Charge pumps, also known as inductorless DC/DC converters, are a special class of switching DC/DC converters that use capacitors as energy storage elements. When compared to "inductive" switching DC/DC converters, which use inductors as energy storage elements, charge pumps offer unique characteristics that make them attractive for certain end-user applications. This article will compare the architecture and operation of a regulated charge pump with that of the most widely used inductor-based DC/DC converters, such as the buck regulator, the boost regulator and especially the Single Ended Primary Inductive Converter (SEPIC).
The regulated charge pump
One of the simplest and most commonly used charge-pump architectures is that of doubling. The doubling charge-pump architecture includes four switches, an external energy storage and transfer capacitor commonly known as the "flying capacitor," and an external output capacitor commonly known as the "reservoir capacitor."
Figure 1 illustrates the doubling charge pump architecture. This architecture's operation consists of two phases -- charging (energy storage) and discharging (energy transfer).
Figure 1: An unregulated, doubling charge pump
During the charging phase, switches S1/S3 are closed (switch on) and S2/S4 are open (switch off). The flying capacitor, CF, charges to the input voltage, VIN, and stores energy that will be transferred during the next discharging phase. The reservoir capacitor, CR, having been charged to 2VIN with energy transferred from CF during the previous discharging phase, supplies the load current.
During the discharge phase, switches S1/S3 are open and switches S2/S4 are closed. CF is now level-shifted by VIN, and since CF has already been charged to VIN during the previous charging phase, the total voltage across CR is now 2VIN (hence, the name "doubling" charge pump). CF then discharges to transfer energy stored during the charging phase to CR, as well as supply the load current.
The frequency of the charge/discharge cycle is determined by the frequency of the clock. It is a common practice to use a higher-frequency clock to reduce the capacitance, and hence the size, of both the flying and reservoir capacitors.
The output voltage of the simple doubling charge pump in Figure 1 is not regulated, as it changes according to the input voltage and load. This is not desirable for applications that require a source with regulated output voltage. However, the addition of a simple feedback loop easily overcomes this. Figure 2 illustrates a very simple doubling charge pump with a regulated output, frequently called a "regulated charge pump."
Figure 2: A regulated charge pump
In Figure 2, a switch, S5, is added to provide additional control on switches S2/S4. A comparator, the output of which is determined by the differential between a fraction of VOUT through a resistor divider R1 and R2) and a highly accurate voltage reference, controls the state of S5. The comparator usually has built-in hysteresis to prevent oscillation. The comparator, the resistor divider, the voltage reference and the S5 switch complete the feedback loop. The feedback loop regulates the output voltage of the charge pump by controlling the on and off states of switches S5 and S2/S4 in the discharge phase.
During the discharge phase, if VOUT is below the preset regulated output voltage, the comparator closes S5, which in turn closes S2 and S4. This allows CF to transfer energy to CR and the load, in order to bring VOUT up to the preset regulated voltage. When VOUT is brought up to the preset regulated voltage, the comparator opens S5, which in turn opens S2 and S4 to terminate the energy transfer. If VOUT cannot be brought up to the preset regulated voltage during this discharging phase, S5, S2 and S4 stay closed until the end of this phase.
On the other hand, if VOUT rises above the present regulated voltage, the comparator opens S5, which in turn opens S2 and S4. This terminates the energy transfer from CF to CR and the load, in order to bring VOUT down to the preset regulated voltage. S5 and S2/S4 stay open if VOUT cannot be brought down to the preset regulated voltage during this discharging phase.
The regulated charge pump can output a regulated voltage that is between ground and 2VIN, by simply manipulating the values of the resistors (R1 and R2) in the divider. This means the regulated output voltage can be either higher or lower than the input voltage. However, this is not the case for the most commonly used DC/DC conversion topologies that utilize inductors as energy storage elements, such as buck (step down) and boost (step-up) regulators.