Charge Pump

A charge pump is a kind of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pump circuits are capable of high efficiencies, sometimes as high as 90-95% while being electrically simple circuits. Charge pumps use some form of switching device(s) to control the connection of voltages to the capacitor. For instance, to generate a higher voltage, the first stage involves the capacitor being connected across a voltage and charged up.

In the second stage, the capacitor is disconnected from the original charging voltage and reconnected with its negative terminal to the original positive charging voltage. Because the capacitor retains the voltage across it (ignoring leakage effects) the positive terminal voltage is added to the original, effectively doubling the voltage. The pulsing nature of the higher voltage output is typically smoothed by the use of an output capacitor. This is the charge pumping action, which typically operates at tens of kilohertz up to several megahertz to minimize the amount of capacitance required.

The capacitor used as the charge pump is typically known as the “flying capacitor”. Another way to explain the operation of a charge pump is to consider it as the combination of a DC to AC converter (the switches) followed by a voltage multiplier. The voltage is load-dependent; higher loads result in lower average voltages. Charge pumps can double voltages, triple voltages, halve voltages, invert voltages, fractionally multiply or scale voltages such as x3/2, x4/3, x2/3, etc. and generate arbitrary voltages, depending on the controller and circuit topology.

The term ‘charge pump’ is also used in phase-locked loop (PLL) circuits. This is a completely different application. In a PLL the phase difference between the reference signal (often from a crystal oscillator) and the output signal is translated into two signals – UP and DN. The two signals control switches to steer current into or out of a capacitor, causing the voltage across the capacitor to increase or decrease. In each cycle, the time during which the switch is turned on is proportional to the phase difference, hence the charge delivered is dependent on the phase difference also.

The voltage on the capacitor is used to tune a voltage-controlled oscillator (VCO), generating the desired output signal frequency. The use of a charge pump naturally adds a pole at the origin in the loop transfer function of the PLL, since the charge-pump current is driven into a capacitor to generate a voltage (V=I/(sC)). The additional pole at the origin is desirable because when considering the closed-loop transfer function of the PLL, this pole at the origin integrates the error signal and causes the system to track the input with one more order.

The charge pump in a PLL design is constructed in integrated-circuit (IC) technology, consisting of pull-up, pull-down transistors and on-chip capacitors. A resistor is also added to stabilize the closed-loop PLL. An internal power source or a charge pump is essential in every system. An embedded system has to perform tasks continuously from power-up to power-off and may even be kept ‘on’ continuously. Certain systems do not have a power source of their own: they connect to an external power supply or are powered by the use of charge pumps.

Network Interface Card (NIC) and Graphic Accelerator are examples of embedded systems that do not have their own power supply and connect to PC power-supply lines. (2) A charge pump consists of a diode in the series followed by a charging capacitor. The diode gets forward bias input from an external signal; for example, from an RTS signal in the case of the mouse used with a computer. Charge pumps bring the power from a non-supply line.

Ninepins COM port has a signal called Request To Send (RTS). It is an active low signal. Most of the time it is in inactive state logic ‘1’ (~5V). The charge pump inside the mouse uses it to store the charge when the mouse is in an idle state; the pump dissipates the power when the mouse is used. A regulator circuit getting input from this capacitor gives the required voltage supply. A charge pump in a contact-less smart card uses the radiations from a host machine when inserted into that.

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