To conclude this short series on charge pumps, I’d like to give you a few circuit ideas, to get you thinking about a basic charge-pump circuit as a building block adaptable to a wide range of applications. If your application needs a nonstandard solution, there’s a good chance you can cobble it together from a few simple components.
Each stage of a charge pump increases the output voltage by approximately the amplitude of the switching waveform. If you want higher (or, in the case of an inverting charge pump, more negative) output voltages, you can cascade more than one charge-pump stage (see Figure 1).
Figure 1: Cascading multiple charge-pump stages
You can also combine regulated and unregulated charge-pump stages. Figure 2 shows such a circuit, in which can regulate the output voltage between approximately 15V and 30V.
Figure 2: Regulated charge pump with additional unregulated stage
Charge-pump circuits use DC voltages to set the voltage across the flying capacitor during the charge phase. If you have them available, you can use different voltages as the supply during the charge phase to improve conversion efficiency or extend the output voltage range.
Figure 3 shows a circuit that uses a 5V input voltage to generate an output voltage of approximately 19.3V from a 15V boost converter.
Figure 3: High-efficiency charge pump
Figure 4 shows a circuit that uses a –5V supply to generate –9.3V from a 5V boost converter.
Figure 4: High-efficiency inverting charge pump
Figure 5 shows a nifty little charge-pump circuit that employs a Dickson multiplier. This uses anti-phase switching waveforms to reduce the number of switching stages needed. In this application, standard logic gates generate a 5V rectangular waveform, which is used to drive the charge-pump circuit to generate an output voltage of approximately 18V (albeit with low current capability).
Figure 5: Using logic gates in a Dickson multiplier circuit
Charge-pump circuits are sometimes used to generate a bootstrap supply in DC/DC converter circuits, as shown in Figure 6. In this circuit, a capacitor connected to the switch node generates a voltage higher than the supply voltage, which enables the use of an N-channel FET for the high-side switch. N-channel FETs are smaller than the equivalent P-channel device, which results in improved performance or lower cost (or some combination of the two). Note that the arrangement shown in Figure 6 only works when the switch node is switching, so it tends only to be used in converters that operate in forced PWM.
Figure 6: DC/DC converter bootstrap capacitor
Charge pumps are useful circuits to have in in your armory. They're low-cost, simple and flexible. Many circuits already have a suitable clock signal available to drive the charge pump, and all you need is a suitable driver stage. For more demanding applications, TI offers a range of dedicated charge-pump devices that offer even better performance than a do-it-yourself charge pump can achieve.
Additional resources:
- Read part 1 of the “Pump it up with charge pumps” series
- Read part 2 of the “Pump it up with charge pumps” series
- Read part 3 of the “Pump it up with charge pumps” series
- Browse the charge pump portfolio.