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Original question:

ISO1212: what is the test condition when testing the ISO1212 temperature rise in the EVM board

ISO1212: Why is ISO1212 threshold voltage so high

Liang Hui
Prodigy 40 points

Part Number: ISO1212
Other Parts Discussed in Thread: ISO6721, ISO7720, ISO1211

Hi ISO team,

I saw the ISO1212 VIH and VIL Calculation formula is shown in blow from datasheet.

We can see in the Calculation formula,  8.25V and 7.1V is ISO1212 Fixed threshold voltage.

I have a questions about the threshold voltage.

Why does it set 8.25V and 7.1V rather than 4V and 3V?

If it sets 4V and 3V, so Rthr need use bigger value to get the some VIH/VIL, which more power dissipation move to Rthr from ISO1212 chip. So ISO1212 temperature rise is less.

What is more, If it sets 4V and 3V, I can use it in 5V input system rather than type1/2/3 and replace opto IC. It can cover more application.

Do you think if it makes sense?

Thanks!

  • 0
    TI__Genius 17100 points

    Hi Liang,

    Thank you for posting to E2E!

    The equation for VIH(typ.) and VIL(typ) thresholds use 8.25V and 7.1V as base values respectively since these are the typical HIGH and LOW threshold values in the default case, when Rsense = 562Ω and Rthr = 0Ω, as shown in Figure 5 and Figure 27 in the ISO1212 datasheet and copied below:



    For different values of Rsense and Rthr, these thresholds will scale as factors of the default Rsense and Rthr case. The 8.25V and 7.1V values in these equations should not be changed for any calculations.

    Power dissipation by the ISO1212 chip is limited by the Rsense resistor value, so decreasing this resistance from 562Ω would increase the current draw and power consumption. The typical current limit can be calculated by Equation 1 in the datasheet, copied below:


     
    Since the minimum threshold values for VIH and VIL are 8.25V and 7.1V respectively, this device could not be used for 3.3V or 5V systems. For those cases, we offer digital isolator devices like the ISO6721 and ISO7720 in multiple channel count and direction options for your systems.

    Please feel free to use our ISO121x Threshold Calculator for 9V to 300V DC and AC Voltage Detection tool to explore different thresholds options for ISO121x devices quickly and easily, and let us know if you have any additional questions regarding ISO1212 or our digital isolator devices as well.


    Respectfully,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 40 points

    Hi Manuel,

    I understood your point, But If the ISO1212 chip is designed that minimum threshold values for VIH and VIL are 4V and 3V rather than 8.25V and 7.1V. I think it is better.

    Because, if design type3 system,  and IC power dissipation= (11V - Rth * IL) * IL.

    So Rth is bigger, IC power dissipation is lower.

    So how to let Rth bigger. Let  IC minimum threshold values for VIH and VIL is lower.

    Because 11V= VIH + IL * Rth. 

    So Rth bigger. VIH is lower.

    So I think it is better if ISO1212 is designed that minimum threshold values for VIH and VIL are 4V and 3V rather than 8.25V and 7.1V. 

  • 0 in reply to Liang Hui
    TI__Genius 17100 points

    Hi Liang,

    Thank you for sharing these comments. For systems that require signal threshold values <5V, our digital isolator devices are typically sufficient.

    Power dissipation in ISO121x devices is dependent on the voltage at the SENSE pin (Vsense) and the current drawn by the device I(INx+SENSEx), not the threshold level, so using larger Rthr and Rsense resistors does help reduce power consumption and current draw.

    The equations below can be used to calculate power consumption of the ISO121x where PD1 is the power consumed on the digital output side and PD2 is the power consumed on the field input side:



    I hope this helps clarify your concerns. Please let us know if you have additional questions.


    Thank you,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 40 points

    Hi Manuel，

    Yes, I aggree for systems that require signal threshold values <5V, our digital isolator devices are typically sufficient.

    Just give this suggestion to ISO1212 with lower threshold level. Lower threshold level, you can using larger Rthr when set up Vth =11V (Type3). Larger Rthr, lower power dissipation according your sharing calculation.

    I just wonder if ISO1212 with lower threshold level, are there some risk for chip?

    Thanks!

  • +1 in reply to Liang Hui
    TI__Genius 17100 points

    Hi Liang,

    Thank you for your suggestion. The ISO121x threshold values were chosen to target applications where thresholds >11V are sufficient, like 24V systems, such that this threshold distributes power moderately well between the device itself and the external components. Moving more power to the resistors would mean an increase in cost and size for those components although the ISO1212 itself would dissipate less power.

    Hence, the chosen thresholds keep the overall design at an optimum point for most applications using ISO121x.

    Liang Hui said:
    I just wonder if ISO1212 with lower threshold level, are there some risk for chip?


    ISO1212 threshold levels cannot be reduced using external components, and we do not currently have lower-voltage versions of this device that are compatible with 3.3V or 5V signals.


    Thank you,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 40 points

    Hi Manuel ，

    one more question, I saw the ISO1212 and ISO1211 have SUB pin,  which  Internal connection to input chip substrate.

    May I know what the SUB function is?

    And datasheet told us they don't need connect anything in the PCB and why let SUB put on the pin in the chip?

  • 0 in reply to Liang Hui
    TI__Genius 17100 points

    Hi Liang,

    The function of ISO1211 and ISO1212 SUBx pins are to dissipate heat only, and they should each be connected to their own floating plane as mentioned in the Pin Configuration and Functions section of these devices' datasheet. These pins should NOT be connected to any other signal or plane of a PCB.

    Please let me know if you have any further questions.


    Thank you,
    Manuel Chavez