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Original question:

ISOW7741: External Supply for VISO_IN?

ISOW7741: Problem with LDO

Bernardo Gama
Prodigy 40 points
Part Number: ISOW7741
Other Parts Discussed in Thread: LP2950

Tool/software:

Regarding the use of a LDO between Visoout and Visoin in figure 10-1.

I tried different 5V LDOs types and can't get Visoout to output voltages greater than 5V so the can output 5V to Visoin.

Is there a sample circuit I can try? Below is one I tried.

Thanks

  • 0
    TI__Mastermind 31655 points

    Hello Bernardo, 

    Thank you for your interest in ISOW7741. ISOW7741's DC-DC converter is recommended for applications where the isolated side needs power, but a larger DC-DC converter cannot be used. Therefore, VISO has internal LDO's and is able to output 3.3V and 5V isolated power rails and it will not be capable of producing voltages larger than 5V at Viso_out. 

    The Viso_out is intended for direct connection to Viso_in as shown in the application example in the datasheet (captured below). As you can see an external LDO is not needed.

    I hope this helps clarify the ISOW7741. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 40 points

    I understand, but I'm trying to follow the datasheet page 51 11.2 states that the output can go to 6V and power the ldo. 

  • 0 in reply to Bernardo Gama
    Prodigy 40 points

    regarding the last post, just to be clear I'm trying to achieve better regulation with this setup, and the ds states this possibility, but in practice I'm unable to attain regulation, because Visoout is not rising above 5V.

  • 0 in reply to Bernardo Gama
    TI__Mastermind 18525 points

    Hello,

    Please allow us an additional 24 hours to get back to you.

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    TI__Mastermind 31655 points

    Hello Bernardo,

    Thank you for the clarifications. The Viso_out will remain at 5V when connected to Viso_in or an enabled LDO.  As described in page 51 of the datasheet the ISOW7741 will only rise to 6V when the Viso_out pin is floating and goes into open regulation mode which is not recommended. 

    I believe the TPS76350 should work well. I think TPS76350 should work well. Feel free to create a new thread regarding this part for an LDO recommendation. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 40 points

    Hi Andrew,

    If it stays at 5V, I don't see how a 5V LDO would work. I tried an LDO (LP2950) similar to TPS76350 and Vout drops to 4.8V. 

    Thanks

  • 0 in reply to Bernardo Gama
    TI__Mastermind 31655 points

    Hi Bernardo, 

    Let me reach out to the larger team to see if we have a solution for this. Please allow me until Monday to look into this further. 

  • 0 in reply to Andrew J
    Prodigy 40 points

    any update on this?

  • 0 in reply to Bernardo Gama
    TI__Guru 55885 points

    Hi Bernardo,

    Thank you for sharing all the information so far.

    Your understanding is correct that the output VISOOUT stays at 5V with tolerance when VSEL is connected to VISOOUT. Due to this, when an LDO is used, the LDO output is going to be <5V and I see that you are observing 4.8V. This is expected behavior and it is not practical to get 5V output from the LDO when its input is 5V.

    The LDO suggestion in section 11 of ISOW7741 datasheet is for the use case where the LDO input is 5V and the output needed is 3.3V, 2.5V or 1.8V. If your requirement for VISOIN is 5V, then you cannot use an LDO between VISOOUT and VISOIN. They need to be connected directly.

    In the schematic you shared, I see that VSEL is connected to VISOIN. Although this might work, we recommend that VSEL is connected to VISOOUT and not to VISOIN irrespective of whether an LDO is used or not.

    Let me know if this answers your question and if you have further questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 40 points

    Hello Koteshwar,

    Thanks for your reply.

    I tried a 3.3 ldo (LT1763-3.3) and connected VSEL to gnd and now VSOOUT doesn't rise above 3.3V and with the LDO between this pin and VSOIN, I now get 2.2V. 

    I saw VSEL connected to VSOIN instead of VSOOUT and I corrected this. With VSEL connected to VSOOUT and the 3.3V LDO I get around 5V on both pins, which is a little bit strange.

    Either way, it doesn't seem to work with a ldo, at least with the ones I have at my disposal. Could you please verify this.

    Thanks,

    Bernardo

  • 0 in reply to Bernardo Gama
    TI__Guru 55885 points

    Hi Bernardo,

    Thank you for your inputs.

    There maybe a misunderstanding here. As you probably already know, the LDOs always need their inputs to be higher than the output, so with 5V input to LDO, 5V output is not possible and with 3.3V input to LDO, 3.3V output is not possible.

    Koteshwar Rao said:
    The LDO suggestion in section 11 of ISOW7741 datasheet is for the use case where the LDO input is 5V and the output needed is 3.3V, 2.5V or 1.8V. If your requirement for VISOIN is 5V, then you cannot use an LDO between VISOOUT and VISOIN. They need to be connected directly.

    As I suggested in previous post, the LDO should be used to converter 5V VISOOUT to 3.3V, 2.5V or 1.8V and not for 5V.

    Bernardo Gama said:
    With VSEL connected to VSOOUT and the 3.3V LDO I get around 5V on both pins, which is a little bit strange.

    Can you confirm which both pins do you see 5V? If you see 5V at LDO input and output then the LDO might not be working well. Please do share the updated schematic along with part numbers / component values, I can review and confirm. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 40 points

    Hello Koteshwar,

    Thanks for clarifying this.

    With VSEL connected to VSOOUT I tried to bump the input to 5.3V, but it doesn't work.

    The 3.3V ldo works, I had a floating ground when changing VSEL, but that's not what we need. Maybe I missed the part in the DS where it says you can't use a 5V ldo.

    I was of the impression that VSOUT could rise above 5V as stated in page 51 when both pins are left open: "If VISOOUT and VISOIN are not connected,
    the DC-DC converter will run open loop and the VISOOUT voltage will drift until the over-voltage clamp clamps at 6".

    Thanks,

    Bernardo

  • 0 in reply to Bernardo Gama
    TI__Guru 55885 points

    Hi Bernardo,

    Thank for further inputs.

    The use of LDO is not really a feature of ISOW7741. The internal DC-DC converter of ISOW7741 produces either 5V or 3.3V based on VSEL connection. VISOIN supports voltages 1.8V, 2.5V, 3.3V and 5V. Based on your application needs, you choose VISOIN voltage to be one of these 4 values.

    If VISOIN voltage and VISOOUT voltage match, then you connect them directly. But if VISOIN is needed to be different from VISOOUT then you can use an LDO to achieve this different voltage and this is a totally the designers choice / preference and the device doesn't require an LDO for normal operation.

    Bernardo Gama said:
    With VSEL connected to VSOOUT I tried to bump the input to 5.3V, but it doesn't work.

    Right, VISOOUT is a regulated output, it will stay 5V irrespective of the increase/decrease in input voltage within ±10% of nominal value.

    Bernardo Gama said:
    I was of the impression that VSOUT could rise above 5V as stated in page 51 when both pins are left open: "If VISOOUT and VISOIN are not connected,
    the DC-DC converter will run open loop and the VISOOUT voltage will drift until the over-voltage clamp clamps at 6".

    The datasheet states that VISOOUT needs to be connected to VISOIN to ensure feedback channel is properly powered to regulate the DC-DC converter. This is a requirement for the device to operate normal. It also states that the device will go to over-voltage and get clamped if these are not connected, which is more like a faulty condition and over-voltage clamp is a protection feature. Prolonged operation in this state could lead to device damage.

    I hope this helps you with clarification and make a relevant decision for your design. Thanks for your patience.


    Regards,
    Koteshwar Rao