Analysis of white light LED backlight dimming scheme

Introduction: This article will briefly explain the LED power supply mode, and then divided into two kinds of LED dimming method and its advantages and disadvantages. At present, many built-in electronic devices are built using WLED backlight. If you do not understand the implementation of various methods and the advantages and disadvantages, it is not easy to find the correct LED dimming method. This article will briefly explain the LED power supply mode, and then divided into two kinds of LED dimming method and its advantages and disadvantages. Through this information, you can choose the appropriate dimming method and LED driver IC application.

Configure voltage regulator to achieve WLED power supply

With the current through the WLED brightness and shows linear change, as the current best serial to WLED accuracy and consistency of brightness, LED driver should adjust the current voltage of LED, rather than the current voltage regulation at both ends of the LED. Figure 1 shows how the LED is greater than the positive voltage in the output voltage and the sum (VLED) decline, reset any adjustable output current regulated DC to DC converter (DC-DC) has become a stable power supply, to drive a plurality of WLED series. After adjusting the voltage (VSENSE) of the current sensing resistor (RSENSE) and not the output voltage (VO), the driver becomes a stable power supply, so that the VO can be adjusted with the change of the VLED of the voltage and temperature. The voltage drop range of WLED is from 3 to 4 volts, which is directly affected by the LED current and inversely proportional to the temperature. The most recent low power driver has replaced the external sense resistor with one or more current inputs, especially the single junction field effect transistor (FET) (Figure 2). The driver has two functions, not only can adjust the driving voltage of drawing in FET to achieve through the appropriate current sink FET (relative to the bias current), and can adjust the integrated DC-DC converter (usually boost converter) output power, the FET is the current required minimum drain source voltage. This class contains an integrated boost converter and eight integrated current sink driver, one of them is TPS61195.

Analysis of white light LED backlight dimming scheme

Eliminate noise is the main task of PWM dimming

In order to improve the display effect and optimize the efficiency of the LED driver with different environment illumination brightness, the new LED backlight electronic device can reach a larger dimming range. There are two methods for LED dimming: pulse width modulation (PWM) dimming and analog dimming (Figure 3). In order to carry out the PWM dimming, digital signal processor (DSP) or micro controller (MCU) sends different load cycle (D) of the PWM signal, as shown in Figure 1 for the driver enables and disables the WLED drive converter, or in Figure 2 enables and disables the drive current sink. Therefore, the average current through the WLED serial is equal to the load cycle multiplied by the maximum current, that is ILEDavg=D * ILEDmax. Because the maximum current through the LED is the same, so the PWM dimming results will show a fairly linear brightness changes. In addition, because the spectral luminous LED will change because the voltage drop, voltage drop will change because the maximum value of ILED, so the color of the LED backlight (i.e., color or hue actually achieved white light level) in the PWM dimming can achieve excellent effect. The key drawback of PWM dimming is noise. If the PWM signal is used to enable and disable the converter, the upper limit of the dimming ratio of the driver will be limited by the time required for the start-up converter, the output capacitor to be charged, and the time required to achieve the maximum current. Even if the WLED drive enables the converter to operate at switching frequencies above 1MHz, the converter control loop response time and / or start-up time also takes hundreds of microseconds to several milliseconds. Therefore, in order to allow the driver to have time to achieve maximum current, PWM dimming frequency can only be hundreds of Hz. The piezoelectric ceramic output capacitor will cause the capacitor to PWM signal frequency in the range of audible noise (20k ~ 20kHz) in charging and discharging, the capacitor will vibrate, and the capacitance and the printed circuit board because of vibration and noise. The amplitude of the vibration, the amplitude of the voltage change and the ceramic capacitor are directly proportional.