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TPS7A78: Is it possible to draw a small current (4mA) from the CSCIN capacitor?

Nick Steven
Prodigy 70 points
Part Number: TPS7A78
Other Parts Discussed in Thread: CC1101

Hi Ti

Thanks for your help with my two previous questions. I have another one I'm afraid.

I'm looking in detail at the 3.3V output variant of the TPS7A78. From the datasheet I think the voltage across CSCIN will be 4 x (VLDO_OUT + 0.6V) or 15.6V. The ripple current at this node will nominally be 1/4 of my application current at 3.3V. I think we need about 60mA at 3.3V and will be using FB mode. So, the capacitor ripple would be about 15mA.

The application needs an auxiliary power rail between 10V and 22V with a current draw of 4mA. Would it be too good to believe that I could rob CSCIN of 4mA to supply this auxiliary power rail without upsetting the TPS7A78 regulation scheme? Naturally I'd expect to adjust the value of CSCIN upwards to accommodate the extra current - and we don't need a rapid start up time.

Thanks again, Nick

  • 0
    TI__Mastermind 43570 points

    Hi Nick,

    Yes. This is possible. the only caveat is that you would have to make sure that you size CS to handle the additional current. 

    I have a calculator that will help select the components. If you send me an e-mail. I can send it to you.

    Note: macros must be enabled.

    Let me know if you have any questions. 

  • 0 in reply to JCHK
    Prodigy 70 points

    Hi John

    Best if you didn't post the following...

    That's really good news! I've been banging my head against this application for years. We have a mains powered wireless device for controlling commercial lighting. It uses the TI CC1101 at 868MHz and we've previously tried low power off line switchers (e.g. Power Integrations). The trouble is that the broadband mush coming from the high voltage buck knocked 20dB off the CC1101 receiver sensitivity. We felt forced down the cap dropper route but it's not ideal from an efficiency point of view. We do something similar to the TPS7A78 in that we hystertically control a coarse voltage with an optomos relay and then linear regulate from there. Quite a few discretes and processor overhead. The TPS7A78 does it neater and better. I'm hoping that the 200kHz switching frequency of the capacitive buck regulator is sufficiently far away from our 100kHz IF frequency that we'll be fine for sensitivity. Harmonics should be even further away.

    To control commercial lighting drivers we simulate a DALI master. The DALI bus needs a 16V (or thereabouts) rail for communication so being able to steal a bit of power from CS is better than I'd hoped for.

    I'm off to buy an evaluation board from Digikey but would really like to have a copy of your calculator to play out some component scenarios. My email address is nick.steven@designwrights.co.uk

    Thanks for all your help so far.

    Best regards, Nick