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SN75188: Output noise under the operation in high temperature

na na78
Guru 16770 points
Part Number: SN75188
Other Parts Discussed in Thread: SN55188

Hi

We are facing the noise issue of SN75188N output under the operation in 50℃ to 55℃.

Input signal : 5V, 9600bps
Power Supply : +/- 12V

W1: Output Port
W2: Input of SN75188N
W3: Output of SN75188N

However, there was no noise under the operation in normal temperature (around 25℃).

And the there was no problem about supply voltage when the noise occurred.

What do you think cause is?
Could you please give us your view?

BestRegards

  • 0
    TI__Mastermind 25705 points
    Could you please give more details about the loading condition, like the component value/model? Is there any load at output port? Is the noise periodic? Is it related to the supply variation? I'll follow up with you. Thanks!
  • 0 in reply to Hao L
    Guru 16770 points

    Hi Hao

    Thank you for your support.

    -component value/model

    C29:1nF, C30:2.2nF, R15:220ohm

    A programmable controller is connected to output port.

    -Is noise periodic?

    No.  The noise seems to occur randomly.

    - Is it related to the supply variation?

    No.  The supply doesn't seem to be varied and not related to noise.

    BestRegards

  • 0 in reply to na na78
    TI__Mastermind 25705 points
    What I meant was if the noise is related to the voltage value of the supply.

    What's the input impedance of the controller?

    Can you also provide rise/fall time of normal operation? In datasheet, RL of rs232 device is between 3k~7kOhm. Thanks!
  • 0 in reply to Hao L
    Guru 16770 points
    Hi Hao

    Sorry for late reply.

    We are checking the impedance of the controller.

    By the way, do you have information of Rjc(junction to case)?

    Or is it OK to use Rja to calculate Tj?

    BestRegards
  • 0 in reply to na na78
    TI__Mastermind 25705 points
    I can request the information for you. You are using PDIP package N, right?

    Is it OK to use Theta JA to calculate junction temperature, if we know the power consumption.
  • 0 in reply to Hao L
    Guru 16770 points
    Hi Hao

    Thank you for your reply.

    >You are using PDIP package N, right?
    Yes.

    >Is it OK to use Theta JA to calculate junction temperature, if we know the power consumption.
    We are taking the data. Let me inform you if the data is collected.

    Regarding to the issue in this case, I noticed one more concern.

    It is input tr/tf time.

    1. Does SN75188N have the spec of input tr/tf time? If a slow tr/tf is input, is it likely to occur this issue?
    We are checking the input signal also.

    2. VIH and VIL are characterized as VIH(min) = 1.9V and VIL(max) = 0.8V.
    Is this specification varied depending on temperature?
    Now, the supply voltage is +12V/-12V.

    BestRegards
  • 0 in reply to na na78
    TI__Mastermind 25705 points
    I don't see any spec on input tr/tf time. Although it's not specified, I think the device is designed to run up to 250k bps. In general what limits the speed would be at the driver output where large load might exist. If you change input rise/fall time, do you see any difference?

    Temperature and supply variation should already be taken into consideration in VIH/VIL spec. If you have concerns, you can also test SN55188 that are characterized wider temperature range.
  • 0 in reply to na na78
    TI__Mastermind 25705 points
    Rjc is 63.0.