What Is a Charge Pump?

A charge pump, also known as a switched capacitor voltage converter, is a DC-DC (converter) that uses so-called "flying" or "pumping" capacitors (rather than inductors or transformers) to store energy.

The charge pump is a

The charge pump can be divided into:

Switching regulator booster pump

No adjustment capacitive charge pump

Adjustable capacitive charge pump ^[2]

The working processes of the three types of charge pumps are: first store energy, and then release the energy in a controlled manner to obtain the required output voltage. Switching regulator boost pumps use inductors to store energy, while capacitive charge pumps use capacitors to store energy.

Capacitive charge pumps achieve voltage boost through switch arrays and oscillators, logic circuits, and comparison controllers, and use capacitors to store energy. Because it works at higher frequencies, small ceramic capacitors (1F) can be used, taking up minimal space and lowering the cost of use. The charge pump can provide ± 2 times the output voltage using only external capacitors. Its losses mainly come from the equivalent series resistance (ESR) of the capacitor and the RDS (ON) of the internal switching transistor. Charge pump converters do not use inductors, so their radiated EMI is negligible. Input noise can be filtered with a small capacitor. Its output voltage is precisely preset during factory production and can be adjusted by the back-end on-chip linear regulator, so the charge pump can be designed to increase the number of charge pump switching stages as needed to provide sufficient activity for the back-end regulator space. The charge pump is very suitable for the design of portable applications. It is a system of reference, comparison, conversion and control circuits. ^[2]

After the DCDC power supply, it doubles the boost and flips the negative power supply to power the LCD. ^[1]

In the past ten years, charge pumps have been widely used, never adjusting a single output IC to a regulating IC with multiple output voltages. Output power and efficiency have also been developed, so today's charge pumps can output up to 250mA with an efficiency of 75% (average). Charge pumps are mostly used in systems that require batteries, such as cellular phones, pagers, Bluetooth systems, and portable electronic devices.

Key applications include driving white light LEDs and milliwatt range digital processors for mobile phone backlights.

How the charge pump works

A charge pump (switched capacitor) IC performs DC / DC voltage conversion by using a switching network to power on or off two or more capacitors. The basic charge pump switching network constantly switches between powering up and down the capacitor. C1 (charge capacitor) transfers charge, while C2 (charge capacitor) stores charge and filters the output voltage.

Additional "fast capacitors" and switch arrays provide multiple benefits.

What are the operating modes of the charge pump

The charge pump IC can be used as an inverter, shunt or booster. The inverter converts the input voltage into a negative output. When used as a splitter, the output voltage is a part of the output voltage, such as 1/2 or 2/3. When used as a booster, it can bring a 1.5X or 2X gain to the I / O. Many portable systems use a single lithium-ion battery or two nickel metal hydride batteries. Therefore, when operating in 2X mode, the charge pump can supply a suitable forward voltage to white LEDs that generally operate in the range of 3.3V to 4.0V.

Is the output voltage of the charge pump regulated?

The basic charge pump lacks an adjustment circuit, so virtually all charge pump ICs used today add linear adjustment or charge pump modulation. The linearly adjusted output has the lowest noise and can provide better performance at lower efficiency. And because the adjustment IC does not have a series pass transistor, the charge pump modulation that controls the switching resistance can provide higher efficiency and provide more output current for a given chip area (or consumption).

What are the main advantages of charge pumps

The charge pump eliminates magnetic fields and electromagnetic interference from inductors and transformers. However, there is still a possible source of small noise, which is the high charging current flowing to the fast capacitor when it is connected to an input source or another capacitor with a different voltage. Similarly, "splitter" charge pumps can improve efficiency on LDOs, but they are not as complicated as inductive buck regulators.

Does the charge pump's output voltage match its input voltage?

The charge pump can continuously change its output voltage based on the battery voltage input. For example, it can run in either 1.5X or 1X mode. When the battery's input voltage is low, the charge pump can produce an output voltage equivalent to 1.5 times the input voltage. When the battery voltage is high, the charge pump operates in 1X mode. At this time, the load charge pump simply transmits the input voltage to the load. This reduces input current and power loss when the input voltage is high.

What happens when the switching frequency of the capacitor is increased

Increasing the switching frequency also increases the quiescent current of the IC, but also reduces the capacitance of C1 and C2. The normal frequency structure provides low noise to regulate the output voltage, while its input noise is lower than that of traditional charge pump regulators. High frequency operation simplifies filtering and further reduces conducted noise.

Which capacitors are best for charge pumps

To achieve optimal performance, use capacitors with low equivalent series resistance (ESR). Low ESR capacitors must be used on the output of the IC to minimize output ripple and output resistance and achieve maximum efficiency. Ceramic capacitors can do this, but some tantalum capacitors may be more appropriate.

What effect the charge pump soft start will bring

Soft start prevents excessive current flow at VIN during startup, thereby increasing the amount of current that can be periodically used to output the charge storage capacitor. Soft start is generally activated when the device is turned off, and is blocked immediately after the device has been adjusted.

How charge pump ICs minimize power consumption

By using pulse frequency modulation, the IC only generates charge when it must be transferred to maintain output regulation. When the output voltage is higher than the target regulation voltage, the IC is idle, and the current consumed at this time is the smallest, because the charge stored in the output capacitor will provide the load current. As this capacitor continues to discharge and the output voltage gradually drops to the target regulated voltage, the charge pump will activate and transfer charge to the output. This charge supplies the load current and increases the voltage on the output capacitor. ^[1]