A typical switched-capacitor charge pump requires no inductors, is easy to design, and can double a positive voltage or convert a positive voltage to an equivalent negative voltage. However, in some applications, only a positive supply is available, and the power-supply system must generate a negative voltage of larger magnitude than the positive supply rail's voltage. The circuit in Figure 1 simultaneously inverts its input voltage and doubles the resulting negative output.
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| Figure 1. | This switched-capacitor inverter derives –10 V from 5 V. |
Normally, the MAX889T voltage inverter, IC1 , converts a positive input to a negative output voltage with an absolute magnitude lower than that of its input. But, in this circuit, Schottky diodes D1 and D2 and capacitors C4 and C5 help produce a higher output voltage. The circuit's nominal output is
VOUT = –(2 × VIN – 2VD – IOUT × RO),
where VIN is the input voltage, VD is a diode's forward-voltage drop, IOUT is the output current, and RO is IC1 's output resistance in free-running mode. For a 300-µA load current, the circuit's output voltage is –10 V. Parasitic inductances inherent in the capacitors and pc-board traces produce a voltage overshoot that charges the output capacitors, delivering more than –11 V at no load (Figure 2).
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| Figure 2. | At light loads, the circuit in Figure 1 delivers more than –10 V; at higher currents, the magnitude of the negative output voltage exceeds its positive input voltage of 5 V. |
