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LM2780TPX データシート(PDF) 5 Page - National Semiconductor (TI) |
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LM2780TPX データシート(HTML) 5 Page - National Semiconductor (TI) |
5 / 7 page Block Diagram 20044610 Operation Description The LM2780 is a switched capacitor voltage inverter that produces a negative output voltage from a positive input voltage. To minimize output voltage ripple, the LM2780 con- tains two charge pump inverters that operate 180˚ out of phase. These two charge pumps can be seen in the block diagram above. A basic switched capacitor inverter is a two-phase charge pump circuit that transfers charge from the input to the output using a flying capacitor. Charge Pump 1 in the block diagram will be used to illustrate the operation of a single charge pump. Disregard Charge Pump 2 in the diagram for now. In phase 1 ( φ1), the CLK1A signal of Charge Pump 1 is high, and MOSFET switches S1A and S1B are ON. For Charge Pump 1, this is the charge phase, as the flying capacitor C1 is charged to the input voltage. In the second phase ( φ2), the positive-charged plate of C1 is connected to GND through switch S1C, and the negative-charged plate is connected to the output through switch S1D. This results in the output voltage being approximately equal in magnitude to the input voltage, but opposite in sign (V OUT ~=-V IN). In this pump phase, charge stored on the flying capacitor sup- plies the current to the load and transfers charge to the output capacitor. This basic inverter has ripple in the output voltage. During the charge phase, the load current is supplied by the charge on the output capacitor. As charge is pulled off C OUT, the voltage across the capacitor drops, and the magnitude of the output voltage ( |V OUT| ) falls. During the pump phase, the flying capacitor transfers charge to the C OUT, elevating both the voltage across the C OUT and the magnitude of the output voltage. The LM2780 contains two charge pump inverters running 180˚ out of phase. Notice in the block diagram that the clock connections on Charge Pump 2 are opposite of the clock connections to Charge Pump 1. Thus, when Charge Pump 1 is in the charge phase, Charge Pump 2 is in the pump phase, and vice-versa. With this double-pump technology, there is always a flying capacitor connected to the output. This re- sults in significantly lower output voltage ripple than that of a single-pump inverter. OUTPUT VOLTAGE RIPPLE Several factors influence the magnitude of voltage ripple on the output of the LM2780. Primary influences are output capacitance, flying capacitance, load current, input voltage, and output capacitor ESR. The magnitude of the output voltage ripple is difficult to predict with equations when the LM2780 is double pumping. Refer to the Typical Performance Characteristic curves for this information. In general, ripple decreases with increased output capacitance, decreased load current, a lower input voltage, a smaller flying capacitance and/or a smaller output capacitor ESR. When only one charge pump is active (C2 removed), the output ripple is more predictable, and can be estimated with the following equation: V RIPPLE = [(0.5xIOUT)/(fSWxCOUT) ] + (2xIOUTxESRCout) www.national.com 5 |
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