General power supply circuits can be classified into three kinds of regulator, such as linear regulators, switched capacitor circuits, and switching regulators. The technologies will be briefly introduced and described as following subsections. Furthermore, we will make a comparison for why we choose boost converter as the power supply unit of LED driver. The considerations of voltage regulators include low quiescent current consumption, low noise, high conversion efficiency, low cost, and so on.
1.3.1. Linear Regulator
The linear regulators use a pass element between input supply voltage and regulative
output voltage. The basic architecture of linear regulator is presented in Fig. 4 [8]. The low dropout voltage regulator provides a regulative voltage between input and output is about 100mv to 400mV. The linear regulator includes an error amplifier to reflect the output and input difference, a power MOSFET to supply the load current, and the resistive feedback network to sensing the output voltage variation. Due to the control circuit of LDO is the simplest one, so the chip size is smaller than other regulators. Furthermore, LDO circuit have no inductors, it can not only reduces the PCB space but also saves the cost.
Besides, it is more suitable to operate at the light-load condition than to operate in the heavy load. The other benefits of linear regulator are low noise and no output ripple. It’s because that it just uses resistive feedback and error amplifier to build the feedback network but not switching signal based. In conclusion, the linear regulator can be used for analog and RF circuit application. The conversion ratio of the LDO is step-down only because of without storage element. The load ability becomes the direct ratio to the size of power MOSFET.
Finally, the efficiency of LDO is proportional to output voltage, so the lower drop out voltage between input and output, the higher efficiency will be arrived.
Fig. 4. The basic structure of linear regulator.
1.3.2. Switching capacitor Voltage regulator
The basic architecture of two-phase switching capacitor voltage doubler is shown in Fig.
5 [9]. The basic structure consists of capacitors and switches and it is also known as charge pump. The switching capacitor voltage doubler can generate a dc voltage higher or lower than the supply voltage or inverting voltage to the supply voltage. The operation principle is described briefly as follow: In the first interval of switching period, clock CK1 is high and CK2 is low. The switch, SW1 and SW2, turning on and the switch, SW3 and SW4, turning off.
The capacitor, C1 is being charged to the supply voltage VDD. In the second interval of switching period, clock CK1 is low and CK2 is high. The switch, SW1 and SW2, turning off and the switch, SW3 and SW4, turning on. The capacitor, C1 is being charged to twice of the supply voltage VDD.
In order to maintain the output voltage, there are many ways to modulate the output voltage of the switched capacitor circuit to the desired value. To regulate the output voltage by a control circuit and an error amplifier is the most straightforward method. The error amplifier senses the variations of output voltage, and the control circuit controls switches, SW1~SW4, from the signal of error amplifier to regulate output voltage to the stable value.
Fig. 5. The basic structure of switching capacitor voltage doubler
The efficiency of switching capacitor voltage regulator is poor at heavy load condition because the load ability depends on the output capacitor. As a result, the larger output capacitor can increase the load ability. However, when the output voltage is a multiple of input voltage, it will have the best efficiency above 90%.
1.3.3. Switching Regulator
The switching regulator includes the boost converter, the buck converter, and the buck-boost converter. The buck-boost converter combines the step-up mode and step-down mode. The conversion ratio of buck-boost converter is depending on the switching duty cycle.
They are widely used in power supply design Fig. 6 [10] shows the basic architecture of switching regulator which is a boost converter. The switching regulator consists of power MOSFET, diode and resistor to feedback the output voltage variance. The error amplifier is utilized to reflect the output error and generate the error signal. The comparator is utilized to compare the error signal and the ramp signal to generate duty cycle depends on the error between output voltage and reference voltage. Therefore, controller will control the timing of switches. Driver is utilized to drive the huge power MOSFET so that it can regulate output voltage to expectative level.
The operation principle can be described as follow: In the first interval of switching period, the switch is closed and the input delivers the energy to the inductor. In the second interval of switching period, the switch is opened and the energy of the inductor delivers from the Schottky diode to output. By through this operation cycle by cycle, the circuit can provide the step-up voltage to the output. Besides, the output voltage is dominated by the reference voltage (VREF) and the feedback resistances. The buck converter is achieved by changing the positions of the inductor, switch, and Schottky diode.
Because of the switching operation, noise and EMI are critical issues for the output voltage in switching regulator design. The efficiency of switching regulator is good in heavy
load, but poor in light load. The detail description of the switching regulator will be
Fig. 6. The basic architecture of switching regulator.
1.3.4. Comparison of voltage regulators
In this section, we list the advantages and disadvantages of the three kinds of voltage
Complexity Low Medium Highest
Output ripple Lowest Medium Medium
Footprint area smallest Medium largest
Load ability Medium Low Highest
Efficiency Low Medium Highest
According to the table, we can realize that not only charge pump but also boost converter can boost the input voltage to higher than output voltage. However, in the application of high
brightness and large number LED driver, high power characteristics should be concerned.
Based on the consideration of the higher efficiency, smaller power consumption, wilder output range for more LEDs in series and stronger loading capacity for more strings connect in parallel. Boost converter is more suitable than charge pump to be a power supply in LED backlight application.