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ISOW7840: Low Frequency Noise

cecilia yang
Prodigy 30 points
Part Number: ISOW7840
Other Parts Discussed in Thread: ISOW7740

Hi expert,

I use ISOW7840 in the system. Below is the SCH.

And I heard some noise from the IC.  I measure the ripple of Viso, and find there is a 10K low frequency noise on it.

I try to remove C20 , and then the IC work normally. There is no low frequency noise.

I find the description in the DS, input decoupling capacitor should be larger than output capacitor by at least 100 µF.

But TI remove the description in 2021.

The Date code of the IC I use is 2145. Could I keep input decoupling capacitor should be larger than output capacitor by at least 100 µF?

Now I remove C20, and everything is OK, can I use like this?

Thanks!

  • 0
    TI__Guru 52660 points

    Hi Cecilia,

    Welcome to TI E2E Forum!

    Thanks for sharing the schematic and details related to your question. ISOW7840 supports variable regulation frequency to offer best efficiency across loads where the regulation frequency increases with increase in output load and decreases with decrease in output load. Due to this you may see low frequency ripple at the output of ISOW7840 when there is no load connected and when there is large output capacitance.

    A large output capacitance under no load conditions keeps helps regulate the output well there by allowing ISOW7840 to decrease its regulation frequency further. If you do not prefer to see such lower frequencies then you could change the value of C20 capacitors from 47µF to 10µF and that should increase the ripple frequency to >10kHz.

    cecilia yang said:
    Could I keep input decoupling capacitor should be larger than output capacitor by at least 100 µF?

    You do not need to add this optional 100µF capacitor at the input, everything should work fine without this capacitor. This is the reason we have removed the suggestion recommending 100µF.

    I would also like to mention that ISOW7840 is our first-generation isolator with integrated DC/DC converter. We also have the second-generation device ISOW7740 that offers many performance improvements over ISOW7840 and we recommend you to use ISOW7740.

    Please note that the DC/DC converter in ISOW7840 integrates power transformer in the device and that is achieved by switching the DC/DC converter at very high frequency. Such high frequency power switching could lead to relatively higher radiated emissions if the PCB design is not optimized. Please see below E2E post that lists the optimizations that we commonly recommend to achieve best emissions results.

    https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/806938/isow7841evm-isow7841evm

    Similarly, the EMI optimizations required due to ISOW7740 are much simpler compared to ISOW7840. Due to this reason, we recommend you to use ISOW7740 instead. Please see below an application note that briefly describes how to achieve best emissions results with ISOW7740. Let me know if you have any questions, thanks.

    How to Meet CISPR 32 Radiated Emissions Limits With ISOW7741


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 30 points

    Hi Koteshwar,

    Thank you very much for your reply. 

    Now I know why the noise appearance.

    Thanks

    Regards