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LM9072 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM9072 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Information (Continued) Input Stability Low dropout voltage regulators which utilize a PNP power transistor usually exhibit a large increase in current when in dropout (V IN < 5.5V). This increase is caused by the satura- tion characteristics ( β reduction) of the PNP transistor. To significantly minimize this increase in current the LM9072 detects when the PNP enters saturation and reduces the op- erating current. This reduction in input current can create a stability problem in applications with higher load current (> 200 mA) where the input voltage is applied through a long length of wire which in effect add a significant amount of inductance in se- ries with the input. The drop in input current may create a positive input voltage transient which may take the PNP out of saturation. If the input voltage is held constant at the threshold where the PNP is going in and out of saturation, an oscillation may be created. This is only observed where a large series inductance is present in the input supply line and when the rise and fall time of the input supply is very slow. If the application and re- moval of the input voltage changes at a rate greater than 500 mV/ms it will move through the dropout region of the regulator (V IN of 3V to 5.5V) too quickly for an oscillation to be established. In the event that an oscillation is present, input bypassing can also help de-tune the resonance. Figure 7 illustrates two input bypassing approaches. The straightforward addition of a larger valued electrolytic capacitor could suffice. in this case however, if reverse battery connections are a possibility it is necessary to add a series protection diode as shown to prevent damaging the polarized input capacitor. An alternative input bypassing scheme is also shown. This eliminates the use of polarized input capacitors and a series protection diode. The values shown were derived empirically in a representative typical application. Appropriate values for any given application require experimentation. DS012906-22 FIGURE 6. Safety Latch-OFF DS012906-23 FIGURE 7. Input Bypassing www.national.com 11 |
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