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LDO regulator with sereis Schottky

Discussion in 'General Electronics Discussion' started by eem2am, Dec 9, 2010.

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  1. eem2am

    eem2am

    414
    0
    Aug 3, 2009
    Hello


    We are using the LM2937-3.3 linear LDO voltage regulator to get 3.3V from 5V.
    Output current is 50mA maximum.

    LM2937-3.3 DATASHEET:
    http://www.national.com/ds/LM/LM2937-2.5.pdf

    The previous engineer has placed a series Schottky diode (D12) following the LM2937 (i.e, the Schottky is immediately upstream of the output cap as in following diagram)

    CIRCUIT DIAGRAM:
    http://i54.tinypic.com/a2py7k.jpg

    do you know why the Schottky is there?

    …..the load has been represented as a resistor, but actually it’s a MC13211 RF Transceiver which has a 2V to 3.4V operating voltage with on-chip voltage regulators….(3.6V max recommended input voltage as per page 46 of the datasheet)

    MC13211 DATASHEET:
    http://cache.freescale.com/files/rf_if/doc/data_sheet/MC1321x.pdf?pspll=1


    Anyway,
    I am wondering why the previous engineer added the series Schottky diode?

    I noticed nothing about putting a Schottky at the output in the datasheet.

    I am wondering perhaps , is it a problem if the input side is at zero volts and yet the output has some hold-up voltage lingering on the output caps.?

    Or, do you know why a Scottky (D12) would be placed here?
     
  2. davenn

    davenn Moderator

    13,671
    1,892
    Sep 5, 2009
    maybe its purely for voltage polarity protection of the MC13211 IC

    That is ... if you accidentally reversed the 3.3V output of the regulator it wouldnt kill the IC

    the other possibility is that 3.3V is getting close to the max voltage of the MC13211
    and so he decided to use the voltage drop across the diode to bring the supply voltage to the
    IC to a safe level in the middle of its operating range, ie. ~ 2.8V

    The datasheet for that diode says its max forward voltage drop is 0.38V :)

    The second possibility is the most likely senario :)

    Dave
     
    Last edited: Dec 9, 2010
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    If the diode is on a separate board from the power supply, then the issue is more likely to be reverse polarity protection. If on the same board, then I'd go voltage drop.

    It is also possible, if the 3v3 reg is powering several other things that the diode is there (along with the capacitor) to help isolate one device from interference emanating from another.
     
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