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MAX865EUA Datasheet(PDF) 5 Page - Maxim Integrated Products |
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MAX865EUA Datasheet(HTML) 5 Page - Maxim Integrated Products |
5 / 8 page Compact, Dual-Output Charge Pump _______________________________________________________________________________________ 5 _______________Detailed Description The MAX865 contains all the circuitry needed to imple- ment a voltage doubler/inverter. Only four external capacitors are needed. These may be polarized elec- trolytic or ceramic capacitors with values ranging from 1µF to 100µF. Figure 2a shows the ideal operation of the positive volt- age doubler. The on-chip oscillator generates a 50% duty-cycle clock signal. During the first half cycle, switches S2 and S4 open, switches S1 and S3 close, and capacitor C1 charges to the input voltage (VIN). During the second half cycle, switches S1 and S3 open, switches S2 and S4 close, and capacitor C1 is level shifted upward by VIN. Assuming ideal switches and no load on C3, charge transfers into C3 from C1 such that the voltage on C3 will be 2VIN, generating the positive supply output (V+). Figure 2b illustrates the ideal operation of the negative converter. The switches of the negative converter are out of phase with the positive converter. During the second half cycle, switches S6 and S8 open and switches S5 and S7 close, charging C2 from V+ (pumped up to 2VIN by the positive charge pump) to GND. In the first half of the clock cycle, switches S5 and S7 open, switches S6 and S8 close, and the charge on capacitor C2 transfers to C4, generating the negative supply. The eight switches are CMOS power MOSFETs. Switches S1, S2, S4, and S5 are P-channel devices, while switches S3, S6, S7, and S8 are N-chan- nel devices. Charge-Pump Output The MAX865 is not a voltage regulator: the output source resistance of either charge pump is approxi- mately 150 Ω at room temperature with VIN = +5V, and V+ and V- will approach +10V and -10V, respectively, when lightly loaded. Both V+ and V- will droop toward GND as the current draw from either V+ or V- increas- es, since V- is derived from V+. Treating each convert- er separately, the droop of the negative supply (VDROOP-) is the product of the current draw from V- (IV-) and the source resistance of the negative convert- er (RS-): The droop of the positive supply (VDROOP+) is the product of the current draw from the positive supply (ILOAD+) and the source resistance of the positive IN a) b) S1 S3 C1+ C1 C3 C1- S2 S4 S5 S6 S7 S8 C2- GND V- RL- RL+ C2+ C4 C2 GND IN IV- GND IV+ V+ V+ Figure 2. Idealized Voltage Quadrupler: a) Positive Charge Pump; b) Negative Charge Pump V = I x RS - DROOP - V - |
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