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Original question:

ISO7740: External Circuit needed to keep ISO7740F from glitching on power-up

ISO7740: Secondary side output outputs H level shortly when the secondary sode power is powered up. Is this no irregular operation?

Kazuya Nakai59
Guru 20100 points
Part Number: ISO7740

 Hello guys,

 One of my customers is evaluating ISO7740 on their own board for their new products.

 At the evaluation, they found the secondary side output of the device outputs H level shortly (1.5V hight and 3~4us width pulse)  when the secondary sode power was powered up with about 200us from 0V to 3V just after the primary side VCC (VCC1) was powered up.

 Then EN2=open and INx="L" level.

 The customer question is the follow.

 Is it normal operation that the short pulse is output at secondaly side VCC (VCC2) powered up inspite of low INx?

 Your reply would be much appreciated.

 Best regards,

 Kazuya. 

  • 0
    TI__Mastermind 49445 points

    Hi Kazuya-san,

    Thank you for sharing details related to customer question.
    Please do note that all the outputs of the isolator first go to failsafe/default state after power-up for a very small duration and then start following the input. Like customer has observed, this duration is going to be a few µs.

    Since typical UVLO+ is about 2V, device will usually power-up at this voltage and if the default output state is HIGH the output voltage will be <2V for a small duration.

    If customer input is LOW after power-up and customer doesn't want to see such a small pulse then customer can use a default LOW device ISO7740F. This will make sure output first goes to LOW and then starts to follow the input. Since default state and input state after power-up are the same, customer will not see any pulse at the output. Similarly, if the input is HIGH after power-up, customer can use default HIGH device ISO7740 (without 'F' suffix) so that the output doesn't see any pulse.

    I hope that clarifies your question. Let me know if you have any other questions, thank you.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Guru 20100 points

     Hi Rao,

     Thank you for your reply.

     I could understand well.

     Thank you again and best regards,

     Kazuya.