The inherent quality of light-emitting diodes (LEDs) allows it to replace cold-cathode fluorescent tubes (CCFLs) as the backlight solution for next-generation televisions, desktops, and notebook displays. The power consumption of LED is much less than CCFL, the life is 5 times longer than the latter, the efficiency is higher, the thickness of the display is thinner, the fineness of brightness adjustment is smaller, the low voltage driver is used, and it is more environmentally friendly because LED is different from CCFL It does not contain mercury or other harmful substances. However, all these features can only be fully realized after a good match between the LED backlight array and the driver IC. Therefore, only after understanding the key features and functions of the driver IC can the designer select the driver IC that best suits the application requirements. If you look at the data sheet of the driver IC, you will find that there are many parameters to consider. The parameters and functions described in this article are the most important. Parameters The first of these specifications is the input voltage that the driver IC can accept. If the input voltage range is narrow, then it can be applied to a relatively small range. In addition, such IC chips may not be able to withstand large input voltage swings and some other transient conditions that always exist in use. led If these differences are not minimized, then there will be significant brightness unevenness on the display. The driver IC controls the current and maintains the variation of the current within a very small range specified by the current matching specification. For notebooks and monitors, the ideal target is ± 2% or less; for TVs is ± 1%. Final considerations Remember that a driver chip with a large number of on-chip functions can reduce the PCB area and bill of materials required for the circuit. They also reduce the complexity of the design because designers no longer need to design external circuits or choose the best components for them. Of course, the importance of each function will be different in different applications, and the selected driver IC is selected based on its price and system performance comparison.
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The maximum output voltage of the driver chip is also critical, because each LED generates a voltage drop of 1 to 4V. The driver chip must have a sufficiently high output voltage to provide the voltage drop generated by multiple LEDs in the array. The maximum output voltage and the number of channels determine the number of LEDs it can support.
This conclusion also applies to the maximum current that the driver chip can provide for each channel. The current it can provide must match each design, with emphasis on the type of LED used.
The LEDs used in most portable applications require 20 to 30 mA of current, while LEDs in displays and TVs usually consume 40 to 120 mA (although in some applications LEDs require up to 350 mA). Generally speaking, the larger the output voltage and output current, the better, but it should be noted that the cost of high-output driver chips is usually higher than that of low-output similar products, so strict matching of driver chips and applications can save design costs. .
The number of channels that the driver IC can provide is from a few to 16, or even more. The choice of several "appropriate" driver chips for the number of channels is entirely determined by system requirements. The goal is to use as few driver chips as possible to meet system requirements to reduce cost and complexity.
However, the number of serial LEDs that the driver chip can support depends not only on its number of channels, but also on the maximum output voltage of the chip. For example, a 16-channel driver chip with a low output voltage can support 80 LEDs, and may only support a series of 5 LEDs; while a 10-channel driver chip with a high output voltage can support 160 LEDs, or perhaps only 16 LEDs .
Depending on the size of the display, the number of LEDs may range from 30 on a 10-inch display to more than 1,000 on a large flat-screen TV. Because the light output of these LEDs depends on the current, it is important to strictly meet the current requirements of all LEDs, although like all other electronic components, the characteristics of each LED are also different.
Because many attributes of LEDs will change with the LED current, the brightness adjustment function should be implemented using pulse width modulation (PWM) control to keep the current in the on state constant. Although some systems use external PWM signals (for example, some notebook computers use direct PWM control), a driver chip with an onboard PWM generator is usually a better choice. Such a device does not require an external PWM generator and can simplify system design. Some driver ICs support both methods.
The PWM frequency used in many existing backlight applications is below 1 kHz. In some cases, if low-cost ceramic capacitors are used, noise may be heard by the human ear. This problem can be avoided by selecting devices that support a wide range of PWM frequencies (including those that are inaudible to the human ear).
For example, Freescale's 10-channel MC34844 LED backlight driver chip meets this requirement. Some driver chips also provide the function of synchronizing with other devices or external sources to reduce the possibility of noise caused by the harmonics and beat frequency generated by the interaction of the devices and eliminate some visual artifacts.
The granularity or accuracy that can be achieved when the driver chip adjusts the brightness of the LED depends on the number of bits in the chip. The larger this number, the smaller the increment by which the PWM signal can be divided, and it can provide better brightness control. For example, the MC34844 LED driver chip is 8-bit, which can adjust the brightness of the LED to any one of 256 levels.
In the specification of the driver chip, the PWM high / low level conversion time should be as short as possible, so as to ensure that even when the duty cycle is small, the output is an accurate square wave pulse. This is necessary to ensure tighter current matching and better linearity in the brightness adjustment range.
At high PWM rates, such as 25kHz, driver chips that can provide linear brightness adjustment as low as one least significant bit (LSB) can provide the best performance. However, the conversion rate should not be too fast, because higher frequencies may cause oscillations and other forms of electromagnetic interference (EMI). A speed of around 50ns can meet this demand while maximizing efficiency.
Although some LED driver ICs do not have a communication interface, the devices used in many backlight applications are best equipped with this function, and this interface is particularly important for devices with onboard PWM generators. This interface simplifies programming, fault monitoring, and other functions, and is the most common type of internal integrated circuit (I2C). For systems that require high-speed updates, interfaces such as low-voltage differential signaling (LVDS) are becoming more common.
Driver chips with onboard boost converters no longer require external circuits to achieve this function. In addition, it is best to use an integrated switch because it eliminates the possibility of electromagnetic interference caused by onboard interconnection, simplifies the bill of materials (BOM), saves PCB area, and no longer requires the designer to specify a good matching driver chip Transistor. The boost frequency range used is very wide and is sometimes programmable.
The advantage of higher frequencies (eg 1.2MHz) is the ability to use smaller inductors and capacitors. Dynamic headroom control (DHC) mode is another important function. It measures all the LED strings connected to the boost converter and automatically adjusts the output voltage to the lowest voltage required to drive these LED strings. Doing so will reduce the voltage drop across the linear driver chip in the current mirror and drive power consumption, thereby increasing the overall efficiency.
A driver chip with optical loop control allows designers to use light sensors to compensate for changes in LED temperature and life. Thermal sensors can also be used to compensate for thermal effects. The light sensor can also be used to adjust the brightness of the backlight as a response to changes in the surrounding environment, and adjust the brightness of the display in a dark environment.
The ideal LED driver IC should contain many functions to protect the driver chip and LED. When a certain LED or LED string fails, the LED short circuit / open circuit protection can keep the backlight working. In addition, over-voltage, over-current and over-temperature protection provide the necessary protection for driving chips and LEDs. Undervoltage lockout is used to ensure that the driver chip will not work outside the specified range, because that may cause the device to not work properly.
