Saturday, December 18, 2010

【 Weak current College 】 LCD backlight with white LED driver solution 】

LCD CCFL is currently relatively often used as a backlight lamp CCFL Backlight, but due to the complex requirements drive lines, and inversion driving voltage high capacity color, plus backlight lamp is the system power consumption is the largest part, so in the increasingly harsh power limit, has gradually been industry discussions will use LED as a substitute.
To meet the demand for energy saving and environmental protection, for different applications and different power ranges, many Governments and Energy Agency's new power consumption standard have also come out of the oven. At the same time, more stringent standards are also developed. Reduce energy consumption has become an important topic cannot be avoided, and so on power management has also put forward higher requirements.
LED control forward current programmes
LED by current-driven devices, the brightness and the forward current is proportional relationship. There are two ways you can control the forward current. The first method is by using the curve, General LEDV-I a voltage of power supply and a rectifier resistors, to determine the yield the desired forward current needed to provide voltage LED. But this approach has some disadvantages, such as LED forward voltage of any change will cause the led current changes.
Assuming that a fixed voltage to 3.6V, current to 20mA, when voltage becomes 4.0V, changes in temperature or manufacturing of particular pressure, then current will be reduced to 14mA. So forward voltage to a larger change, will lead to greater forward current changes, in addition, the voltage drop and power consumption will be a waste of power and reduce the battery life. The second method is to use fixed current to drive the LED. Fixed current eliminates the forward voltage variations caused by the current change, therefore, produces fixed LED brightness. Use fixed current only requires through current detecting resistors voltage without adjusting the power supply's output voltage.
Power supply voltage and current sense resistor values determine the led current, LED the drive more, simply concatenated to each LED to achieve fixed current. In driven parallel LED must each LED string, place a rectifier resistance, but doing so will result in loss of efficiency and power.
As a result of portable applications, the battery life is key to the overall application. So high efficiency LED drive must be reached. However, the LED drive efficiency measurement and measurement of typical power efficiency is somewhat different. Typical power efficiency measurement, is the power output divided by the input power. For led driver, the output power was not a relevant parameter, instead of the expected LED brightness required input power value is the key. At this point you can use LED power divided by input power to get the answer.
Over voltage protection
In fixed current mode, the led driver components must provide overvoltage protection. No matter how much load can produce fixed current. But if the load resistance increase, the relative power output voltage must also be increased. When the power detected excessive load resistance, or load disconnect, then the output voltage may exceed the IC, or other components of the maximum operating voltage range. Therefore, in the drive, you must provide overvoltage protection. For example: you can use the Zener diode and LED connected in parallel, this way you can limit the output voltage Zener diode Breakdown voltage and power. When the pressure, the output voltage is increased to Zener diode breakdown point and Zener diodes, and then to the Earth's current detecting resistors, Zener diode and LED connected in parallel, you can securely deliver output current.
Alternatively, you can use the monitoring output voltage, before reaching the over-voltage power down. When an over-voltage condition appears, LED drive can reduce power consumption and longer battery life.
PWM dimming
Many portable LCD backlight applications need to adjust the brightness to a limited degree. In this part you can follow two methods, dimmable is analog or PWM method. Analog dimming, just as we are familiar with in LED to increase of 50 per cent of current, so you can increase the brightness of the 50 per cent. But this approach is flawed, and that's LED color offset arise and require the use of analog control signal, therefore, this pattern in general usage. While using PWM adjust the brightness of the key is to ensure that the consumer's eyes do not see the phenomenon of pulsed PWM, PWM signal frequency must be greater than the maximum 100Hz, PWM frequency depends on the power to activate and response time.
Load disconnect
Load disconnect is led driver power in an often overlooked feature, because of a power failure, you can use load disconnect will disconnect with power LED. This kind of functionality in the following two situations are very important, that is, power loss and PWM dimming. For example: in the boost converter loses power, the load is still through the inductor and diode connected to the input voltage. Because input voltage remains connected with LED, power failure, it still would have continued to produce small current, when long time leakage current phenomenon will shorten battery life. In addition, load disconnect in PWM for brightness control is also very important. Because in the PWM is not in operation, the power supply in case of failure, but the output capacitor still connect with LED.
If you do not load disconnect, output capacitor still will provide PWM LED power until the turn on the power again. Because capacitance at each start of PWM cycle, will discharge phenomenon, a power supply must be in each of the PWM cycle start the output capacitor charging, will each generate PWM cycle occurs or surge pulses. The current project will result in system efficiency, and enter a momentary voltage on the bus. But if you have load disconnect function, the LED will be disconnected from power supply, so when the power failure does not occur, and the leakage current PWM cycle for brightness adjustment, output capacitors are full.
At present, the world's largest companies are active in the development of a more complete, backlight more efficient white LED driver. So as the mobile phone to continue moving all-in-ChiCan turn direction, therefore, it is expected that the demand for LED drive will continue to increase. For example: ordinary mobile phones generally only takes 2 to 4 stars LED drive, but richer dual screen camera phones need 7 ~ 9 stars LED the drive to meet lighting requirements.
Supertex second generation high voltage LED driver IC HV9911
Supertex second generation high voltage LED driver IC HV9911, provides engineers with the LCD color backlight, automotive lighting and battery-powered LED lights and other products backlight designed application. HV9911 is closed loop of switch-mode led driver IC design for the led current and pulse width adjustment dimming good transient response. LED to multiple drives, you can synchronize subharmonic vibration prevention system.

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