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LM7912: Ext. Diode for protection

Brian Wang0928
Genius 14020 points

Part Number: LM7912

In the On-semi Spec, it specifies that there is D2 protection diode should be added. however, in the LM79xx series, it doen't mention about this. I'm not sure if it's neccesary or not? Or is it integrated in LM series? 

SCD7915

cid:image007.jpg@01D26A95.FAF6B7F0

  • 0
    TI__Genius 14020 points

    Here is from On-semi Spec:

  • 0 in reply to Brian Wang0928
    TI__Genius 12630 points

    Brian,

    When external capacitors are used with any IC regulator, it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Most 10-μF capacitors have low enough internal series resistance to deliver 20-A spikes when shorted. Although the surge is short, there is enough energy to damage parts of the IC. When an output capacitor is connected to a negative output regulator and the input is shorted, the output capacitor pulls current out of the output of the regulator. The current depends on the value of the capacitor, the output voltage of the regulator, and the rate at which VIN is shorted to ground. The bypass capacitor on the adjustment terminal can discharge through a low current junction. Discharge occurs when either the input, or the output, is shorted.

    The datasheet for LM79xx recommends "For output capacitance in excess of 100μF, a high current diode from input to output (1N4001, etc.) will protect the regulator from momentary input shorts." For adjustable negative regulators, such as LM137/LM337, it is recommended to add diode (shown as D2 in above Figure 11) from Vin to Vout if Cload is larger than 20uF to protect in case the input supply is shorted. If the adjust to gnd capacitor is larger than 10uF and -Vout is larger than -25V, a diode from Vout to adj will protect in case the output is shorted. For the fixed version negative regulators, there is no adj pin so the D1 diode from Vout to GND shown above is used to protect against output shorts.

    Even though LM7912 is a fixed negative voltage regulator, you can still apply the same protection as the adjustable versions (which is the same protection seen in Figure 11 of the On-Semi spec). It is a case by case basis and depends on the capacitance at the input/output and operating conditions. If you are certain that the input/output won't be shorted and are using smaller capacitors, the diodes are not necessary. If unsure, you can add them to be safe.

    I hope this helps! Please let me know if you have any more questions.

    -Michael