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LM2662 Folha de dados(PDF) 8 Page - National Semiconductor (TI) |
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LM2662 Folha de dados(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Application Information (Continued) CAPACITOR SELECTION As discussed in the Simple Negative Voltage Converter section, the output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the output resistance, and the power efficiency is Where I Q(V+) is the quiescent power loss of the IC device, and IL 2R OUT is the conversion loss associated with the switch on- resistance, the two external capacitors and their ESRs. Low ESR capacitors ( Table 3) are recommended for both capacitors to maximize efficiency, reduce the output voltage drop and voltage ripple. For convenience, C 1 and C2 are usually chosen to be the same. The output resistance varies with the oscillator frequency and the capacitors. In Figure 3, the output resistance vs. oscillator fre- quency curves are drawn for four difference capacitor values. At very low frequency range, capacitance plays the most important role in determining the output resistance. Once the frequency is increased to some point (such as 100 kHz for the 47 µF capaci- tors), the output resistance is dominated by the ON resistance of the internal switches and the ESRs of the external capacitors. A low value, smaller size capacitor usually has a higher ESR compared with a bigger size capacitor of the same type. Ceramic capacitors can be chosen for their lower ESR. As shown in Figure 3, in higher frequency range, the output resistance using the 10 µF ceramic capacitors is close to these using higher value tantalum capacitors. TABLE 3. Low ESR Capacitor Manufacturers Manufacturer Phone Capacitor Type Nichicon Corp. (708)-843-7500 PL, PF series, through-hole aluminum electrolytic AVX Corp. (803)-448-9411 TPS series, surface-mount tantalum Sprague (207)-324-4140 593D, 594D, 595D series, surface-mount tantalum Sanyo (619)-661-6835 OS-CON series, through-hole aluminum electrolytic Murata (800)-831-9172 Ceramic chip capacitors Taiyo Yuden (800)-348-2496 Ceramic chip capacitors Tokin (408)-432-8020 Ceramic chip capacitors DS100003-36 FIGURE 3. Output Source Resistance vs Oscillator Frequency www.national.com 8 |
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