[Peter Green/Odile Ronat]
The backlight of a liquid crystal display (LCD) has a considerable impact on power consumption, image quality, and panel life. Although LED backlight arrays have improved in these areas, stable photometric output and equalization are required. Enhanced color LED current control technology is required for colors, as well as functions that are not possible with traditional current limiting resistors such as better heat dissipation. This paper proposes a new technology called "Time Delay Hysteretic Current Control", which is accurate and highly flexible, and can achieve LEDs of up to 5 watts (W) and 1.5 amps (A) in a single chip. control.
In many cases, flat-panel displays need to support multiple modes of operation, data display, high-resolution images, full-motion video, or high-quality images. Recent analysis by Lumileds indicates that different modes require different backlighting characteristics to provide The best image quality, for example, when running a personal computer Windows application, the optimal color temperature of the backlight is about 9,600K, and when playing a movie, it is better to use a color temperature of about 6,500K, compared to the traditional cold cathode. The lamp tube (CCFL) uses LED arrays to make it easier to control and optimize backlight characteristics. In addition, LEDs also provide lower power consumption, heat generation and longer life, while eliminating the need for The frequency converter simplifies circuit design and reduces noise.
The LED backlight used in the LCD panel is efficiently coupled to the light guide plate by using red, green and blue LEDs to produce a white backlight. Adjusting the output of each color allows the design engineer to precisely control the characteristics of the backlight and bring more rich colors. Output and achieve better control of color temperature, wavelength and illumination, and use this flexibility to provide a more vivid display output to enhance the experience.
Power requirements for high-brightness LED backlights
Since the brightness and color of each LED are directly related to the current flowing through the LED itself, in order to take full advantage of the LED backlight, precise control of the current is required. However, this is the traditional way of using a current limiting resistor. It is not easy to achieve, especially for high-brightness LEDs used in backlight applications where there is a large difference in forward voltage drop, as shown in the data sheet of Lumileds' Luxeon III, which is rated at 3.70 volts (V). The voltage may vary between 3.03 and 4.47 volts, in addition to the temperature, such as the forward voltage temperature coefficient of the same product is -2mV / °C.
When considering the use of Luxeon DCC arrays as LCD backlight applications, each array consists of one or more strings of Luxeon LEDs connected in separate colors. One of the models is LXHL MGEA, which consists of two reds. The light LED, two strings and 11 green LEDs and two strings are composed of a combination of four blue LEDs. The requirements of the power supply are to provide constant current for each of the red, green and blue LED strings, and to adjust to the best photometric output characteristics, while meeting the maximum forward voltage specification requirements of each LED string, to the green in the LXHL MGEA array. The LED string is an example, which means that the bus voltage (VBus) requires a minimum of 40.5 volts plus a safe affluent space.
If the current of such a LED string is controlled by only one resistor, the difference in forward voltage during production and the change due to temperature will affect the operation efficiency and even cause the LED backlight to malfunction. Take 6 high-brightness LED strings with a rated forward voltage of 3.7 volts as an example. Under the 24VDC bus power supply, the current will be 750 mA through a 2.57 Ω resistor. The overall operating efficiency is 92.5%, but if the LED When the forward voltage is lowest, the current will rise to 2.4 amps, and the efficiency will drop to 76%. On the contrary, at the highest forward voltage, the overall voltage drop of the high-brightness LED string will be higher than the bus voltage, thus causing High-brightness LEDs cannot emit light.
Constant current control reduces output current variation
A more efficient and accurate solution is to use a high voltage DC-to-DC (Buck-O) step-down converter that includes a high voltage side switch or metal oxide semiconductor field effect that connects the bus power supply (Vbus) to the high brightness LED. The sense resistor between the transistor and the high-brightness LED and ground, which must be configured with a floating switch at the high voltage end to continuously monitor the load current and adjust it.
Under normal operating conditions, the output current is adjusted by the feedback voltage on the IFB pin, typically 0.5 volts. When VIFB is lower than the reference voltage (VIFBTH), the high voltage terminal MOSFET is turned on, and the high brightness LED is DC. The bus is powered, and the energy is stored in the LC resonant circuit during the IFB pin voltage rise. When the voltage on the IFB pin reaches the critical value VIFBTH, the high voltage terminal MOSFET is disconnected after a fixed time delay of the built-in circuit. .
At this point, the circuit will begin to release the pre-stored energy to provide high-brightness LED power. When the voltage on the IFB pin falls to a fixed critical point, the MOSFET will turn on again, but the built-in fixed circuit time delay will cause VIFB to The MOSFET is actually turned on and allows the system to be below the threshold before repeating the cycle action.
With the help of this fixed time delay, the continuous switching action of the circuit allows the current flowing through the high-brightness LED to be maintained at an IOUT (AVG) that can be calculated using the ratio between VIFBTH (usually 0.5 volts) and the RCS sense resistor. The average current output value is as long as the output resonant circuit consisting of LC can maintain low chopping (usually below 0.1 volts) on the IFB pin. Figure 3a and Figure 3b depict the ultra-low output current variation that can be achieved using fixed-time delay hysteresis current control, showing that the variation of the bus voltage is between 40 and 170 volts, and the variation is within ±0.3% at 1,400 mA. At 15 to 30 volts, the change at 1,400 mA is less than ±0.1%.
This circuit also limits spike currents, so it can be combined with small MOSFETs and small inductors to provide a precise and versatile high-efficiency control solution that allows designers to enhance LCD display quality with next-generation high-brightness LED backlighting.
It is worth noting that this method also eliminates the need for a fixed-frequency oscillator because the chip continuously compares the load current with the threshold value and performs MOSFET switching control according to the result. The frequency can be freely selected and depends on the LC and the input-output voltage. Therefore, this circuit not only compensates for differences in production and temperature drift, but also applies to a wide range of input and output voltages, as well as different high-brightness LED strings or array configurations.
High-brightness LEDs outside the backlight
Control is also quite suitable
A continuous-mode time-delayed buck regulator on a single chip brings a single-chip solution for high-power LED constant current control, with International Rectifier's IRS2540 and IRS2541 as examples, which can be 200 volts and 600, respectively. Volt's rated supply voltage operation, whether powered by a DC bus or direct AC, provides high-power LED string precision current control, saving both size and cost compared to conventional converters.
The time-delayed lag current control used in the two components is quite suitable for controlling the luminous intensity, operation mode and color characteristics of the new generation of high-brightness LEDs in various applications, and is not limited to the backlight of the LCD panel. High-brightness LEDs on the market today can reach output levels of approximately 5 watts and 1.5 amps, which will enable design engineers to apply LEDs in advertising signage, architectural lighting, decorative lighting, entertainment applications, automotive lighting, and more.
(The author of this article works at International Rectifier Corporation)