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LM317LZ Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM317LZ Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 17 page Application Hints (Continued) Protection Diodes 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 10µF capacitors have low enough in- ternal series resistance to deliver 20A spikes when shorted. Although the surge is short, there is enough energy to dam- age parts of the IC. When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regu- lator, and the rate of decrease of V IN. In the LM317L, this dis- charge path is through a large junction that is able to sustain a 2A surge with no problem. This is not true of other types of positive regulators. For output capacitors of 25 µF or less, the LM317L’s ballast resistors and output structure limit the peak current to a low enough level so that there is no need to use a protection diode. The bypass capacitor on the adjustment terminal can dis- charge through a low current junction. Discharge occurs when either the input or output is shorted. Internal to the LM317L is a 50 Ω resistor which limits the peak discharge current. No protection is needed for output voltages of 25V or less and 10µF capacitance. Figure 3 shows an LM317L with protection diodes included for use with outputs greater than 25V and high values of output capacitance. LM317L micro SMD Light Sensitivity Exposing the LM317L micro SMD package to bright sunlight may cause the V REF to drop. In a normal office environment of fluorescent lighting the output is not affected. The LM317 micro SMD does not sustain permanent damage from light exposure. Removing the light source will cause LM317L’s V REF to recover to the proper value. DS009064-9 D1 protects against C1 D2 protects against C2 FIGURE 3. Regulator with Protection Diodes www.national.com 7 |
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