Figure 9-2 Voltage Doubling Principle

The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals 2 V+. The output resistance R_out is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, and the capacitance and ESR of C₁ and C₂. Because the switching current charging and discharging C₁ is approximately twice the output current, the effect of the ESR of the pumping capacitor C₁ is multiplied by four in the output resistance. The output capacitor C₂ is charging and discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the output resistance. A good approximation of R_out is:

Equation 1.

where

• R_SW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 9-2.

The peak-to-peak output voltage ripple is determined by the oscillator frequency as well as the capacitance and ESR of the output capacitor C₂:

Equation 2.

High capacitance, low ESR capacitors can reduce both the output resistance and the voltage ripple.

The Schottky diode D₁ is only needed to protect the device from turning on its own parasitic diode and potentially latching up. During start-up, D₁ also quickly charges up the output capacitor to V_IN minus the diode drop thereby decreasing the start-up time. Therefore, the Schottky diode D₁ must have enough current carrying capability to charge the output capacitor at start-up, as well as a low forward voltage to prevent the internal parasitic diode from turning on. A Schottky diode like 1N5817 can be used for most applications. If the input voltage ramp is less than 10 V/ms, a smaller Schottky diode like MBR0520LT1 can be used to reduce the circuit size.