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ISOW7841: VISO output

David Zhao
Intellectual 1820 points
Part Number: ISOW7841

Hi

My customer is adopting ISOW7841F. Schematic is as below. ADUM5401 is pin compatible with ISOW7841F. Input is 3.3V. When test VISO between PIN16/15, the voltage is not stable as below. But when change to ADUM5401 solution, the VISO output is good. Please help comment the reason. Thanks a lot!

Regards

David

  • 0
    TI__Intellectual 2325 points
    Hi David,

    Thanks for using E2E.

    To double-check, are you referring to the ripple in the output waveform? If yes, then some ripple is common and normal for both parts in question.Both ADUM5401 and ISOW7841(F) specify output ripple in their respective datasheets.

    -In the first case, there is a typical output ripple of 50mV specified at 54mA load current, 20 MHz bandwidth, and with output caps of 0.1 µF||10 µF.
    -In ISOW7841F, in section 7.13 of the datasheet (link), a typical output ripple of 90mV (pk-pk) specified at 75mA load current , 20MHz bandwidth and with output caps of 0.1 µF|| 20 µF. This is for 3.3V input and 3.3V output.
    -There are also some sample scope shots shown in Figure 30 and Figure 31.

    Please let us know if this addresses your question or if you need additional clarifications.
    Regards,
    Abhi
  • 0 in reply to Abhi Aarey
    TI__Intellectual 1820 points

    Hi Abhi

    Thanks for quick response!

    In fact, it should be not ripple in VISO output. It is not regular as ripple. Also the PK to PK value is about 300mV. It is not acceptable by customer.

    Regards

    David

  • 0 in reply to David Zhao
    TI__Intellectual 2325 points
    Hi David,

    I see two things going on there. There is a thin, solid yellow line (periodic sawtooth shape) and noise (maybe from the probes?) riding on the top of that and varying irregularly from left to right of the picture. At the left, it hits the upper cursor; and on the vertical division just right of the origin, it hits the bottom cursor, so this band is indeed 300mV but it spans across multiple cycles.

    Is it possible to get a cleaner scope shot? Please feel free to email me if you'd like us to review any additional schematics and waveforms.
    When reviewing for noise, it is also helpful to know more board level details and end-application details if possible. The internal switching frequencies of the internal converters in these two products are different, so analyzing the setup will be helpful.

    Thanks and happy holidays!
    Abhi