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TPS25942A: How to get Reverse Current Protection in Power MUXing application?

Art Lashbrook
Prodigy 215 points
Part Number: TPS25942A

I am using two TPS25942s for Power MUXing, with EN/UVLO pins tied to the input voltage.  I don't need Over/Undervoltage Protection or Priority MUXing and I have the DMODE inputs floating.

The circuit is properly routing power from the active supply to the System Load as desired.

Based on the data sheet, I expected this setup to provide Reverse Current Blocking.

HOWEVER, I found that the operational supply is sourcing current to the non-operational supply!  This is not a good situation since one of the supplies is a battery!

I discovered today that Application Note SLVA811A.PDF says on page 11 "Note that if using two TPS25942 eFuses for power MUXing, then when the primary supply is disconnected, the master TPS25942 will not turn off. This means that reverse current will be able to flow through the device and bring up the input voltage of the master TPS25942."

REALLY???  Why isn't Reverse Current Blocking functional?  That is a big surprise!

My Question is, what circuit changes are required so that Reverse Current Blocking works in a Power MUX application?  (Note that I don't want to use Diode Mode due to the 0.7V drop and associated power dissipation.)

  • 0
    TI__Guru** 105361 points
    Hi Art,

    Reverse current blocking becomes functional after detecting reverse voltage of 10mV (typ) across the device. When the VOUT is raised above IN2 (battery in your case), there will be spiky small reverse current from VOUT to IN2. The spiky small reverse current should generate 10mV reverse voltage to activate reverse current protection. If VOUT raises very slowly, there will not be enough magnitude of reverse current to cause 10mV drop and hence fails to detect and cutoff.
    We are in the process of updating the datasheet to clarify this function.

    Can you provide information of your system to suggest the changes
    Do you want always the higher voltage source (out of active supply and battery) to power the load?
    what is the operating voltage range of active supply and battery?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 215 points

    From the datasheet, I expected that there would be no reverse current when used in the Power MUX configuration.  I thought that was a big part of this IC's function.  YES, the datasheet absolutely needs to be updated to reflect the true situation!

    In my case, I want to use the battery if there is no external power supply present and the external power supply if it is present.  

    I don't want the battery to be sourcing current into the external power supply when it is shut off (as is presently the case.)

    I also don't want to have any current from the external power supply going into the battery (that could be dangerous if the battery needs to be charged intelligently.)

    One mitigating factor in our design is that the external power supply voltage is higher than the battery's voltage.  This being the case, I suspect I can use a voltage divider to the Overvoltage and Undervoltage pins in order to manage which of the two Power MUXes are turned on based on the external power supply's voltage.

    Your thoughts?

  • 0 in reply to Art Lashbrook
    TI__Guru** 105361 points
    Hi Art,

    Since the external power supply voltage is higher than the battery's voltage, the external power supply's voltage threshold (UVLO, OVP, PG output) can be used to control the devices.
    This is the only solution to mitigate the issue with minimal external components.

    Regards
    Rakesh