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TPS7A20: Can voltage be present at the output when the regulator is disabled?

Denis Kotlar
Intellectual 820 points
Part Number: TPS7A20

Tool/software:

I would like to use the output from the TPS7A20 as an aux supply that would be controlled via its EN input. I would like to know when the TPS7A20 is disabled, whether the nominal voltage from another source can be present at the output or I will have to make an OR-ed connection to isolate them.

  • 0
    TI__Guru 50025 points

    Hi Denis,

    When the device is disabled and the other supply is powered, is the TPS7A20 input voltage still up and larger than the other supply? Just want to be sure there's no reverse polarity condition causing reverse current. 

    I did some testing on the EVM and I got some somewhat unexpected results. For the test I powered the input with 5V and the output is fixed at 3.3V. I used a second supply and connected a series resistor with that supply connected to the output so that I was essentially pulling up the output when the device is supposed to be disabled. I had the output at 2V before enabling the device, and what I saw is that the output ramps up by 3.3V on top of the 2V that was already there, and so the output rose to 5.5V which is above the input voltage. This was with no load. If I connected a load the output ramped up to 3.3V as expected. So, the device does start up when there is a pre-bias (some devices can't start up if the output isn't close to ground), but if there is no load there could be an issue. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 820 points

    Thanks Nick for the quick reply. Yes, when the TPS7A20 is disabled and the other supply is powered, the voltage at its input is still higher than the other supply. Likewise, the load will also be continuously connected.

  • +1 in reply to Denis Kotlar
    TI__Guru 50025 points

    Hi Denis,

    Understood. It seems that it could work then, but you will need to validate it in your system. I haven't done any characterization to determine how much load is required to let the LDO ramp up to the appropriate voltage - I used something like 30mA resistive load for testing (I don't remember exactly).  

    Regards,

    Nick