Simple circuit converts 5V to –10V

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.

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 V_OUT=–(2×V_IN–2V_D–I_OUT×R_O), where V_IN is the input voltage, V_D is a diode's forward-voltage drop, I_OUT is the output current, and R_O is IC₁'s output resistance in free-running mode. For a 300-µA load current, the circuit's output voltage is –10V. Parasitic inductances inherent in the capacitors and pc-board traces produce a voltage overshoot that charges the output capacitors, delivering more than –11V at no load (Figure 2).

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