The versatile switched-capacitor charge pump is easy to use and requires no inductor. It can double a positive voltage or convert a positive voltage to negative.
For some applications in which only a positive supply is available, the system must generate a negative voltage of larger magnitude than the positive rail. For that purpose, the circuit of Figure 1
inverts the input voltage and doubles the resulting negative output at the same time. The voltage inverter shown (IC1) converts a positive input to a negative output voltage, normally with an absolute magnitude lower than that of the input. But in this circuit, the two Schottky diodes and the two capacitors at the output produce a higher output voltage.
Figure 1. This simple circuit derives -10V from +5V.
The expected output is VOUT
), where VD
is the voltage drop across a diode, IOUT
is the output current, and RO
is the output resistance. While the maximum expected voltage is -10V, overshoot across the capacitors due to parasitic inductance in the capacitors and traces produces more than -11V at no load (Figure 2
Figure 2. The circuit in Figure 1 above produces more than -10V at no load, and more than 200mA while the output voltage remains greater than the input.
This design idea appeared in the May 26, 2005 issue of EDN