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Original question:

TLV733P: Input Voltage On-off Test problem

TLV733P: VDO VS VOUT

Lin_Charles
Expert 4260 points
Part Number: TLV733P

Hi, all. When I learn the basic knowledge of LDO, I got one question about VDO && VOUT. Whne the output current is the same of TLV73318 and  TLV73333, why TLV73333 has a lower VDO? Why VGS voltage is larger in this device? 

  • 0
    TI__Guru 55940 points

    Hi ,

    Please note that TLV733P has a PMOS pass element.  Note that the source of the the PMOS is connected to IN for the LDO.  In order for an LDO to be in dropout, Vin must approach the nominal Vout.  Therefore, a LDO with a lower Vout will need a lower Vin to go into dropout.  Since opamp is trying to drive the FET so that the center tap of the feedback resistors is equal to the reference voltage, the gate will be driven to the negative rail of the opamp which is tied to GND.  As the gate voltage is 0 V for both LDOs but the lower output TLV73318 requires a lower source voltage to enter dropout, Vgs will be lower for TLV73318.

    Very Respectfully,

    Ryan