General power management circuits used in portable devices can be classified into three technologies: linear regulators, switched capacitor circuits, and switching regulators.
These voltage regulators will be introduced in briefly in this section, and comparisons will be given finally, including load ability, efficiency, circuit complexity, efficiency and so on.
1.2.1 Linear Regulator
The basic structure of linear regulator is shown in Fig. 3 [3], it’s also called low drop-out (LDO) voltage regulator because of a drop out voltage (Vdropout) between input and output voltages about 100~500mV. An error amplifier controls the gate voltage of the pass transistor (MPWP) with respect to a reference voltage (VREF). These devices are constructed in a feedback configuration to maintain the output voltage irrespective of the load current variations. The power transistor has equivalent resistor (RDS) from input to output, so the size of power transistor should be well designed to fit the regulated output voltage and load ability.
The advantages of linear regulator are described as follows. One is the circuit structure is simple, make the die size is the smallest in all kind of voltage regulators, another is linear regulator only uses feedback resistors and error amplifier’s output analogy signal to control power transistor, it makes a purely analogy operation environment without any digital base circuits. So there is no Electro Magnetic Interference (EMI) issue and no output ripple.
There are two major disadvantages described as follows. Firstly, because of without dual storage components, the linear regulators only can do buck regulation. Secondly, 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 across
input and output voltages. The efficiency that depends on the difference of input and output voltages is given by (1).
OUT LOAD OUT
OUT LOAD IN OUT LOAD IN
V I V
V I V V I V
(1)The major applications that use linear regulators can be classified to digital and analog.
The digital applications include Digital Signal Processors (DSP), Input/Output (I/O) modules and memory type devices. Analog applications include signal-path applications such as power amplifier (PA) and Phase lock loop (PLL).
Fig. 3. The basic structure of linear regulator.
1.2.2 Switched Capacitor Circuit
The switched capacitor circuit is also called chare pump, is used to generate a dc voltage higher or lower than the supply voltage or opposite in polarity to the supply voltage in low power applications. Charge pumps use capacitors as energy storage devices. The capacitors are switched to deliver energies to obtain desired output voltage. Fig. 4 illustrates a conventional switched capacitor voltage doubler [4].
The switches S1 and S3 are closed during the first interval of the switching period, charging capacitor CS to supply voltage (VBAT). During the second interval of the switching
period, the switches S2 and S4 are turned on and the voltage across capacitor CS is placed in series with the input to generate an output voltage that is twice the input voltage. In order to maintain the output voltage, there are many ways to modulate the output voltage of a switched capacitor circuit to a desired value. 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.
Depending on the hysteric feedback control and reference voltage, both buck and boost type can be used in switched capacitor circuits, and the circuit complexity is between linear regulators and switching regulators. But there is major drawback which is the load ability is depend on the size of output capacity (CO) and switching frequency. That is to say, larger capacitor makes stronger load ability but consumes more chip size.
The charge pumps are useful in many different applications including low-voltage circuits, dynamic random access memory circuits, Electrically Erasable Programmable Read-Only Memory (EEPROM), and transceivers.
Fig. 4. The conventional structure of voltage doubler.
1.2.3 Switching Regulator
Switching regulators are widely used in power management system, because it has high efficiency and power handing capability. The conventional structure of buck type voltage mode control switching regulator is shown in Fig. 5 [5]. The regulator consists of a couple of complementary power transistors (MPWP MPWN), passive storage inductor (L), output capacitor (CO), feedback resistors (RFB1 RFB2), and control circuits.
The circuit operation is described as follows. The error amplifier receives the output voltage variation and produces the error signal (VEA). The inputs of comparator receive the error signal from error amplifier and the ramp signal (VRAMP) from ramp generator, then compares the quantity between VEA and VRAMP to decide the duty cycle. After generating the control signal, the logic block produce the detail timing to avoid short through current.
Finally, control signals are sent to gate drivers to drive huge complementary power transistors. At the first subinterval, power PMOS (MPWP) turns on and power NMOS (MPWN)
turns off then supply source charges the inductor and the output capacitor. At the second subinterval, power NMOS (MPWN) turns on and power PMOS (MPWP) turns off then the inductor will discharge to the output capacitor and load. As mentioned above, the switching regulator adjusts the output voltage error and regulates to correct voltage.
The advantages of switching regulators are described as follows. Firstly, the load ability is very large which in the range from no load to several amps. Secondly, the power conversion efficiency is high in medium to heavy load condition, up to 90%. Thirdly, switching regulators can operate in different kinds of type including buck, boost, and buck-boost type.
Nevertheless, there are some drawbacks described as follows. Firstly, the power conversion efficiency is poor at light load condition. Secondly, too many external components enlarging PCB size and cost. Finally, there are EMI and noise issues should be considered due to digital switching.
Fig. 5. The structure of buck type voltage mode control switching converter.
1.2.4 Comparison
A comparison table of power management circuits is listed in Table I. From Table I, 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 I. Comparative table of power management circuits.
Linear Regulation Type Buck Buck/Boost Buck/Boost/Buck-Boost
Efficiency Low Medium High
Power Capability Medium Medium High
Footprint Area Compact Moderate Large
Cost Low Medium High
Complexity Low Medium High
Noise Low Medium High