10052017 Switchedcapacitor voltage multiplier achieves 95% efficiencyAnalog Devices » ADG849, ADG888Marian Stofka EDN A capacitor that you charge through a resistor operates at 50% efficiency; hence, many engineers avoid using switchedcapacitor dc/dc converters. That efficiency figure holds true only for capacitors with no initial voltage, however. If you decide to switch a precharged capacitor, you can transfer energy to an output with a powerefficiency approaching 100%. To attain a fourthirds multiple of an input supply voltage, you can charge three capacitors to onethird of each supply voltage and then add that onethird of the input voltage to the input voltage to yield an output voltage that is fourthirds of the input voltage. You seriesconnect three capacitors of equal value and then charge this series string to a voltage equal to the input voltage. Because the values are equal, each of the capacitors charges to onethird the input voltage. The circuit then connects these three capacitors in parallel on top of the input voltage and switches this increased voltage to the output (Figure 1). The circuit repeats these two phases of operation at clock frequency F.
C_{IN} and C_{OUT} are filtering capacitors at the input and output, respectively. R_{P} is a protective resistor, which limits the inrush current to the capacitors at poweron. As the output voltage rises, it closes IC_{5} and shorts out this resistor. Schottky diodes D_{1} and D_{2} allow you to power the ICs using the input voltage until the output rises, at which time the higher output voltage powers the ICs. CDC is a storage and decoupling capacitor for this power bus. The higher powersupply voltage is necessary for proper operation, and it lowers the onresistance of the analog switches. The 0.4 Ω onresistance of IC_{1} results in low circuit losses and highefficiency operation. IC_{1}, IC_{2}, and IC_{3} exhibit a breakbeforemake operation, which is essential in this case. For a 50%dutycycle clock, you can calculate the theoretical powerefficiency of the converter, according to the following equation: If the value of C_{OUT} is equal to the value of C, the power loss due to charging of the three capacitors is about twothirds of the power loss during the discharging phase. The power consumption of the control circuit reduces the efficiency of this calculated value. For CMOS circuits, the power consumption rises linearly with the operating frequency. By choosing the operating frequency, you can optimize the efficiency of the circuit. The optimum frequency is inversely proportional to the load resistance, R_{L}. Fortunately, the efficiency maximum is flat, so you can achieve efficiencies higher than 90% over a wide range of values for R_{L}. You can attain 94% efficiency driving a 120 Ω load over clock frequencies of 100 to 400 kHz. If you set a 229kHz operating frequency, an input of 2.2 V yields a 2.87 V output at an efficiency of 95.9%. The optimum clock frequency shifts to lower values at lighter loads. You can drive a 269 Ω load at 100 kHz and achieve an output of 2.88 V. Materials on the topic 

