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LP38798-ADJ LDO Application question -- Input power

Jeremy Huffman
Prodigy 80 points
Other Parts Discussed in Thread: LP38798

I am using the LP38798-ADJ LDO in my design and have an interesting situation.

I would prefer to set the VIN of the LDO to just above the max dropout voltage of my preferred VOUT (2.5V nominal).

Based on my projected worst case load of about 0.2A and factoring in an estimated maximum required dropout voltage of about 175mV at 85 degrees ( I am interpolating based on the dropout spec. and Figure 23), this would make my minimum required VIN be 2.675V, so that would be my preferred VIN to minimize the dropout.

 

Given my specific application, here are some violations I appear to be making:

  • My preferred VIN is 2.675V whereas the specified operating range of VIN is 3.0V to 20.0V and the datasheet says “Operating Range conditions indicate the conditions at which the device is functional and the device should not be operated beyond such conditions.”
  • It seems like I am violating the dropout voltage specification per my reading of Note 3 under the specs, which states “Dropout voltage (VDO) is defined as the differential voltage measured between VOUT and VIN when VIN, falling from VIN = VOUT + 1V, causes VOUT to drop 2% below the value measured with VIN = VOUT + 1V. Dropout voltage specification does not apply when the programmed output voltage is below the Minimum Operating Input Voltage.”

 

Given these violations, would TI have any more detailed insight into the implications of my preferred VIN and VOUT requirements.  I can proceed to do some testing with the part myself, but it would be great if TI already had some data readily available to applies to my situation.  I know I am possibly “stretching the limit” of this part, but I am strongly pressed to do two things in this design:  minimize the power consumption and keep the output noise as low as possible with a COTS LDO.

  • 0
    TI__Mastermind 22610 points

    " ... My preferred VIN is 2.675V ..."

    My first concern is that your 2.675V VIN is inside the UVLO min/max window (2.47V to 2.83V), so the LP38798 might not even start up.

    If it does start up once, it might not start again when the LP38798 is warmer as the UVLO threshold goes higher as the device warms up. Not a lot higher, but probably just enough to get really annoying.

    The limiting factor here is the UVLO max limit, Vin must be at, or above, 2.83V to be sure that the LP38798 always starts every time.

    Between Vin=2.83V and Vin=3.00V, you will see some problems with output voltage accuracy, and perhaps transient response, as certain parts of the internal biasing circuits start to go just a bit ~non-linear~ when Vin is less than 3V.

     "... would TI have any more detailed insight into the implications of my preferred VIN and VOUT requirements ..."

    Sorry, but when the design team called out the minimum operating Vin to be 3.0V, there was not much characterization done below that voltage (other than the UVLO thresholds).