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REG104-A Current limit

Timothy Friend
Prodigy 30 points
Other Parts Discussed in Thread: REG104

I am using the REG104-A to power a small incandescent lamp (2.9V @ 700mA).  Because a cold lamp filament presents a very low resistance, the current surge as the lamp turns on can be rather high.  The REG104 will current limit this to1.7A (typical), which is actually desirable, but I noticed that as the input/output differential falls below ~0.7V , the current limit rises suddenly to about 2 Amps.  At this point the lamp reaches operating voltage and its current draw falls as its impedance rises.

There is nothing in the datasheet to suggest that the current limit of this device depends somewhat on input/output differential.  Is there any additional data available or does anyone have any experience with this?  I would like to know what the limits on this behavior are and what it does over temperature.

Thanks

~Tim

  • 0
    TI__Expert 8890 points

    Hello Timothy,

    I will look into this further but the data sheet shows that the current limit is dependent on temperature and on how near the output voltage is to nominal (a condition analogous to the output under a shorting condition).   The figure on page 6, Current Limit vs Temperature, shows that the current limit is lower when the output is being pulled down by one volt (ICL at room temp = 1785mA ) compared to when it is pulled down by only 0.9*Vout_nominal (ICL = 1830mA).  The Electrical Characteristics show that the Current Limit under worst case conditions is as high as 2.2Amps.  I will check to see if the Current Limit is dependant on the dropout voltage as well.

    Regards

    Bill

  • 0 in reply to Bill Stokes
    TI__Expert 8890 points

    Timothy,

    Our conclusion is that it is not unusual for this LDO to experience changes in the current limit due to input and or output voltage/load/temperature dependencies - yet remain within specifications under all conditions.  The exact reason could only be known with some degree of reverse engineering effort by the original designer who was not immediately available.

    Regards

    Bill

  • 0 in reply to Bill Stokes
    Prodigy 30 points

    Bill, thanks for looking into this and replying so quickly.

    ~Tim