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LM2665M6 Datenblatt(PDF) 6 Page - National Semiconductor (TI) |
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LM2665M6 Datenblatt(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Application Information (Continued) voltage source equals 2V+. The output resistance R out is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance and ESR of C 1 and C2. Since the switching current charging and dis- charging C 1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C 1 will be multi- plied by four in the output resistance. The output capacitor C 2 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 R out is: where R SW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 2. The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor C 2: High capacitance, low ESR capacitors can reduce both the output resistance and the voltage ripple. The Schottky diode D 1 is only needed for start-up. The inter- nal oscillator circuit uses the OUT pin and the GND pin. Volt- age across OUT and GND must be larger than 1.8V to insure the operation of the oscillator. During start-up, D 1 is used to charge up the voltage at the OUT pin to start the oscillator; also, it protects the device from turning-on its own parasitic diode and potentially latching-up. Therefore, the Schottky di- ode D 1 should have enough current carrying capability to charge the output capacitor at start-up, as well as a low for- ward 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 10V/ ms, a smaller Schottky diode like MBR0520LT1 can be used to reduce the circuit size. Split V+ in Half Another interesting application shown in the Basic Applica- tion Circuits is using the LM2665 as a precision voltage di- vider. . This circuit can be derived from the voltage doubler by switching the input and output connections. In the voltage divider, the input voltage applies across the OUT pin and the GND pin (which are the power rails for the internal oscillator), therefore no start-up diode is needed. Also, since the off-voltage across each switch equals V in/2, the input voltage can be raised to +11V. Shutdown Mode A shutdown (SD) pin is available to disable the device and reduce the quiescent current to 1 µA. In normal operating mode, the SD pin is connected to ground. The device can be brought into the shutdown mode by applying to the SD pin a voltage greater than 40% of the V+ pin voltage. Capacitor Selection As discussed in the Positive Voltage Doubler 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 resis- tance, and the power efficiency is Where I Q(V+) is the quiescent power loss of the IC device, and I L 2R out is the conversion loss associated with the switch on-resistance, the two external capacitors and their ESRs. The selection of capacitors is based on the specifications of the dropout voltage (which equals I out Rout), the output volt- age ripple, and the converter efficiency. Low ESR capacitors (Table 1) are recommended to maximize efficiency, reduce the output voltage drop and voltage ripple. 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 Other Applications Paralleling Devices Any number of LM2665s can be paralleled to reduce the out- put resistance. Each device must have its own pumping ca- pacitor C 1, while only one output capacitor Cout is needed as shown in Figure 3. The composite output resistance is: www.national.com 6 |
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Ähnliche Beschreibung - LM2665M6 |
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