How a charge pump circuit works and an example of a voltage doubler circuit

Learn more about switched capacitor circuits by learning about charge pump circuits, what they are, how they work, their advantages and disadvantages, and their applications.

What is a charge pump circuit?

A charge pump circuit or charge pump regulator is a DC-DC converter that utilizes switched capacitor technology to increase or decrease the input voltage level.

As shown in Figure 1, these circuit blocks typically consist of just a capacitor and a switch (i.e., a clocked field-effect transistor, or FET), and work by carefully timing and controlling these switches to exploit the charge transfer properties of the capacitor.

Figure 1. Simple charge pump circuit schematic. Image courtesy of Texas Instruments

By alternately charging and discharging the capacitor, the charge pump converts the input voltage into a stable output voltage.

From a lower-level perspective, charge pump circuits exploit the inherent behavior of capacitors and their inability to change states instantaneously. As defined by the capacitor IV equation, in order for a capacitor to instantaneously change its voltage, it requires an infinite amount of current.

Since this is physically impossible, we see that a capacitor cannot suddenly change the voltage across its terminals. A charge pump exploits this behavior by using carefully timed switches to control the voltage across the capacitor.

Charge Pump Voltage Doubler Circuit Example

To better understand how a charge pump works, let's now look at a basic example: a voltage multiplier circuit.

As shown in Figure 2, our voltage doubler circuit consists of a single capacitor controlled by four surrounding switches.

Figure 2. Voltage doubler circuit schematic

The operation of this circuit is divided into two phases: gain phase and common phase. During the gain phase, SW1 and SW2 are closed, while SW3 and SW4 are open. As shown in Figure 3, at this stage, the positive and negative terminals of C1 are connected to Vin and GND respectively.

Figure 3. During the gain phase, the capacitor charges to Vin

Therefore, the capacitor is charged until the voltage on its terminals is equal to Vin. Now C1 charges to Vin and we switch to the common phase shown in Figure 4.

Figure 4. In common phase, the capacitor maintains the voltage across it by raising its positive terminal to 2*Vin

In common phase, SW1 and SW2 are open and SW3 and SW4 are closed. Here, the negative terminal of C1 is connected to Vin and the positive terminal is connected to Vout.

As mentioned before, the voltage across a capacitor cannot change instantly. Therefore, the capacitor will try to maintain an equivalent Vin voltage across itself. In order to hold this Vin on itself, the capacitor forces the voltage at Vout to be equal to 2*Vin, making the equivalent voltage across the capacitor equal to Vin.

The output voltage is referenced to ground, and the voltage multiplier circuit effectively accepts the input Vin and produces an output voltage of 2*Vin.

Non-ideal behavior in charge pump circuits

It is worth noting that our discussion so far has assumed ideal capacitors and ideal switches, neither of which are realistic in practical applications.

Some sources of non-ideal behavior in charge pump circuits include:

clock feedthrough

MOSFET (Metal Oxide Semiconductor Field Effect Transistor) switching losses

Capacitor Equivalent Series Resistance (ESR)

No match currently

charge leakage

cost sharing

Each of these non-ideal situations can cause the charge pump circuit to be less efficient and behave slightly differently than what our equations and examples have modeled so far.

Charge Pump Regulators: Advantages, Disadvantages, Applications

One of the main advantages of charge pump regulators compared to switching regulators is their significant reduction in size since no inductor is required.

Inductors are notorious for requiring a lot of silicon area on integrated circuits because their performance is very dependent on geometry: inductance value is directly related to the number of turns, and more turns require more area. Charge pumps, on the other hand, do not require the use of an inductor and are therefore much smaller than switching converters.

Table 1 below shows some of the key advantages and disadvantages between charge pumps, inductor-based switch-mode regulators, and low dropout (LDO) circuits.

Table 1. Compares the advantages and disadvantages of charge pumps, switching regulators, and LDOs.

Charge pumps also have the advantage over linear regulators that they offer higher efficiency and can step down and step up the input voltage.

Charge pumps, on the other hand, tend to be less efficient than switching regulators and have high levels of output ripple and noise, making them a worse regulator than linear regulators. For these reasons, charge pumps are best suited for applications requiring low load current and moderate input-output voltage differences.

Some popular applications of charge pump circuits include:

bias circuit

Successive approximation ADC

Electrically Erasable Programmable Read-Only Memory (EEPROM)

H-bridge high-side driver

Magnetoresistive Random Access Memory (MRAM)

phase locked loop

In this article, we discuss an overview of charge pump circuits, how they operate, and show an example of a voltage doubler circuit. In addition to this, we also discuss the trade-offs of the charge pump regulator and discuss how it compares to other popular types of regulators.