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TPS62740: Host-controlled VSEL

Chris Schramm
Intellectual 590 points

Part Number: TPS62740
Other Parts Discussed in Thread: TPS62840

Dear TI-Team,

TPS62740 works perfectly well but it would be awesome if it was possible to control VSEL1 - VSEL3 via MCU. The MCU runs fine with 2.6V, so VSEL4 will always be tied to VIN.

The recent d/s (July 2014) says that the pins have to be terminated but an older revision (2013) shows "Digital Configuration" for VSEL on page 7 which could be (but I'm not sure) what I want.

Obviously, the VSELs will only float a short time - as soon as the MCU (which is fed by TPS62740) turns on, the pins will have a known state.

Is this possible? If not, what is (in terms of low power) the recommended pull-down value?

Thanks,

Chris

  • 0
    TI__Guru**** 152430 points

    Hi Chris,

    As digital inputs, it is always recommended that they be terminated to a high or low logic level.  If your MCU can always boot up at 2.6V and is ok if it receives 3.3V, it might be ok to connect the pins directly.  You would need to check if there is any issue in your system.

    This app note, found on the product page, explains how to calculate the pull-up/down resistors based on the leakage current: http://www.ti.com/lit/an/slva485/slva485.pdf

    We also just previewed the 60 nA Iq TPS62840.  You cannot adjust the voltage on the fly, but the lower Iq may provide more power savings for your system.

  • 0 in reply to Chris Glaser
    Intellectual 590 points

    Hi Chris,

    thanks. Meanwhile I tried on a demo board and it seems everything is fine: if VSELs are floating, VOUT is 2.6V

    TPS62840 looks really really awesome.

    How does VOUT behave if VOUT > VIN (because VOUT = 2V6/3V3 which might be fed externally) if VIN is floating? TPS62740 works perfectly with this so we can save a diode or similar protection. By the way, I noticed that VLOAD > VIN won't work, I guess there's a short (about 40mA excess current) - which, again, is fine but good to know :)

    What's expected behavior for TPS62840 in this case? There's not yet a block diagram, but maybe EN/STOP pins would help with that?

    Thanks,

    Chris

  • 0 in reply to Chris Schramm
    TI__Guru**** 152430 points

    Hi Chris,

    This app note addresses your case, I believe: http://www.ti.com/lit/an/slyt689/slyt689.pdf It applies to nearly all devices, including these 2.  If something is still unclear, let me know.