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Vih for lm49450 I2S interface

Lucas McKeehan1
Intellectual 755 points
Other Parts Discussed in Thread: LM49450

Looking at the LM49450 specification for Vih on the digital inputs, I read a minimum of +2.8V is required.

However the datasheet also says that IOVDD can be as low as +1.8V: “IOVDD powers the I2S and I2C, allowing the LM49450 to interface with lower voltage digital controllers”.

So that +2.8V requirement does not make sense and should instead be a percentage of IOVDD.

Could you Clarify whether there is a different Vih specification?

 

Another related question.

Making the lower limit of IOVDD +1.8V is confusing because that does not really include a +1.8V supply.

To be able to power IOVDD with a typical +1.8V regulator/supply, the IOVDD spec should be >= +1.7V to account for the regulator/power supply variation.

Any chance LM49450 can actually be specified for an IOVDD >= +1.7V ?

Thanks For the Help,

Lucas McKeehan

  • 0
    TI__Guru 52115 points

    Hi Lucas,

    The LM49450 datasheet only specifies the I/O voltage range for specific conditions. The I2S specification is specified by originally Philips. See the attached i2s spec. This device should follow this spec as well.

    I2SBUS philips spec.pdf

    Andy

  • 0 in reply to Andy Liu (BMS)
    TI__Intellectual 755 points
    Hi Andy,

    Thanks, this clarifies the first question. To the second question of IOVDD supporting a minimum input of 1.8V, what is the tolerance here? If i put a standard 1.8V regulator on the input, you could easily see 1.7-1.9 V. What will happen during an I2S communication when the supply drops to 1.7V?

    BR,
    Lucas
  • 0 in reply to Lucas McKeehan1
    TI__Guru 52115 points
    The specified operating range for IOVDD is from 1.8V to 4.5V, so the 1.8V should be the min guaranteed voltage. From a QA's perspective, below 1.8V is not guaranteed by TI. If your regulator's accuracy is +/-100mV, you should set the center voltage 1.9V.
  • 0 in reply to Andy Liu (BMS)
    TI__Intellectual 755 points

    HI Andy,

    Couple questions:

    1.  requiring an input voltage of 1.9V may break some standards for 1.8V signaling. Can you take a look at what is limiting the IOVDD from supporting a 1.8V rail?
    2. Can you please calculate the following Vihs for the different input voltages?
      1. What is the Vih for the digital interface when IOVDD is +2.5V?
      2. What is the Vih for the digital interface when IOVDD is +1.8V?



  • 0 in reply to Lucas McKeehan1
    TI__Guru* 84915 points

    Hi Lucas,

    1. I compared the LM49450 IOVDD behavior with another devices. It seems that the IOVDD min value is limited to the VIH min value. The VIH min value depends of IOVDD (you may see the next answer for details), but it has a limit where VIH cannot be lower. So, that limit is taken as IOVDD min voltage. Unfortunately, we don't have an EVM to make a test.

    2. The VIH min value can be calculated as a percentage of IOVDD. The min value is obtained with 0.7 x IOVDD. I guess that for LM49450 datasheet, the VIH was calculated with IOVDD = 4V.

    I hope this helps you. If you still have questions, please let me know.

    Best regards,

    Luis Fernando Rodríguez S.

  • 0 in reply to Luis Fernando Rodriguez S.
    TI__Intellectual 755 points

    Hi Luis,

    I am still slightly confused. Does this mean that the 1.8V on IOVDD spec is the minimum Vih supported by the device, therefor the true minimum input voltage is 1.8V/0.7=2.6V, or that the 1.8V IOVDD spec mean that the absolute minimum Vih is 1.26V?

    BR,

    Lucas McKeehan

  • 0 in reply to Lucas McKeehan1
    TI__Guru* 84915 points

    Hi Lucas,

    I mean, the 0.7 x IOVDD condition only works when IOVDD > 2.6V. When IOVDD < 2.6V, the min VIH value must be 1.8V. This is a behavior that I could see in another devices where the min VIH and min IOVDD values were 1.1V.

    I apologize, I don't have an EVM, so I can't guarantee it.

    Best regards,

    Luis Fernando Rodríguez S.