# LED Current Balance Using a Variable Voltage Regulator with Low Dropout \(v_{DS}\) Control
## 1 Introduction  
The improving of lighting efficiency is one of the direct ways to contribute to energy-saving and green environment initiatives. Furthermore, mercury-free requests for avoiding the release of pollution into the environment also form part of the vital work at present. Accordingly, light-emitting diodes (LEDs) have become the necessary option in the lighting environmental renovation. Even the power LED has advantages, including a high-fidelity, high rendering, and low power consumption. Currently, the most urgent task is to understand how to make LED luminance become a surface light source, such as a fluorescent lamp. The most effective way to do this is by using a much lower-power LED to implement the surface source. To date, effective control strategies for balancing LED currents only include the linear balancing way and the digital pulse balancing way, such as is shown in Figure 1[1; 1; 1; 1; 1; 10]; in which Figure 1a,b demonstrate linear balancing ways and Figure 1c illustrates the digital pulse balancing way. In Figure 1a, all LED strings are supplied by a constant voltage source (CVS). In this situation, the LED current balance can be easily determined by a voltage detected from a sense resistor. However, this technique may cause a sense voltage change, especially for the CVS supply, since the LED forward voltage will vary with the ambient temperature [11]. As shown in Figure 1b, a shunt current balance configuration is built with a constant current source (CCS) supply, in which all of the string LED currents are collected into the controller and then individually compared with the reference current. This can achieve more accurate current balancing between adjacent LEDs, in order to emit uniform light. In Figure 1c, a digital pulse shunt current balance circuit is implemented with a supply of either voltage source (VS) or current source (CS). In this case,LED current balancing mainly uses a PWM duty cycle control, such as sequential phase-shift duty control [12; 13], burst mode [14; 15], self-adaptive control [16], and series-connected mode [17], etc. In this case, the LED driven by the pulse to emit luminance for a human is based on the persistence of vision. Although the LED current balance by pulse drives can be conducted by multiplexing, it is difficult to make all of the string LED currents balanced for obtaining a uniform light source. To improve this problem, the luminance's area should be fed back to adjust the adjacent string currents, producing a uniform light source. In spite of the fact that the mentioned current balance strategies are available, an uncertainty of the balance between the adjacent string LEDs always exists, mainly due to temperature variation and the dimming process. Further, the cost-effective way to perform a large scale lighting display is by using a shunt current balance configuration by linear current regulators (LCR) with a single voltage source [13; 18]. Above all, the MOSFET of LCR may dissipate more conduction loss, due to an uncertain drain-to-source voltage on the \(i_{DS}\)-\(v_{DS}\) plane [19]. Besides, many balance cells for LEDs directly using versatile dc/dc converters or resonant converters have been successively developed [18; 20; 21; 22; 23; 24; 25]. However, in terms of application for a large scale display, it is difficult to implement a large lighting area using such a large amount LED cells, needed to acquire a balanced luminance. In fact, the cost-effective way to perform a large scale lighting display is to use multiple shunt current balance configurations, controlled by multiple linear current regulators (LCR) with only a single voltage source. In this paper, to improve the characteristics of LCRs for driving the string LEDs, all operation points of the MOSFETs are separately placed on their \(v_{GS}\)-controlled characteristic curves; curves which are produced from an array of IC values and which are expected to present correspondingly similar results [19].