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ISOW7742: Do I need a Y1 cap? Where do I place it?

Elder Costa
Prodigy 141 points
Part Number: ISOW7742
Other Parts Discussed in Thread: MAX3232E, ISOW7821, ISO7821

For context, it is a medical device. ISOW7742 is part of an isolated serial port (which includes a MAX3232E). Isolation requirement is 3kVac for 60s. We want the port to pass IEC60601-1-2 ed 4 requirements (8kV contact, 15kV through the air).

In a previous design based on ISOW7821 I was advised to place the Y1 capacitor between GNDISO and GND (pins 15 and 2). I made provision for a Y1 capacitor (SMD) capacitor in the schematics, DNP by default unless it is needed for EMC. The PCB is small and circuitry is dense, so I am wondering if I really need the Y1 in the design.

Could you advise if Y1 cap is deemed necessary, and if affirmative, should I place it between GND1 and GND2 (similar to ISO7821 design) or between the floating and non-floating GND planes?

PS: picked "Industrial" application as it is closest to mine.

 

  • +1
    TI__Mastermind 31865 points

    Hello Elder, 

    The ISOW7742 should have less emissions concerns than the ISOW7821 series. The ISOW7742 was designed to minimize EMI emissions, and its internal construction and layout are optimized to reduce radiated and conducted emissions. Please see How to Meet CISPR 32 Radiated Emissions Limits With ISOW7741. If the emissions results shown with the ISOW7742 app note are not acceptable, then we can discuss further mitigation techniques similar to the ISOW78xx.

    In general, a Y1 cap is not preferred since it can affect the isolation performance of the device.

    Best,
    Andrew 

  • 0 in reply to Andrew J
    Prodigy 141 points

    Hello, Andrew.

    Thank you for your quick response. Emission looks just fine, our requirements are CISPR 11 class A, so ISOW7742's performance in this regard is excellent.

    Re the Y1 cap, I thought of it as a mitigation feature for ESD (to divert ESD voltage from the isolator). It provides 2xMOOP and can stand 4kVac. However, I understand ISOW7742 itself can stand +/-8kV ESD (contact) so I suppose I do not need the Y1.

  • 0 in reply to Elder Costa
    TI__Mastermind 31865 points

    Hi Elder, 

    Yes, that is correct. Also, if higher ESD protection is needed I believe it's preferred to discharge the surge to the local ground using one of TI's ESD protection diodes rather than discharging across the barrier using a Y-cap.