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LP3856ESX-1.8 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LP3856ESX-1.8 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 18 page  Application Hints V IN RESTRICTIONS FOR PROPER START-UP To prevent misoperation, ensure that V IN is below 50mV before start-up is initiated. This scenario can occur in sys- tems with a backup battery using reverse-biased "blocking" diodes which may allow enough leakage current to flow into the V IN node to raise it’s voltage slightly above ground when the main power is removed. Using low leakage diodes or a resistive pull down can prevent the voltage at V IN from rising above 50mV. Large bulk capacitors connected to V IN may also cause a start-up problem if they do not discharge fully before re-start is initiated (but only if V IN is allowed to fall below 1V). A resistor connected across the capacitor will allow it to discharge more quickly. It should be noted that the probability of a "false start" caused by incorrect logic states is extremely low. EXTERNAL CAPACITORS Like any low-dropout regulator, external capacitors are re- quired to assure stability. These capacitors must be correctly selected for proper performance. INPUT CAPACITOR: An input capacitor of at least 10µF is required. Ceramic or Tantalum may be used, and capaci- tance may be increased without limit OUTPUT CAPACITOR: An output capacitor is required for loop stability. It must be located less than 1 cm from the device and connected directly to the output and ground pins using traces which have no other currents flowing through them (see PCB Layout section). The minimum amount of output capacitance that can be used for stable operation is 10µF. For general usage across all load currents and operating conditions, the part was characterized using a 10µF Tantalum input capacitor. The minimum and maximum stable ESR range for the output capacitor was then measured which kept the device stable, assuming any output capacitor whose value is greater than 10µF (see Figure 1 below). It should be noted that it is possible to operate the part with an output capacitor whose ESR is below these limits, as- suming that sufficient ceramic input capacitance is provided. This will allow stable operation using ceramic output capaci- tors (see next section). OPERATION WITH CERAMIC OUTPUT CAPACITORS LP385X voltage regulators can operate with ceramic output capacitors if the values of input and output capacitors are selected appropriately. The total ceramic output capacitance must be equal to or less than a specified maximum value in order for the regulator to remain stable over all operating conditions. This maximum amount of ceramic output capaci- tance is dependent upon the amount of ceramic input ca- pacitance used as well as the load current of the application. This relationship is shown in Figure 2, which graphs the maximum stable value of ceramic output capacitance as a function of ceramic input capacitance for load currents of 1A, 2A, and 3A. For example, if the maximum load current is 1A, a 10µF ceramic input capacitor will allow stable operation for values of ceramic output capacitance from 10µF up to about 500µF. If the maximum load current is 2A and a 10µF ceramic input capacitor is used, the regulator will be stable with ceramic output capacitor values from 10µF up to about 50µF. At 3A of load current, the ratio of input to output capacitance required approaches 1:1, meaning that whatever amount of ceramic output capacitance is used must also be provided at the input for stable operation. For load currents between 1A, 2A, and 3A, interpolation may be used to approximate values on the graph. When calculating the total ceramic output capaci- tance present in an application, it is necessary to include any ceramic bypass capacitors connected to the regulator out- put. SELECTING A CAPACITOR It is important to note that capacitance tolerance and varia- tion with temperature must be taken into consideration when selecting a capacitor so that the minimum required amount of capacitance is provided over the full operating tempera- ture range. In general, a good Tantalum capacitor will show very little capacitance variation with temperature, but a ce- ramic may not be as good (depending on dielectric type). Aluminum electrolytics also typically have large temperature variation of capacitance value. Equally important to consider is a capacitor’s ESR change with temperature: this is not an issue with ceramics, as their 20030970 FIGURE 1. ESR Curve for C OUT (with 10µF Tantalum Input Capacitor) 20030985 FIGURE 2. Maximum Ceramic Output Capacitance vs Ceramic Input Capacitance www.national.com 11 |
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