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ISO7220 Question

Mark Pieper
Expert 4640 points
Other Parts Discussed in Thread: ISO7420FE, ISO7420FCC, ISO7221C

My customer has observed that when input side Vcc is powered UP, and the output side is powered down, when the output Vcc is turned on and rises, The output is high (follows Vcc) until it gets to it's minimum Vin. This signal is used to drive a PWM signal and he would like to have it stay low during power up. Please tell me if a pull down resistor could be used to keep the output low during power up.

  • 0
    TI__Mastermind 28265 points

    Mark,

    your customer can use a pull-down resistor but must also ensure that the input voltage is low during the power-up ramp.

    Reason:

    When the input supply is already powered-up and the output supply is slowly ramping up then

    1) Until the output supply reaches its POR-level (Power-On-Reset), the output is high-impedance independent of the input level. During this time a pull-down resistor is effective.

    2) Once the output-supply is above the POR-level, the output follows the input level. So if the input is high, the output is high too.

    Hope this helps.

    Regards,

    Thomas

     

  • 0
    TI__Genius 11765 points

     

    Please take a look at ISO7420FE or ISO7420FCC devices. Datasheet is available on ti.com. These devices have default output LOW whereas ISO7220 has default output HIGH.

    ISO7420FE is released to production while ISO7420FCC is expected to release in May. I prefer ISO7420FCC for low speed applications because this device has integrated input noise filter for noisy application environments. We can send you samples.

     

  • 0 in reply to Saleem Marwat
    TI__Mastermind 28265 points

    I guess he would have considered these devices if they would come up when clicking the "Failsafe-Low" option.

    Unfortunately only ISO76xx devices appear under that option.

    When will the TI-site be updated to reflect ALL iso devices with failsafe-low output?

  • 0 in reply to Thomas Kugelstadt
    Intellectual 600 points

    Hello Thomas,

    I'm using a ISO7221C for a similar application (isolated gate driver) and I need to know the "POR-level" value, which I couldn't find in the datasheet.

    I believe that the "POR-level" is the same threshold used for the "failsafe" feature. Please confirm me that I am right.

    Are these values consistent?

    Thanks for your help.

    Regards,

    Max

  • 0 in reply to Max Sonnaillon
    TI__Mastermind 21800 points

    Hi Max,

    You are correct. There is also a graph on page 19 of the datasheet that shows the undervoltage of the device for rising and falling supplies across temperature. I would use the rising edge as the same as the POR.

    http://www.ti.com/lit/gpn/iso7221c

    This is a typical graph, but it should be helpful.

    Let me know if you have any other questions,

    John