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TPS7A4501-SP VOUT when SHDN = 0V

LeonardEllis
Guru 71526 points

Design uses the 10-pin -SP version of the TPS7A4501 with a VIN of 18V and VOUT of 15V 20mA.  Everything works properly except when SHDN is connected to GND, a voltage of 5.1V is present at OUT when there is no load.  If a 50-ohm load is connected, VOUT is 0V as expected.  Note that even when there is no load, there is the voltage divider to set ADJ which places about a 50k-ohm load on OUT. 

Where is this voltage coming from?  And how do we eliminate it, other than the 50-ohm load?

~Leonard 

  . 

  • 0
    TI__Genius 15780 points

    Hi Leonard,

    Based on the information, I would say that this is a result of the leakage current through the device.  Taking a look at the chart below, when VIN is 18V, there is about 230uA of leakage current through the device.  

    For 5.1V to be on the output, there would need to be a resistive load of about 22kΩ on the output (V=IR).  Because of this leakage, a pull down resistance is needed to guarantee 0V.  If a lower voltage can be tolerated, a weaker pull down can be used.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    TI__Guru 71526 points

    I'm noticing in the -SP datasheet, the same graph is not present.  The only thing close is a spec for "Quiescent current in shutdown" and it has a 50uA MAX (at VIN = 6V).  In this design, the resistance OUT to GND is 48k, which would equate to about a 100uA leakage.

    I'm also not clear on how Quiescent Current with SHDN = 0 is the result of leakage at the OUT pin.  Is this the only path?  I wouldn't think so.

    ~Leonard  

  • 0 in reply to LeonardEllis
    TI__Genius 15780 points

    Hi Leonard,

    The spec at VIN = 6V does not apply to higher voltage values, only voltages up to 6V.  We added this graph to the TPS7A4501 datasheet when we updated it since we were receiving questions about it.  It looks like it didn't make it into the -SP datasheet.

    While this is not the only leakage path, it is a large contribution to the leakage into the device.

    Thanks,

    Alek Kaknevicius