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LP38798: Drop out voltage & Design recs

Abigail Leonardi
Expert 3760 points
Part Number: LP38798


Tool/software:

Hi,

My customer is trying to figure out a few different questions to see if this device will be suitable for their application

1) Is it possible to estimate what the dropout would be for 12V output? Figures 22 and 23 from the datasheet show the dropout voltage against current but only for output voltages of 5V or 3,3V

2) Will the LDO work given the following conditions (and the dropout condition)?

3) With an output current of 200mA @125°C and Vout = 3V the dropout would be 100mV (figure 22), with Vout = 5V the dropout would be 75mV (figure 23). Is it safe to assume that the dropout @12V would be about 75mV or less for 200mA?

Customer conditions: 

    • (Vin_max = 12.85V and Vin_min = 12.15V)
    • Vout_max = 12. 48V and Vout_min = 11.55V
    • R1 = 135k +/-1%
    • R2 = 15k +/-1%
    • Vfb_min = 1,176V / Vfb_max = 1,224V (Typical 1,2V)
    • Vos_max = 0,016V

     

    The LDO will source around 465 mA when all low noise loads are active, so I am trying to understand if I really have a problem or if the design is safe as it is.

    • 0
      TI__Expert 7330 points

      Hi Abigail,

      1) Dropout is linear with output current here; you should be able to scale back the 420mV limit linearly from 800mA to your expected load (0.465V/0.8V*0.42V=244mV) to estimate an upper bound on dropout voltage. The dropout voltage is essentially defined by RDS_ON of the pass element in triode, if you needed a better estimate you would have to calculate that.

      2) Unfortunately, that range may enter dropout because the output range defined by the part accuracy overlaps with the input range.  I don't know the customer's requirements or how fixed the output and input requirements are, so I can't make many suggestions. However, increasing R2 to 15.8k +/- 1% will give you 11.03V-11.89V which has just enough headroom to avoid dropout entirely within your conditions.

      3) Yes, I would expect dropout to be marginally better at higher output voltages, so slightly less than 75mV.

      Best,

      Gregory Thompson