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LM2665M6 Datasheet(PDF) 10 Page - Texas Instruments |
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LM2665M6 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 20 page 4176 = 4176 KBA=? D ./2665 0QI>AN KB &ARE?AO ß = 2176 2+0 = +. 24 . +. 24 . + +. 24 176 + +3(8+) 8 4+22.' = + . &15% × %2 + 2 × + . × '54%2 4 176 24 59 + 2 &15% × %1 + 4'54 %1 + '54%2 10 LM2665 SNVS009H – NOVEMBER 1999 – REVISED MARCH 2016 www.ti.com Product Folder Links: LM2665 Submit Documentation Feedback Copyright © 1999–2016, Texas Instruments Incorporated 9.2.1.2 Detailed Design Requirements 9.2.1.2.1 Positive Voltage Doubler 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, the capacitance and equivalent series resistance (ESR) of C1 and C2. 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 will be 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 when 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 10. (1) The peak-to-peak output voltage ripple is determined by the oscillator frequency, 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 for start-up. The internal oscillator circuit uses the OUT pin and the GND pin. Voltage across OUT and GND must be larger than 1.8 V to insure the operation of the oscillator. During start-up, D1 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 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. 9.2.1.2.2 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 IC 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 are recommended to maximize efficiency, reduce the output voltage drop and voltage ripple. 9.2.1.2.3 Paralleling Devices Any number of LM2665 devices can be paralleled to reduce the output resistance. Each device must have its own pumping capacitor C1, while only one output capacitor COUT is needed as shown in Figure 12. The composite output resistance is: (4) |
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