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LM2664M6X Datasheet(PDF) 6 Page - Texas Instruments |
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LM2664M6X Datasheet(HTML) 6 Page - Texas Instruments |
6 / 9 page Application Information SIMPLE NEGATIVE VOLTAGE CONVERTER The main application of LM2664 is to generate a negative supply voltage. The voltage inverter circuit uses only two external capacitors as shown in the Basic Application Cir- cuits. The range of the input supply voltage is 1.8V to 5.5V. The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals −(V+). 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 discharging C 1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C 1 will be multiplied by four in the output resistance. The output ca- pacitor C 2 is charging and discharging at a current approxi- mately 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. High capacitance, low ESR capacitors will reduce the output resistance. The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor C 2: Again, using a low ESR capacitor will result in lower ripple. SHUTDOWN MODE A shutdown (SD ) pin is available to disable the device and reduce the quiescent current to 1µA. Applying a voltage less than 20% of V+ to the SD pin will bring the device into shutdown mode. While in normal operating mode, the pin is connected to V+. CAPACITOR SELECTION As discussed in the Simple Negative Voltage Converter section, the output resistance and ripple voltage are depen- dent 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 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 www.national.com 6 |
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