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SN65176B: about capacitor between Vcc and GND

ACGUY
Guru 11170 points
Part Number: SN65176B


Hello E2E,

Please let me know about capacitor between Vcc and GND.
In the datasheet, that capacitor is 1.5-uF. However, the datasheet does not say that why.
Our customer is using 0.1-uF cap between Vcc and GND, R-output has bounce during operation.
They changed the capacitor's value from 0.1-uF to 1.5-uF, it has been resolved.

Why should the capacitor value be 1.5-uF?
Is there any documentation that mentions this?

Regards,
ACGUY

  • 0
    TI__Guru 74322 points

    ACGUY,

    When you say R-output bounce during operation, do you mean there was ringing, or the signal voltage dropped and then came back? In general, the capacitor on VCC will help keep the supply voltage stable when there is a sudden current change, like what would happen when driving communication on this device. 

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    Guru 11170 points

    Erick,

    The signal voltage dropped.

    0.1uF Ceracap is often used for Vcc of general bipolar processing ICs.
    The customer used 0.1-uF CAP according to that convention.
    The SN65176B datasheet does not have a detailed description of the Vcc cap, nor is there a "recommended value" for it.
    However, the layout diagram just says 1.5-uF.
    I would like to know "How to" to determine the value of the Vcc cap.

    Regards,
    ACGUY

  • 0 in reply to ACGUY
    TI__Guru 74322 points

    ACGUY,

    Good question, even with our newer RS422 and RS485 transceivers we only recommend 0.1uF to 0.22uF. Since this is an older device, I need dig into the history of it to understand why the previous team recommended 1.5uF. I'll get back to you by Monday.

    Regards,

    Eric Hackett

  • +1 in reply to Eric Hackett
    TI__Guru 74322 points

    ACGUY,

    When I looked into this, there isn't anything inherently wrong with this device that makes it require more capacitance. The reasoning behind the higher capacitance recommendation is due to the fairly large current consumption when transmitting at 40mA to 70mA. The bulk capacitance allows the voltage to stay high even with the sudden large current load when transmitting high and low bits. 1.5uF was the recommendation based on simulations and what was the minimum amount that would cover the majority of power supplies powering this device.

    Please let me know if you have any other questions.

    Regards,

    Eric Hackett