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 NXP Semiconductors

  • Charge Pump Voltage Controlled Oscillator

  • Created: Sep 08, 2014

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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, a two-stage cycle can be used to generate a higher pulsed voltage from a lower-voltage supply. In the first stage of the cycle, a capacitor is connected across the supply, charging it to that same voltage. In the second stage of the cycle, the circuit is reconfigured so that the capacitor is in series with the supply to the load. Ignoring leakage effects, this effectively provides double the supply voltage to the load (the sum of the original supply and the capacitor). The pulsing nature of the higher voltage output is typically smoothed by the use of an output capacitor. An external or secondary circuit drives the switching, typically at tens of kHz up to several MHz. The high frequency minimizes the amount of capacitance required, as less charge needs to be stored and dumped in a shorter cycle. The capacitor used as the charge pump is typically known as the "flying capacitor".

The charge pump has a lot of applications. Not only that it is very useful in power supply applications, it is also very handy in voltage controlled oscillator. Some of the many useful features of VCO include single positive power supply operation, positive VCO reference voltage, fast response, high linearity, temperature compensation and open collector output. These features are present simultaneously most of the time. Although the component count is considerably high compared to the IC devices in the market, there is a guarantee of minimum cost because all the components are garden variety. Central is the low-cost LM339 quad single-supply comparator.

The NCX2222 provides a dual, low voltage, low-power comparator with open-drain outputs. The NCX2222 has a very low supply current of 5 A per comparator and is guaranteed to operate at a low voltage of 1.3 V. It is fully operational up to 5.5 V, which makes it convenient for use in both 3.0 V and 5.0 V systems.



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