In this section, three types of voltage regulators will be introduced briefly, including linear regulators, switched capacitor circuits and switching regulators. Finally, a brief comparison will be given about three types of voltage regulators. The comparisons included circuit complexity, cost, efficiency, load ability and so on.
1.2.1 Linear Regulator
Linear regulator is also called LDO voltage regulator because there is a low drop-out voltage between input and output pin about 100~500mV [2]. LDO regulators are widely used as power management ICs in portable communication systems since they occupy a small chip area and can convert Li-Ion battery voltage to a low-noise and high-precision voltage to noise-sensitive analog blocks for ensuring high performance. The characteristic
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of LDO is compact without complex control unit, results in smaller chip size and cost. As shown in Fig. 3, the construction of LDO regulators usually utilizes a pass device, an error amplifier and a resistive feedback network. The pass device typically uses a p-type MOSFET for low dropout voltage. The error amplifier is used to regulate output voltage by controlling the pass device to supply load current. That is, LDO regulator utilizes the feedback network to construct shunt negative feedback effect to regulate output voltage.
Therefore, LDO regulator does not need oscillation clock, then the output noise can be minimized and the output voltage does not have ripple. However, the disadvantage of this type regulator is the conversion efficiency, which is about the output voltage dividing input voltage. The highest efficiency occurs when output voltage is near input voltage. Besides, the supply load current ability is dependent on the pass device’s size.
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OUTFig. 3. The basic structure of linear regulator.
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1.2.2 Charge Pump
The features of charge pump are described as follows [3] [4]. Firstly, the charge pump can be operated in both buck and boost mode, it depends on the reference voltage of the hysteric feedback control, but it is more efficiently in boost mode operation. Secondly, the circuit complexity of charge pump is between linear regulator and switching regulator.
Thirdly, the electromagnetic interference (EMI) and switching noise problem are not heavier than switching regulator because of lower switching frequency. Finally, the load ability is weak because of output capacitor and switching frequency.
Fig. 4. The basic structure of charge pump.
Fig. 4 shows the basic structure of two phase charge pump regulator. It consists of four switches, two capacitors, one hysteric comparator, one oscillator and resistive feedback. The operation concept is charging and discharging the capacitor C1 in complementary phases.
Firstly, during phase Φ1, switches S1 and S3 are closed and switches S2 and S4 are opened, the capacitor C1 is charged by input to Vin approximately. Secondly, during phase Φ2,
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switches S1 and S3 are opened and switches S2 and S4 are closed. Vin added the voltage on capacitor C1 to charge output capacitor Co, so Co maintains an output voltage close to 2Vin. With the hysteric feedback control, the output voltage level can be regulated at a desired value.
1.2.3 Switching Regulator
Switching regulators are mixed-signal circuits which have both analog and digital block in feedback loop. An analog signal which is error signal feeds back to produce a digital signal at a certain frequency rate which calls duty cycle. Output capacitor and inductors use duty cycle to regulate output voltage. The basic structure of buck type voltage mode switching regulator is shown in Fig. 5 [5]. The power stage of switching regulator consists of a couple of complementary power MOSFET (MP MN), passive storage elements inductor (L) and capacitor (C) and resistive feedback network (R1 R2). Detailed operation is described as follows; the resistors R1 and R2 sensing the variation of output voltage and error amplifier receives the voltage variation information then brings the error signal (VC). The comparator’s inputs receive the error signal from error amplifier and the ramp signal (VRAMP) from ramp generator, then compares the quantity between the error signal and the ramp signal to decide the duty cycle. After generating the control signal, the PWM generator control the detail timing to avoid short through current. At last, the purposes of gate drivers are driving huge complementary power MOSFET.
Generally speaking, the conversion efficiency of switching regulator can achieve above 90% under heavy load condition. Meanwhile, with higher switching frequency in the range from hundreds of Kilo-Hertz to several Mega-Hertz, the storage components can be designed smaller to save the cost. But the EMI and noise problems become critical. The supply load
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ability is the largest always in the range about hundreds of milliamps to several amps.
Fig. 5. The basic structure of buck type voltage mode switching regulator.
The characteristics of switching regulator are described as the following. Firstly, due to the storage components such as inductor and capacitor, the switching regulator can operate in three kinds of type including buck, boost and buck-boost mode. But the more external components cause the bigger PCB size and cost. Secondly, it suffers from EMI and switching noise problem due to switching based circuits.
1.2.4 Comparison
As the above description, three types of voltage regulator have its own advantages and disadvantages. How to choose the best voltage regulator as power supply depend on the electronic applications characteristics and specifications. The comparison of different type
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voltage regulator is listed in TABLE I.