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SN6501: Schematic Review

Lauren Mayfield
Expert 8155 points

Part Number: SN6501
Other Parts Discussed in Thread: ISO6742

Hi team,

Could you please review this part on the schematic? It is on page 4, U3.

One thing that looks strange to me is I believe this can only provide a 3.3V or 5V isolated rail. However, the LDO (U4) slightly downstream provides a 5.2V rail. This seems odd to me. 

Can you let me know if I'm correct about the SN65x providing the 3.3V or 5V rail, and if you see any issues with this implementation?

TIDA-010939_Schematic.PDF

Thanks,
Lauren

  • 0
    TI__Guru 56585 points

    Hi Lauren,

    Thank you for sharing the schematic for review.

    Please find my inputs below, thanks.

    1. The schematic mostly looks good.
    2. The VCC pin of SN6501 currently has two 1µF capacitors, we recommend that one of them is changed to 0.1µF or add a new 0.1µF capacitor.
    3. The 0.1µF capacitor provides the transient current required by SN6501 for reliable switching and should be placed close to VCC pin. 1µF capacitor is a bulk capacitor which should be placed close to transformer center-tap.
    4. Transformer 760390015 supports 3.3V to regulated 5V through LDO. Without LDO, the output of SN6501 based solution is going to be between 5.6V to 6.6V. If 5.2V output LDO is used with very low drop-out voltage, then it shouldn't be an issue to generate 5.2V. Please see below the output voltage vs output current plot for 760390015 from SN6501 datasheet.
    5. I also reviewed the schematic of ISO6742 and it looks good.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 8155 points

    Thank you Koteshwar.

    Can you explain why you recommend decreasing the capacitance on VCC pin? Section 8.2.2.4 on the datasheet says that capacitance should be between 1uF-100uF, it's at 2uF right now with the two 1uF caps.

    The LDO drop out is 250mV at 200mA. Let me know if you think that is enough. I will also ask the LDO team to review because this is supposed to be fixed at 5V but it is listed as supplying 5.2V.

    Thanks,

    Lauren

  • +1 in reply to Lauren Mayfield
    TI__Guru 56585 points

    Hi Lauren,

    Thanks for your inputs.

    Lauren Mayfield said:
    Can you explain why you recommend decreasing the capacitance on VCC pin? Section 8.2.2.4 on the datasheet says that capacitance should be between 1uF-100uF, it's at 2uF right now with the two 1uF caps.

    I am not suggesting to decrease rather I am asking you to introduce one 0.1µF capacitor either adding a new one or replacing one of the existing capacitors. Most power supplies and/or digital circuits typically require one bulk cap and one transient / decoupling cap. The 1µF cap serves as a bulk cap whose value can be anywhere between 1µF to 100µF while the 0.1µF serves as the decoupling cap.

    Lauren Mayfield said:
    The LDO drop out is 250mV at 200mA. Let me know if you think that is enough. I will also ask the LDO team to review because this is supposed to be fixed at 5V but it is listed as supplying 5.2V.

    Yes, 250mV sounds good. The LDO team will be able to suggest the best device for this requirement. Thanks.


    Regards,
    Koteshwar Rao