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ISOW7840: Oscillation frequency of internal DC/DC

MESH
Intellectual 880 points
Part Number: ISOW7840
Other Parts Discussed in Thread: ISOW7741

Hello,

I am considering circuit board used ISOW7840.

I am considering of using the ISOW7840 as the SPI communication interface with the DA converter and the VISO power output as the digital power supply to the DA converter.

And I am considering of adding an LC filter to the output of the VISO power supply for ripple reduction.

I want to know the oscillation frequency of the ISOW7840 built-in DC/DC converter for the design of this LC filter. (to determine cut off frequency)

Conditions are below.

- VISO voltage: 3.3V

- ISOW7840 operation: All 4ch input / output are used 10Mbps SPI communication

- Output current range of VISO to DAC digital power input: approx 0.1mA to 5mA

Regards,

MESH

  • +1
    TI__Guru 55845 points

    Hi MESH,

    Thanks for reaching out to us and also for your interest in ISOW7840.

    The switching frequency of DC/DC converter is ~60MHz but the output voltage ripple observed would be seen at the regulation frequency. The device use variable frequency regulation to achieve the good efficiency even at low output load currents. Hence, the regulation frequency is going to be 120kHz at max load current and can go as low as 10kHz for low load currents. Since the load current requirement in your application is very low, I would recommend choosing cut-off frequency as 10kHz or lower.

    An alternate and better solution would be to use ISOW7741 which is the second generation device while ISOW7840 is from the first generation. ISOW7741 offers significantly better performance than ISOW7840 including lower output voltage ripple and radiated emissions. Please refer to the below E2E post for a comparison between ISOW7741 and ISOW784x.

    https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/985520/isow7741-emc-inquiry

    Let me know if you have any further questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Intellectual 880 points

    Hi Koteshwar,

    Thank you for your reply.

    Your reply is very helpful for me.

    I would use this 10 kHz as a guide to consider LC filter.

    And I will check ISOW7741.

    Regards,

    MESH