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LM2766 Datasheet(PDF) 10 Page - Texas Instruments |
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LM2766 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 20 page , = POUT PIN = IL 2R L IL 2R L + IL 2R OUT + IQ (V+) V RIPPLE = I L OSC × C2 + 2 × I L × ESRC2 R OUT 2RSW + 2 OSC × C1 + 4ESR C1 + ESRC2 LM2766 SNVS071C – MARCH 2000 – REVISED SEPTEMBER 2015 www.ti.com The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals 2 V+. The output resistance Rout is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, and the capacitance and ESR of C1 and C2. Because the switching current charging and discharging C1 is approximately twice 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. A good approximation of Rout is: where • RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 12. (1) The peak-to-peak output voltage ripple is determined by the oscillator frequency as well as the capacitance and ESR of the output capacitor C2: (2) High capacitance, low ESR capacitors can reduce both the output resistance and the voltage ripple. The Schottky diode D1 is only needed to protect the device from turning on its own parasitic diode and potentially latching up. During start-up, D1 also quickly charges up the output capacitor to VIN minus the diode drop thereby decreasing the start-up time. Therefore, the Schottky diode D1 must have enough current carrying capability to charge the output capacitor at start-up, as well as a low forward voltage to prevent the internal parasitic diode from turning on. A Schottky diode like 1N5817 can be used for most applications. If the input voltage ramp is less than 10 V/ms, a smaller Schottky diode like MBR0520LT1 can be used to reduce the circuit size. 9.2.2.2 Capacitor Selection As discussed in Positive Voltage Doubler, 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 • IQ(V+) is the quiescent power loss of the device; and • IL 2R OUT is the conversion loss associated with the switch on-resistance, the two external capacitors and their ESRs. (3) The selection of capacitors is based on the specifications of the dropout voltage (which equals IOUT ROUT), the output voltage ripple, and the converter efficiency. Low ESR capacitors (Table 2) are recommended to maximize efficiency, reduce the output voltage drop and voltage ripple. Table 2. Low ESR Capacitor Manufacturers MANUFACTURER WEBSITE CAPACITOR TYPE Nichicon Corp. www.nichicon.com PL & PF series, through-hole aluminum electrolytic AVX Corp. www.avxcorp.com TPS series, surface-mount tantalum Sprague www.vishay.com 593D, 594D, 595D series, surface-mount tantalum Sanyo www.sanyovideo.com OS-CON series, through-hole aluminum electrolytic Murata www.murata.com Ceramic chip capacitors Taiyo Yuden www.t-yuden.com Ceramic chip capacitors Tokin www.tokin.com Ceramic chip capacitors 10 Submit Documentation Feedback Copyright © 2000–2015, Texas Instruments Incorporated Product Folder Links: LM2766 |
Similar Part No. - LM2766_15 |
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Similar Description - LM2766_15 |
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