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ISO1212: Vcc1 hysteresis

Davut Uzun
Prodigy 40 points
Part Number: ISO1212

Dear TI Team,

what does the hysteresis (VCC1) mean, or how should I understand it?

Because if my VCC1 increases from 0V up to 3.3V and the IC can be operated. But if the voltage decreases up to 2.00V will it be turned off because the UVLO threshold hysteresis (VCC1) is 0.2V or must it be below 1.7V? But in this case, why is hysteresis (VCC1) is 0.2V? 

I guess the hysteresis (VCC1) value is typical and could vary. Therefore, the difference between 2.25V and 1.7V (the positive and negative-going UVLO thresholds) is greater than 0.2V.

Under these assumptions:

VCC1 from 0V to 3.3V is powered up and will be powered up until the voltage drops to 1.7V and not 2.05V.

VCC1 from 0V to 2.20V  is NOT powered up.

How should I understand the power up or down for the state when 1.7 V < VCC1 < 2.25 V.

Regards

Davut Uzun

  • 0
    TI__Genius 16305 points

    Hello,

    Thank you for your question. 

    Please give us an additional 24 hours to respond.

    Regards, 

    Aaditya Vittal

  • 0 in reply to Aaditya V.
    TI__Genius 16305 points

    Hello,

    Thank you for your patience.

    The Vhys (hysteresis) of the device is mainly a tolerance value for the device when it is powering up or powering down. When the device powers in, the current it draws can cause the supply voltage to drop. If there is no hysteresis, then the device would switch off immediately due to the voltage drop.

    The Vhys is a voltage "tolerance" or range that the voltage drop can occur but the device will not immediately off. In order for the device to actually turn off, the voltage would have to cross the entirety of the Vhys threshold and enter the VUVLO- range.

    In terms of the ISO1212, device will power on when the voltage is greater than or equal to 2.25V and will power off when voltage less than or equal to 1.7V. The Vhys represents the gap between the max value of VUVLO- and VUVLO+, which is 0.2.

    Please read this Application Note for more information on UVLO and hysteresis: Understanding Undervoltage Lockout in Power Devices (Rev. A)

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    Prodigy 40 points

    Hello Vittal,

    thanks for the application note, but according to this application note, I need the minimum value of UVLO threshold hysteresis (VHYS(UVLO1)) and the maximum value of negative-going UVLO threshold (VIT-(UVLO1)) to make a worst-case analysis. It is possible to provide these values.

    Regards

    Davut Uzun

  • 0 in reply to Davut Uzun
    TI__Genius 16305 points

    Hi Davut,

    Thanks for your reply.

    The values for VIT+UVLO1(max) and VIT-UVLO1(min) provided in the datasheet are the "worst-case" for the device. This means that the device is guaranteed powered on when it crosses the 2.25V VIT+UVLO1(max) value and is guaranteed powered down when it crosses the 1.7V VIT-UVLO1(min) value.

    Vhys(typ) of 0.2V is a window that occurs between the VIT+UVLO1(max) and VIT-UVLO1(min). Operating the device with VCC between 1.7V and 2.25V could result in an undetermined state for the device.

    Keep in mind that this wide range is provided to account for the process variations that could occur. But, The VIT+UVLO1(max) and VIT-UVLO1(min) are the "worst-case" thresholds that account for all devices.

    In order to make sure the device is fully powered up and operating, we recommend to have VCC between 2.25V to 5.5V, as shown in the recommended operating conditions table in the datasheet.

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    Prodigy 40 points

    Hello Vittal,

    I now understand that the operating voltage between 1.7V and 2.25V is undetermined for the device, unlike the application note, which splits this into two regions—functional and undecided. Therefore, the values given in the datasheet are worst-case. But for the threshold hysteresis (VHYS(UVLO1)), the minimum value is needed for a worst-case analyisis, could you provide this? Because, as written in the application note: Typical values are of limited use, because robust system design requires known limits over temperature, and typical values are (a) not limits and (b) only valid at 25°C.

    Regards

    Davut Uzun

  • 0 in reply to Davut Uzun
    TI__Mastermind 26765 points

    Hello Davut, 

    The values provided in the datasheet are the worst-case values. The application note defines this undetermined region as "functional" and "undecided" for reference purposes and it is not recommended to design an application to operate in these regions.

    It is only recommended to operate the device within the recommended operating conditions as listed in the datasheet. Outside of this region the device will not behave as characterizes in the datasheet. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 40 points

    Hello Andrew,

    thanks for the reply, but my question is: the  threshold hysteresis (VHYS(UVLO1)) is given only for TYP, and with this, it is not possible to make a worst-case calculation. Therefore, we need the MIN value of it.

    In the Application Note on UVLO and hysteresis: Understanding Undervoltage Lockout in Power Devices (Rev. A) as explained there.

    "Typical values are of limited use, because robust system design requires known limits over temperature, and typical values are (a) not limits, and (b) only valid at 25°C."

    and also in this Application Note

    "If some parameters are missing, contact the factory via the E2E forum for clarification."

    Therefore, it would be very helpful if you could share the min value of the  threshold hysteresis (VHYS(UVLO1))

    Regards

    Davut Uzun

  • +1 in reply to Davut Uzun
    TI__Mastermind 24945 points

    Hello Davut,

    The hysteresis is a typical value as you mentioned, given a worst case hysteresis is not provided, the worst case that can be assumed would be VHYS(UVLO1) = 0.0V. Essentially the worst case would be that there is no hysteresis and therefore there is a possibility that (given noise on the supply) the device could turn on and off very close to the same voltage. 

    For example, the device could have an actual UVLO of 2.0V with no hysteresis, so upon startup the device would turn on at 2.0V, and if the supply dips then the device would turn off at 2.0V as well.

    Essentially the device is guaranteed to be on >2.25V and guaranteed to be off <1.70V. There is some hysteresis to help prevent switching between on/off states rapidly on power up/down if there is noise on the supply, but this hysteresis does not guarantee that the device will stay on if the supply goes to 2.25V and then drops below 2.24V. It is possible by the datasheet spec (but also extremely unlikely in reality) that the device could be off at 2.24V.

    I hope this helps to clarify the situation, let us know if we can help further.

    Best,

    Michae