In this section, three kinds 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
The basic structure of linear regulator is shown in Fig. 3[2], it also called low drop-out (LDO) voltage regulator because there is a drop out voltage (Vdropout ) between input and output pin about 100~500mV. The power MOSFET has equivalent resistor (RDS) from input to output, so the power MOSFET size should be well designed to fit the regulated output voltage and load ability. The linear regulator main control circuit was error amplifier, it could adopt output voltage information form resistive feedback network (VFB) then compare to
reference voltage (VREF). After error amplifier operation, it could immediately adjust input and output difference then control the gate of power MOSFET to supply load current.
The features of linear regulator are described as follows. Firstly, the linear regulator whole circuit is simple and compact, so the die size is smaller than other voltage regulators.
And secondly, linear regulator is easy to use, instead of using inductor to transfer energy, the linear regulator just adding two capacitors at input and output pin respectively. As a result, it not only can reduces Printed-circuit board (PCB) area but also cost down. Thirdly, linear regulator only uses resistive feedback network and error amplifier output analogy signal to control power MOSFET, it doesn’t use any switching base circuits. So this kind of regulator has no Electro Magnetic Interference (EMI) and no output ripple, there are very suit for audio, analog and RF circuit applications. Finally, because of without dual storage components, the linear regulator only can do buck regulation. The efficiency is proportional to output voltage and the highest efficiency occurs that output voltage is near to input voltage.
Fig. 3. The basic structure of linear regulator.
1.2.2 Charge Pump
The basic structure of two-phase charge pump regulator is shown in Fig. 4[3] [4]. Power stage consists of capacitors (C1 C2) and switches (S1 S2 S3 S4). Detailed operation is described as follows, during the first phase, switches S1 and S2 turn on and switches S3 and S4. The input voltage charges capacitor C1 to the input voltage level (VIN). Than during the second phase, switches S3 and S4 turn on and switches S1 and S2. Because the capacitor C1 still maintained the charge from the previous phase, the output voltage equals to input voltage adding voltage across the capacitor C1, ideally obtains twice input voltage. Adding hysteric feedback control, the output can be regulated at desired voltage level.
The features of charge pump are described as follows. Firstly, the charge pump can be
Fig. 4. The basic structure of charge pump.
reference voltage, but it’s more efficiently at boost type. Secondly, the circuit complexity of charge pump is between linear regulator and switching regulator, which is more compact than switching regulator but more complicated than switching regulator. Thirdly, due to digital rail-to-rail switching clock control, the charge pump suffers from EMI and output noise problems. But this problem doesn’t heavier than switching regulator because of lower operation frequency. Finally, the load ability of charge pump is weak because the ability depends on the output capacitor C2 and switching frequency. That is to say, the larger output capacitor causes the powerful load ability. Because of light load ability typically, the charge pump is very suit for displaying applications, such as driving the gate of MOSFET to on or off.
1.2.3 Switching Regulator
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. At the first subinterval, upper power MOSFET (MP) turns on and lower power MOSFET (MN) turns off then input voltage source charge the inductor and the capacitor. At the second subinterval, lower power MOSFET (MN) turns on and upper power MOSFET (MP)
turns off then the inductor will discharge to the capacitor and load. By the above-mentioned, the switching regulator adjusts the output voltage error and regulates to correct voltage.
Fig. 5. The basic structure of buck type voltage mode switching regulator.
The features of switching regulator are described as follows. 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, because of switching based circuits, it suffers from EMI and noise problems critically, it will take circuit layout into consideration to avoid EMI and noise problems. Finally, the load ability of switching regulator is the largest which is in the range about hundreds of milliamps to several amps, so the efficiency of switching regulator is the highest at heavy loading, up to 90%. But operating at light loading will decline.
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 voltage regulator is listed in TABLE I.