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CDC3S04: Is VBAT necessary for operating?

Nir Alon
Intellectual 600 points
Part Number: CDC3S04

Hello all,

I'm using the CDC3S04 in a very small size PCB, and I wish to reduce the BOM as much as I can.

I already have an external 1.8V supply for powering the TCXO, so I don't really need to use the internal LDO.

At first, I considered leaving VBAT and VLDO unconnected.

But later on, I read this note: 

CDC3S04 datasheet Page 3, under VDD_ANA DESCRIPTION:

1.8-V power supply for sine-wave buffers. For correct sine-wave buffer function, all three power supplies (VBAT, VDD_DIG VDD_ANA and VDD_ANA) must be on. 

I assume that the VBAT pin is not only connected to the LDO input (as described in the component block diagram), except it feeds additional internal circuits of the IC. 

First, I want to make sure that VBAT indeed needs to be connected, even if the internal LDO is not in use.

Second, if VBAT must be connected to a power supply, even if the internal LDO is not is use - is it possible to connect VBAT to a power source without the need of additional components? (decoupling caps, ferrite bead etc...)

If only the electrical connection is needed, I can consider this solution.

Thanks 

Nir

  • 0
    TI__Genius 16845 points

    Hi Nir,

    Due to the age of these devices, design support is limited. I looked into your question but couldn't find a definitive answer.

    Would it be possible for you to try and test without the VBAT connection?

    Kind regards,
    Lane

  • 0 in reply to Lane Boyd
    Intellectual 600 points

    Hi Lane

    I've used the CDC3S04EVM in order to operate the buffer without VBAT:

    I've removed R106 so the IC VLDO is not connected.

    The TCXO was connected to an external 1.8V supply

    L7 was disconnected in order the Supply VBAT From external supply.

    1- Powering up all supplies, and enabling all four outputs - OK

    2- VLDO is turned off via software - OK

    3- Removing VBAT completely - All buffers were disabled

    4- Restoring VBAT - All four outputs are once again operative

    So, VBAT is indeed required even if the internal LDO is not in use.

    I guess that my second question will be much more difficult to answer.

    VBAT should be in the range of 2.3 - 5.5V, that is for a operative LDO, which is not my case...

    So I've tried lowering VBAT voltage until the outputs are disabled again, and I've reached down to 1.2V (current consumption in VBAT is 100nA at this stage) 

    So, the quickest and easiest solution for me is to connect VBAT directly to VDD_DIG (1.8V) which is the nearest power supply to VBAT (in terms of board design)

    I understand the IC is old and support is limited, but is there any way to confirm that applying 1.8V to VBAT (I remind you that VLDO is disabled) is OK?

    I guess that the IC needs a certain voltage in VBAT in order to make sure that all power supply are valid (like a "Power Good" internal indication)

    I would be very happy if you could provide the answer to the above question,

    Best Regards,

    Nir

  • 0 in reply to Nir Alon
    TI__Genius 16845 points

    Hi Nir,

    Thank you kindly for sharing the test results!

    Its good that 1.8V VBAT appears to function on the single device that you have tested. I will try to affirm whether 1.8V could suffice for any given device, but can't promise anything at the moment.

    I agree with your understanding, 1.8V might be OK, however this would fall outside of the recommended range. 

    Kind regards,
    Lane

  • 0 in reply to Lane Boyd
    Intellectual 600 points

    Hi Lane,

    I agree that only one test on one device is not satisfactory. 

    A real test should be performed on several devices, at all temperature range etc...

    The 1.8V is outside the specifications, that if you are actually using the internal LDO (of course the LDO cannot regulate 1.8V output from 1.8V input)

    But if this voltage is only needed to notify the chip that VBAT is present, so maybe this would be sufficient.

    Anyway, I'm waiting for you to check if this is somehow reasonable.

    Thanks again!

    Nir

  • 0 in reply to Lane Boyd
    Intellectual 600 points

    Hi Lane,

    Any news regarding my issue?

    Best Regards,

    Nir 

  • 0 in reply to Nir Alon
    TI__Genius 16845 points

    Hi Nir,

    Nothing to report yet. I am awaiting response from the backend team.

    Kind regards,
    Lane 

  • 0 in reply to Lane Boyd
    TI__Genius 16845 points

    Hi Nir,

    Sorry for the delay. Thank you for your patience.

    VBAT is also supplying internal Bandgap voltage which is used to generate reference currents for the Sine wave output stages.

    Reducing the VBAT down to 1.8V might create voltage headroom issues at weak corner.

    I think output stages will still function, but performance may be affected. Thus we advise against using 1.8V on VBAT

    Kind regards,
    Lane

  • 0 in reply to Lane Boyd
    Intellectual 600 points

    Hi Lane,

    Thank you for investigating this...

    This is a bit disappointing to understand that the IC requires three power supply in order to operate...

    This is a very good chip, even after long time in the market, but it turns out as very spoiled one :)

    It's one thing if could use the internal LDO in order to supply both 1.8V rails...but, I noticed that this cannot be done:

    https://e2e.ti.com/support/clock-and-timing/f/48/t/711032?tisearch=e2e-sitesearch&keymatch=CDC3S0

    Anyway, thanks again for helping!

    Nir