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Charge pump

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A charge pump is an electronic circuit 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.

Another use of charge pump is in phase-locked loops (PLL). 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 (hence the names of the signals). A low-pass filter is placed in parallel with the capacitor to smooth out abrupt changes on the capacitor's voltage and to ensure the PLL's closed-loop stability. The output of the charge pump is used to tune a voltage-controlled oscillator (VCO), generating the desired output signal frequency. 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 and resistors.

Applications

  • A common application for charge pump circuits is in RS-232 level shifters where they are used to derive positive and negative voltages (often +10 V and -10 V) from a single 5 V or 3 V power supply rail.
  • Charge pumps can also be used as LCD or white LED drivers, generating high bias voltages from a single low-voltage supply, such as a battery.
  • Charge pumps are a key component in EEPROM and flash memory devices. These devices require a high voltage pulse to "clean out" any existing data in a particular memory cell before it can be written with a new value. Early EEPROM and flash memory devices required two power supplies: +5V (for reading) and +12 V (for erasing). As of 2007, commercially available flash memory and EEPROM memory requires only one external power supply -- generally 1.8 or 3.3V. A higher voltage is used to erase cells, but that higher voltage is generated internally by an on-chip charge pump.

See also

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