Microelectromechanical system (MEMS) devices often require high DC-bias voltages (40V to 100V) at low current (< 1mA), but the available supply voltage may be < 12V. A DC-DC converter that combines inductive and capacitive step-up circuitry (Figure 1) achieves the high voltage without need for a costly transformer.
U1 is a step-up DC-DC converter capable of output voltages to +36V, and a 2-stage charge-pump converter (D2-D5 and C1-C4) nearly triples the U1 output. The R1/R2 divider provides feedback for regulating the circuit's output voltage. Use these equations to calculate the resistor values:
For lower output voltages (< 70V), remove D4, D5, C3, and C4 to produce a single-stage charge pump. Similarly, you can add another stage to produce voltages higher than 100V. An optional post filter (R3 and C5) reduces the output ripple to < 10mVP-P. Figure 2 illustrates the variation of output voltage with output current.
A similar version of this article appeared in the August 18, 2002 issue of EET magazine.