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TPSM846C23EVM-806: Issue with reading READ_IOUT

fatih cingoz
Intellectual 660 points
Part Number: TPSM846C23EVM-806
Other Parts Discussed in Thread: TPSM846C23

With no load connected I always read 0xE018. The MSB of 0xE0 makes sense but I do not understand why do I read the LSB as 0x18 as you can see from the snapshot below ?

Thanks in advance...

  • 0
    TI__Genius 14455 points

    Hi Fatih,

          We will take a look at this and get back to you shortly.

    Regards,

    Gerold

  • 0
    TI__Mastermind 48180 points

    Fatih,

    The TPSM846C23 reports READ_IOUT using the LINEAR11 data format.  The upper 5 bits (15:11) are a 2s compliment value for the exponents and the lower 11 bits are the twos-compliment Mantissa, where the represented value is 2^Exp * Mantissa.

    E0 represents an exponent of -4

    018 represents a mantissa of 24d

    2^-4 * 24 = 24 / 16 = 1.5A

    Read Iout is reporting 1.5A, representing the no load offset of the current sense measurement.  This is consistent with the +/- 3A accuracy specification for the digital current reporting.

  • 0 in reply to Peter James Miller
    Intellectual 660 points

    Thanks Peter,

    It looks like even when I connect a load there is always 1.5A offset.

    I should always take this 1.5A offset into account in my calculations right?

  • 0 in reply to fatih cingoz
    TI__Mastermind 48180 points

    Fatih,

    The specific offset exhibited by an individual module will vary from part to part and across applications and assemblies, but once measured, the offset can be cancelled by entering the offset value into the PMBus Command IOUT_CAL_OFFSET and storing this value to NVM with STORE_USER_ALL.  The TPSM546C23 module will add the offset to all measured READ_IOUT responses to cancel the offset.

    Since there can be some non-linear effects close to 0A, it is best to measure the READ_IOUT offset under a reference load current, such as 10A.

  • 0 in reply to Peter James Miller
    Intellectual 660 points

    Peter,

    Thanks for the clarification. A quick question regarding the nonlinear effects at low currents. In our design we will mostly draw around 1A and it will never exceed 3A. It makes sense to me taking the reference load current as 1A but it is close to the 0A. Does that mean we can not perform precise current measurements under 1A due to the nonlinear effects?

    Thanks...

  • 0 in reply to fatih cingoz
    Intellectual 660 points

    Peter,

    Thanks for the clarification. A quick question regarding the nonlinear effects at low currents. In our design we will mostly draw around 1A and it will never exceed 3A. It makes sense to me taking the reference load current as 1A but it is close to the 0A. Does that mean we can not perform precise current measurements under 1A due to the nonlinear effects?

    Thanks...

  • 0 in reply to fatih cingoz
    TI__Mastermind 48180 points

    Fatih,

    I am sorry about the delay getting back to you. I am currently on vacation, but noticed this was lingering without response for a while.

    The TPSM546C23 uses the low-side FET as the current sense element, that FET has an Rdson of less than 1mOhm and the current sense circuitry is not well suited for accurate 0-3A measurements.  While many TPSM546C23 modules may be able to provide precise current measurements in the 0A to 3A range after some calibration, I can not recommend it as a mass solution.  It would be possible for some TPSM546C23 modules to report near zero current over the entire load range from 0A to 3A of load.