General power supply circuits used in portable applications can be classified into three technologies: linear regulators, switched capacitor circuits, and switching regulators. In the following subsections, these technologies are briefly introduced and described. 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 Regulators
Fig. 2 shows the schematic of a linear regulator with a pass transistor operates M1 used as a variable resistor, which operates linearly to maintain the output according to the value of
power MOSFET with equivalent resistor (RDS)) between the input supply voltage and the regulated output voltage. An error amplifier controls the gate voltage of the pass resistance with respect to a reference voltage. With the 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.
Load
Fig. 2. The schematic of a linear regulator.
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.
But since the output current must pass through the series transistor which consumes the dropout voltage between the output and input voltages, the efficiency is low for large voltage difference between input and output voltages. The efficiency that depends on the difference of input and output voltages is given by (1):
( )
Efficiency OUT LOAD OUT
OUT LOAD IN OUT LOAD IN
V I V
V I V V I V
= ⋅ ≅
⋅ + − ⋅ (1)
1.2.2 Switched Capacitor Circuits
The switched capacitor circuit, also known as charge pumps, is usually used to obtain a dc voltage higher or lower than the supply voltage or opposite in polarity to the supply voltage in low power applications. Charge pump circuits use capacitors as energy storage devices. The capacitors are switched in such a way that the desired voltage conversion occurs.
The basic structure of two-phase charge pump regulator is shown in Fig. 3. Power stage consists of capacitors (C1 C2) and switches (S1 S2 S3 S4). Detailed operation is described as follows. The switches S1 and S2 turn on and the switches S3 and S4. turn off during the first interval of the switching period, charging capacitor CS to the input voltage level (VIN). During the second interval of the switching period, the switches S1 and S2 turn off and the switches S3
and S4. turn on, the voltage that across capacitor CS is placed in series with the input to generate an output voltage that is twice the input voltage.
The most straightforward method is to use a control circuit and an error amplifier. The error amplifier senses the output voltage variations via the feedback resistors. The control circuit fed from the error amplifier controls switches S1 ~ S4 to regulate output voltage to a stable value through a voltage control oscillator.
Fig. 3. The schematic of a switched capacitor voltage doubler.
The features of charge pump are described as follows. Firstly, 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. Secondly, 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 regulators
Switching regulators are widely used in power supply design, because it has high efficiency and power handing capability. The basic structure of boost type voltage mode switching regulator is shown in Fig. 4. The power stage of switching regulator consists of a
couple of complementary power MOSFET (MP MN), passive storage elements inductor (L) and capacitor (CO) and resistive feedback network (R1 R2). Detailed operation is described as follows; the resistors R1 and R2 sensing the variation of output voltage as the feedback signal (VFB), the 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, lower power MOSFET (MN) turns on and upper power MOSFET (MP) turns off then input voltage source charge the inductor. At the second subinterval, lower power MOSFET (MN) turns off and upper power MOSFET (MP) turns on 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.
on/off two power transistors M1 and M2, alternately. These current pulses are then converted to continuous or discontinuous current by means of an inductive and capacitive filter.
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.The most advantage of switching regulators over the linear regulators is the higher efficiency because the output pass transistors are operated as switches. When the output pass transistor is operated in the cutoff region, it dissipates no power. When the output pass transistor is operated in triode region, it is nearly a short circuit with little voltage drop across it, and it dissipates little power. In this manner, almost all power from input supply voltage is transferred to load, and high power efficiency can be achieved typically in the range from 80% to 90%, relatively independent of input to output voltage differences.
1.2.4 Comparison
A comparison table of power supply is listed in Table 1. From Table 1, we can conclude that switching regulators are best choices for power supplies driven portable application because of their high efficiency and large power handling capability.
Table 1. Comparative table of power supply circuits.
Characteristics Linear Regulator Switching Regulator Charge Pump
Regulation Type Buck Buck/boost/buck-boost Buck/boost
Chip Area Compact Large Moderate
Efficiency Minimum Maximum Medium