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TL7700: /RESET H/L chattring problem

Kazuya Nakai59
Guru 20780 points
Part Number: TL7700
Other Parts Discussed in Thread: TPS3808, TPS3890

 Hello guys,

 One of our customers is evaluating TL7700 for their new product. Then they faced a problem that /RESET signal repeated H/L level a few times when VCC goes down.

 We have 3 questions as the follow.

 Q1: Why SENSE pin voltage is not shift down immediately by the internal constant current source when /RESET is goes L level?

 Q2: Our customer added this resistor (R163) to avoide /RESET H/L chattering and as the result, /RESET H/L chattering problem was removed. Is this countermeasure not problem for TL7700?

 Q3: Is there any way to avoid the RESET signal H/L chattering except R163 addition?

 I attached the customer circuit and  several waveform of the problem.

 Could you please take a look the attached file and please give me your comment and solution for the customer?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai. 

TL7700_Waveforms_Schematics.pdf

 

  • 0
    TI__Genius 12630 points

    Kazuya,

    The device reset should not be chattering like that. Has this been observed on multiple devices?

    Also, since Customer is evaluating devices for new product, I would recommend them look at TPS3808 or TPS3890. These devices are voltage supervisors with programmable delay just like the TL7700 but offer much better performance in terms of device accuracy, Iq, and size. TPS3890 is the best performance and smallest size for these type of voltage supervisors.

    -Michael

  • 0 in reply to Michael_DeSando
    Guru 20780 points
    Michael,

    Thank you very much for your reply.
    Yes, the chattering was observed on about 10 boards.

    Do you have any idea about the phenomenon cause? Your comment would be helpful for the customer and me.

    Also we've already introduced those devices as replacing devices.

    Thank you very much and best regards,
    Kazuya Nakai.

    Thank you and
  • 0 in reply to Kazuya Nakai59
    Guru 20780 points
    Michael,

    About Q2, the chattering problem is removed when R163 is attached between /RESET pin and SENSE pin of TL7700 because SENSE voltage becomes lower when /RESET goes low. Electrical wise, is this countermeasure not problem for TL7700? In other word, is TL7700 not broken by R163 addition?

    Your reply would be much appreciated.

    Thank you and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Genius 12630 points
    I do not believe adding a resistor (R163) from SENSE input to /RESET output is a problem. Essentially you are simply increasing the hysteresis of the sense voltage. This technique is outlined in an app note called Adding Hysteresis to Supply Voltage Supervisor www.ti.com/.../slva360.pdf

    Please refer to "Rh" resistor in Figure 1 on page 2. Although the device is different, the idea is the same. Please let me know if you have any questions or need any additional support. Thanks!

    -Michael
  • 0 in reply to Michael_DeSando
    TI__Genius 12630 points
    Kazuya,

    I also notice that datasheet uses 2.2kohm pull-up resistor but customer is using 10kohm pull-up resistor. I don't think this is responsible for the chatter but I think adding R163 to add hysteresis is a good way to prevent chatter and false triggers.

    -Michael
  • 0 in reply to Michael_DeSando
    Guru 20780 points
    Nichael,

    Thank you very much for your reply and comments. It is very helpful for the customer and me. I introduced the app note base on your comment.

    Thank you again and best regards,
    Kazuya Nakai.