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Original question:

BUK7908-40AIE Kelvin Source

LDO Current Divider For Current Sense

Fred Doe
Intellectual 655 points

Can anyone shed some light on how I might be able to implement in a discrete manner main driving transistor in linear LDOs the common base/gate and common drain/collector or  emitter/source circuit used in some LDOs for current sense for current limit? My application is a discrete LDO (as I am a hobbyist I am building it with discrete components).

By the way I think it is usually drawn as a single transistor with (for the case of PNP) one base one emitter and two collectors with  the collector on the left usually being the current  sense one and the collector on the right being the main power output.  

Also is this what a "Kelvin Source" is for?

  • 0
    TI__Mastermind 37065 points

    Hi Fred,

    There are multiple ways to achieve current limit in a discrete design.

    One way would be to use a current sense resistor (usually a very low value, such as 10m ohms or whatever your circuit can tolerate).
    Place the current sense resistor in series with the pass element.  Use a current sense amplifier that meets the needs of your design to sense the voltage across the current sense resistor.  Then use the output of the current sense amplifier to control the feedback node when you exceed some threshold.  If the current sense amplifier does not have a comparator, then you will probably need to add that.  So:

    Current sense resistor -> sense voltage with a current sense amplifier -> output of amplifier goes into some small RC filter -> Comparator to set the current limit with some hysteresis -> feedback node

    TI has a long list of current sense amplifiers.  Please see the following link:

    www.ti.com/.../universalsearch.tsp

    The output of the comparator is high impedance until current sense is tripped.
    Using just about any technique will add series impedance between the input - output of your discrete LDO.  So make sure you have the headroom (Vin-Vout) to operate the device while in current limit.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Intellectual 655 points

    Thanks for the help!