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TPS7A26: Can we use TPS7A26 LDO together with TPS63900 DC-DC to minimise current draw?

Ron Segal1
Prodigy 150 points
Part Number: TPS7A26
Other Parts Discussed in Thread: TIDA-00393, TPS63900, TPS709

Inspired by TI's design article (TIDA-00393) "Adding an LDO for Increased Standby Mode Efficiency", we are considering using a TPS7A26 LDO in conjunction with a TPS63900 DC-DC converter to get the best of both worlds, minimising current draw when an mcu is in deep sleep (< 10uA) to driving a transceiver at around 180mA burst from a 3.6 nominal volts Lithium Thionyl Chlorate (LTC) cell, with 3.3 volts operating target.   In this case we can (hopefully) just program the mcu to enable the LDO and disable the DC-DC via their enable pins immediately before entering deep sleep, then disable the LDO and enable the DC-DC converter just before executing a high load function.  LTC cells tend to drop their voltage significantly when driving loads as high as 180mA, hence the need to boost (or else implement some kind of capacitance reservoir approach).

Question is whether the above can be achieved by simply connecting the voltage outputs of the two devices or whether there is likely to be contention or undue current draw by doing this, particularly in deep sleep mode?

Any advice would be appreciated.

Thanks.

  • 0
    TI__Mastermind 37025 points

    Hi Ron,

    Interesting idea.  I can help you with the linear regulator, but for the switcher we will want to consult the switching team as they are more familiar with this exact device.  The issue you will have with the linear regulator will be the body diode of the internal pass fet.  Without reverse current protection, this will conduct even with the LDO turned off, and will at a minimum result in reverse current flow through the device.  This can damage or destroy the linear regulator.  A series diode on the output can correct this.  In my experience, when doing something like what you are describing (as opposed to paralleling converters to add additional current) it is common to use series diodes between the power supplies, with the obvious concern being the added diode drop.  So schottky diodes are typically used.

    The resistor feedback network can be connected to the output of the series output diode.  This way the linear regulator will attempt to cancel out the voltage drop of the diode, although it will be very small as the linear regulator will not provide a lot of current.  I would recommend an output capacitor before the series diode for stability purposes, and since the LDO is not providing the transient current (the switcher will be doing that).

    You may also want to sequence the enables of the linear regulator and switching converter such that you first disable both devices for a very short period of time, then enable the device you want to provide power to the MCU.  In the small amount of time that no power supply is turned on, the load capacitance will be holding up the MCU, but if the current draw is very low this may not result in a significant drop.  If it is microamps this may not even be measurable.  By sequencing the power supplies you can ensure that the supplies are not somehow fighting each other.  This can happen through modes such as the Active overshoot pulldown circuitry inside the linear regulator (see the copied text from the datasheet below).  This is another reason to use a series diode on the output of the linear regulator.

    "This device has pull down circuitry connected to VOUT. This circuitry is a 100-μA current sink, in series with a 5.5-kΩ resistor, controlled by VEN. When VEN is below VEN(LOW), the pull down circuitry is disabled and the LDO output is in high-impedance mode."

    I would highly recommend using two eval boards of the components you are reviewing, and trying out the idea with your MCU (especially if you have an eval board of that also).  This provides a quick way to test the concept with low risk.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Mastermind 37025 points

    Hi Ron,

    One more comment.
    I took another look at the paper you linked to.
    The linear regulator used there is the TPS709, which has reverse current protection and does not include an active overshoot protection circuit.
    So the series diode on output of the LDO is not required if you design the two converters as described in the app note.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 150 points

    Hi Stephen

    Brilliant, thank you very much for your clear walkthrough with follow-up and suggestions regarding sequencing. The TPS709 should work, as the LDO will be off with higher current switcher on, whenever a load over 150mA is required. Good point about both off briefly before changeover. Which will certainly need to be tested, particularly as the DC-DC converter will take a while to start up. Generally I've found though that with the mcu in deep sleep, even after manually removing the power supply completely, if it is quickly reconnected the mcu often continues.  It might also depend on brownout protection settings.

    You mentioned to start '..for the switcher we will want to consult the switching team..'.  So should I leave this question open for the time being?

    All the best, Ron

  • 0 in reply to Ron Segal1
    TI__Mastermind 37025 points

    Hi Ron,

    I've sent this to one of the engineers in the group which handles the TPS63900 for comments.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 150 points

    Thanks again Stephen. See what happens.

    Cheers, Ron.

  • 0 in reply to Ron Segal1
    TI__Guru 65200 points

    Hello Ron,

    Sorry for the late response. I think the best solution you could use is detailed in this app note:

  • 0 in reply to Brigitte
    Prodigy 150 points

    Hi Brigitte,  That's great, thanks. Design note is spot on.  So from a very quick read of the paper so far, a dual voltage approach using the new TPS63900 will support ultra low current during long periods of deep sleep (when there is almost no load) vs the short periods when the radio, sensors and other parts that are external to the mcu need to be powered, which is of course exactly what we need. So I have to mark this as the best solution.  Cheers Ron.