________________________________________________________________

Q2. Circuit will be disabled when an Alert occurs from INA237, since alert pin is designed as one of the circuit switches.

So, Voltage / Current / Power value will be zero after INA237 make Alert.

We wonder is there any feature that we can read (1)right-before-alert-V/I/P-value after alert occurred. - not (2)value

i.e. Can INA237 store or hold the V/I/P value if alert condition is meet?

Thank you,

Lee

  • +1
    TI__Mastermind 22065 points

    Hello Lee,

    Thank you for using the TI forum, here are my responses to your questions:

    1) Yes, your understanding and calculations are correct.

    2) There is no direct feature to disable device conversions after an ALERT event, however the result registers will not change until a new conversion has taken place. Because of this, you have a couple options based on your environment and how fast you are reading data. 

    a) When using Continuous Conversions: If your conversion time is large enough and your MCU is fast enough, then you would be able to read the existing value before the next conversion has completed (this would be triggered from the ALERT as well to prioritize the MCU). It is important to note, that the ALERT can respond before the conversion that triggered the ALERT is even complete, giving you a little extra time. Optionally, you can also disable conversions after receiving the alert.

    b) When using Triggered Conversions: This method is more simple since all conversions are started manually. Simply don't start another conversion when there has been an ALERT and the data will remain in the register until you are ready to read it. 

  • +1 in reply to Mitch M
    TI__Mastermind 22065 points

    Hello Lee,

    One more side note about Q2. If you are using the averaging feature, then it's possible that the value that triggered the alert will never show up in the register.