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ISOW7741: Issues Achieving 3.3V Isolated Output

Amanda Muller
Prodigy 10 points

Part Number: ISOW7741
Other Parts Discussed in Thread: ISOW7844,

We recently received samples of the ISOW7741 as a potential replacement for the ISOW784X family. We used jumper wires on an existing board with the ISOW7844 footprint to power the ISOW7741 DC/DC converter for very basic EMC comparison testing. With the digital isolator pins disconnected/floating (VIO, EN_IO1/2, and all digital channels), we measured 5V on the secondary/isolated side despite inputting 3.3V at VDD, pulling EN_DCDC to VDD, connecting Visoin to Visoout, and shorting VSEL to GND2.

After pulling the chip off the board and powering it directly from a 3.3VDC power source (bypassing all of the input/output caps, ferrites, etc), we couldn't get more than 0.8V on the isolated side. We reconnected the full circuit (digital isolator included) to our test board, but now we are measuring 0.8V between Visoout and GND2. The final circuit is included below: existing PCB circuit in red, remaining connections completed using jumper wires.

Any idea why we can't seem to get 3.3V at Visoout?

 

  • 0
    TI__Intellectual 1960 points

    Hi Amanda:

    Thanks for posting your question on E2E!

    Unfortunately, your circuit diagram didn't get attached properly. However, unlike our ISOW7844, ISOW7741 requires VIOSOUT to be externally connected to VIOSIN at all times to complete the feedback loop to the primary side of the DC-DC. If this feedback is broken, the device output will go into over-voltage condition (and also has the potential to sustain permanent damage).

    To ease the evaluation of ISOW7741 in existing designs with ISOW784x footprint, we've developed an adapter board. You'll be able to solder this adapter board directly onto the existing ISOW7844 footprint. Please let me know if you're interested in obtaining this adapter board for your EMC and functional evaluations - I can reach out to you via your email address on myTI account.

    Regards,

    Dushmantha

  • 0 in reply to Dushmantha Rajapaksha
    Prodigy 10 points

    Hopefully this image actually appears. Regardless, we do have Visoout connected to Viosin, but we still aren't measuring the correct voltage. An adapter board would be perfect!

     

  • 0 in reply to Amanda Muller
    Prodigy 10 points

    Everything outside of the 3.3V and 3.3V_ISO markers is on the existing circuit. The 5k pull up resistor and all of the chip connections were made with jumper wires.

  • 0 in reply to Amanda Muller
    TI__Intellectual 1960 points

    Hi Amanda:

    Looks like the ISOW7741 had sustained permanent damage since VISO measures only 0.8V. Please evaluate ISOW7741 using the adapter board that I'll be shipping to you. Please feel free to click on "Ask a related question" or post a follow up question here if you have any questions after evaluating ISOW7741 on the adapter board.

    Regards,

    Dushmantha