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LM2660M Scheda tecnica(PDF) 9 Page - Texas Instruments |
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LM2660M Scheda tecnica(HTML) 9 Page - Texas Instruments |
9 / 20 page LM2660 www.ti.com SNVS135D – SEPTEMBER 1999 – REVISED MAY 2013 The switching frequency of the converter (also called the charge pump frequency) is half of the oscillator frequency. NOTE OSC cannot be driven by an external clock in the voltage-doubling mode. Table 1. LM2660 Oscillator Frequency Selection FC OSC Oscillator Open Open 10 kHz V+ Open 80 kHz Open or V+ External Capacitor See Typical Performance Characteristics N/A External Clock External Clock (inverter mode only) Frequency 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 (3) Where IQ(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. Since the switching current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C1 is multiplied by four in the output resistance. The output capacitor C2 is charging and discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the output resistance. However, the ESR of C2 directly affects the output voltage ripple. Therefore, low ESR capacitors (Table 2) are recommended for both capacitors to maximize efficiency, reduce the output voltage drop and voltage ripple. For convenience, C1 and C2 are usually chosen to be the same. The output resistance varies with the oscillator frequency and the capacitors. In Figure 17, the output resistance vs. oscillator frequency curves are drawn for three different tantalum capacitors. 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 20 kHz for the 150 μF capacitors), 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. For lower ESR, use ceramic capacitors. Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2660 |
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