Sequential low-voltage detecting method for multistring LED BLU circuit

Abstract

A new low-voltage detecting method for a multichannel LED back lighting unit is proposed. The proposed method detects the lowest voltage drops among the multichannel LED strings with a reduced number of devices, and it provides feedback voltage to control the stages of the boost converter for minimising the power consumption in the LED driving circuit.Introduction: A backlight unit is needed in LCD applications, and two major backlighting sources are widely used: the cold cathode fluorescent lamp (CCFL) and the light emitting diode (LED). Recently, LEDs have gradually substituted CCFLs as the backlighting source for their better colour gamut, longer lifetime and thin structure. LED back lighting units (BLUs) can also provide a local dimming function that significantly enhances the contrast ratio (CR) in LCD TVs, and its colour spectrum is even larger than NTSC [1,2].In general, the LED backlighting system consists of a boost converter and current driving channels. The boost converter supplies sufficient voltage to ensure the normal operation of LED strings, and the current drive channels regulate the current of each LED string. LEDs have an inherent forward voltage drop deviation. Moreover, the forward voltage drop of LEDs depends on the junction temperature. For these reasons, the channel voltage drop in the LED driving circuit should be kept as low as possible to minimise the power consumption. To achieve this, a low-voltage detection circuit is required to control the output voltage of the boost converter in an LED BLU.The most common method for controlling low voltage in LED strings is sensing the cathode voltage of each LED string with small-signal diodes and regulating the voltage of the boost converter [3,4]. Although they are simple to implement, these methods are inaccurate because sensing diodes also have variations in forward voltage drop as well as temperature dependence, which results in higher cost and poor sensing accuracy.

Design Aspects of a Single-Output Multi-String WLED Driver Using 40 nm CMOS Technology

“…As a result, customized WLED drivers are necessary. When it comes to designing a WLEDs driver, there are different architectures to consider, including boost-based [4,5], buck-based [6,7], and charge pump-based WLED drivers [8], as well as many others [9][10][11]. Boost-based architectures are preferable for smartphone applications, as the available input voltage, i.e., the battery voltage (3.2-4.2 V), is well below the required output voltage (20-40 V).…”

A Multi-Output Multi-String High-Efficiency WLED Driver Using 40 nm CMOS Technology

“…The traditional architecture of multi-string WLED drivers has all strings connected to the same output node, which is V BOOST , as shown in Figure 1. Despite its simple architecture with a common controller, single OVP circuitry, and a minimum number of switches per string [5][6][7][8], it suffers from lower power efficiency and current mismatching among strings. This is particularly true in an asymmetrical string arrangement and low process nodes, such as a 40 nm process node.…”