Backlighting LED driver design required for new automotive displays

According to a study by Strategies Unlimited, the market for high-brightness LEDs is expected to reach $8.2 billion in 2010, and is expected to grow to $20.2 billion by 2015, with a compound annual growth rate (CAGR) of 30.6%. The LEDs currently used to backlight the display are the main drivers of this growth, including high-definition televisions, automotive displays, and a large number of handheld devices. To maintain high growth rates, LEDs need to provide higher reliability, lower power consumption, and smaller/flatter form factors, as well as improved contrast, image quality, and color accuracy. In addition, automotive, avionics and marine displays are suitable for a variety of ambient lighting conditions, such as bright daylight and dim nights. These transistor liquid crystal displays (TFT-LCDs) are used in a wide range of applications, from infotainment systems, meters to meters, to backlighting these displays with LEDs, to optimize displays in a wide variety of ambient lighting conditions. The legibility brings with it some unique LED IC driver design challenges that require LED drivers to provide a very wide dimming range and high efficiency, and to withstand the harsh and harsh automotive electrical and physical environments. Finally, these solutions must be flat and compact in size to increase cost efficiency.

The rapid growth of LEDs in the automotive lighting market stems from several factors: First, LEDs are more than 10 times more efficient than incandescent lamps, almost twice the efficiency of fluorescent lamps, including cold cathode fluorescent lamps (CCFLs), thus reducing the availability. A quantity of light output (in lumens) of the required electrical power and the amount of heat it produces. With the further development of LEDs, the efficiency of generating lumens with electrical power will only continue to increase. Secondly, LED lighting does not need to handle, contact and remove toxic mercury vapors commonly found in CCFL/fluorescent lamps, which is a good requirement for green environmental protection. Third, the incandescent lamp needs to be replaced every 1000 hours, and the continuous use time of the fluorescent lamp reaches 10,000 hours, compared with the life of the LED exceeding 100,000 hours. In most applications, this allows the LED to be permanently embedded in the final application without the need for an attachment. This is especially important for backlighting of instrument/navigation/infotainment system displays embedded in the interior of the car, as they never need to be replaced during the life of the car. In addition, LEDs are orders of magnitude smaller and flatter than other illuminators, so the LCD display can be very thin, so the space required inside the car is minimal. In addition, an unlimited number of colors can be provided by using red, green and blue LED configurations. The LEDs are also capable of dimming and turning on/off at a much faster rate than the human eye can detect, significantly improving the backlighting of the LCD display while achieving extremely high contrast and resolution.

One of the biggest challenges facing automotive lighting system designers is how to optimize all the benefits of the latest generation of LEDs. Since LEDs typically require an accurate and efficient DC current source and a dimming method, LED driver ICs must be designed to meet these requirements under a wide variety of conditions. Power solutions must be efficient, reliable, compact, and affordable. It can be said that for LED drivers, the most demanding applications will be found in avionics, marine and automotive infotainment systems for TFT-LCD backlighting applications, as these applications must face the harsh automotive electrical environment and must compensate for changes. A wide range of ambient lighting conditions must be placed in a very confined space while maintaining an attractive cost structure.

Automotive LED backlighting

Advantages such as small size, extremely long life, low power consumption and enhanced dimming capabilities have led to the widespread use of LED TFT-LCD backlighting in today's cars, aircraft and ships. Infotainment systems typically have an LCD display mounted somewhere in the center of the dashboard so that drivers and passengers can easily view locations, perform audio tuning, and perform a variety of other tasks. In addition, many cars have LCD screens for passengers in the back seat, offering movies, video games and more. Historically, these displays used CCFL backlighting, but the replacement of these relatively large CCFL lamps with very flat white LED arrays is becoming more common, with white LEDs providing more accurate and adjustable backlighting, and It is easy to exceed the service life of the car.

The benefit of using LEDs in this environment is that it can have several positive effects. First, they never need to be replaced because they have a reliable lifespan of more than 100,000 hours (11.5 years of use), exceeding the life of the car itself. This allows automakers to permanently embed them into the backlighting system in the car without having to worry about easy replacement. The style may also be a significant change, as the LED lighting system does not require as much space or as much space as a CCFL lamp. LEDs are also generally more efficient than fluorescent lamps in providing light transmission (in lumens) from the input source. There are two positive effects. First, the electrical power provided by the car bus is less expensive, and it is equally important that this reduces the amount of heat that needs to be dissipated in the display, eliminating the need for any bulky and expensive heat sinks.

Another important benefit of LED lighting is the wide dimming ratio capability offered by high performance LED driver ICs. Due to the wide range of ambient lighting conditions inside the car, including every change from direct daylighting to complete darkness, LED backlighting systems must be able to provide very large dimming ratios, typically up to 30,000:1. With a suitable LED driver IC, this large dimming ratio is relatively easy to achieve, while CCFL backlighting is not possible. Figure 1 shows a typical LCD-based analog dashboard.

Figure 1simulationdashboard

Automotive LED lighting design parameters

To ensure optimum performance and long operating life, LEDs require an efficient drive circuit. These driver ICs must rely on the harsh automotive power bus. In order to maintain a longer working life of the LED, the design requirements do not exceed the LED current and temperature limits.

One of the major challenges in the automotive industry is to overcome the demanding electrical environment on automotive power buses. The main challenges are transient conditions such as the well-known load dump and cold start. The load dump condition means that the battery cable is disconnected while the alternator is still charging the battery. This can happen when the battery cable is loose and the car is driving, or when the battery cable is broken and the car is driving. This sudden disconnection of the battery cable can produce transient voltage spikes of up to 40V because the alternator attempts to fully charge a battery that has been disconnected. Transient voltage suppressors on alternators typically clamp the bus voltage to approximately 36V and absorb most current surges. However, the DC/DC converter downstream of the alternator is subject to these 36V to 40V transient voltage spikes, ensuring that these converters are not damaged in the event of such transient events, and that the output voltage is stable and can be used externally. Different alternative protection circuits are implemented, such as the transient voltage suppressors that are often chosen, but these aspects add cost and weight and take up space.

“Cold car launch” refers to what happens when the car engine is in a cold or freezing temperature for a period of time. The engine oil becomes extremely viscous and requires an engine starter to provide greater torsional force, which in turn draws more current from the battery. This high current load may pull the battery/main bus voltage below 0.4V during ignition and then typically return to the nominal 12V.

At present, there is a new solution to help get rid of this dilemma. Linear Technology has introduced the LT3599, which can be damaged in both cases and adjust a fixed output voltage. The device's 3V to 30V input voltage range and 40V transient protection make it suitable for automotive environments. Even when VIN is above VOUT (which can occur at 36V transient), the LT3599 will still regulate the desired output voltage.

Most LCD backlighting applications require 10 W to 15 W of LED power, and the LT3599 meets this power requirement, and it can boost the car bus voltage (nominally 12V) to 44V to drive up to 4 parallel LED strings (per The string contains 10 series connected 100mA LEDs). Figure 2 shows a schematic of the LT3599 driving four parallel LED strings, each consisting of 10 80mA LEDs for a total of 12W.

The LT3599 uses an adaptive feedback loop design that modulates the output voltage to a voltage slightly above the maximum voltage of the LED string, which minimizes the power lost in the ballast circuit and optimizes efficiency. Figure 3 illustrates the efficiency of the LT3599 with an efficiency of 90%, eliminating the need for any heat dissipation and achieving a very compact flat solution. Equally important in driving an LED array is the ability to provide accurate current matching to ensure that the backlight brightness is consistent across the display. The LT3599 guarantees a LED current change of less than 2% over a temperature range of -40°C to 125°C.

Figure 2 12W LED backlight circuit with 90% efficiency using LT3599

Figure 3 Figure 2. LED current matching and efficiency of the LT3599

The LT3599 uses a fixed frequency, constant current boost converter topology. Its internal 44V, 2A switch is capable of driving 4 LED strings (each string consisting of 10 100mA LEDs in series). Its switching frequency is programmable and synchronizable from 200kHz to 2.5MHz, allowing it to maintain the switching frequency outside of the AM radio band while minimizing external component size. If fewer LED strings are used, the design also allows the device to run one of four LED strings, and each string provides additional LED current. Each LED string can use the same number of LEDs, or can operate in an asymmetrical fashion with a different number of LEDs per string.

The LT3599 offers a direct PWM mode with a dimming ratio of up to 3,000:1 and analog dimming with a dimming ratio of up to 20:1 via the control pins. In applications where a dimming ratio of 30,000:1 is required, these two dimming functions can be combined to achieve the desired dimming ratio. Some of the emerging automotive, marine, and avionics applications often require this very high dimming ratio to compensate for the very wide range of ambient light on the LCD display.

In addition, the LT3599 has integrated protection features including open and short circuit protection and an alarm pin. For example, if one or more LED strings are open, the LT3599 adjusts the remaining LED strings. If all LED strings are open, it still regulates the output voltage and, in both cases, signals the OPENLED pin. Similarly, if a short circuit occurs between VOUT and any of the LED pins, the LT3599 immediately disconnects the channel and sets a SHORTLED flag. This channel is prohibited to protect the LT3599 from high power heat dissipation and to ensure reliable operation. Other features that optimize reliability include output disconnection during shutdown, programmable undervoltage lockout, and programmable LED temperature derating. The high voltage capability and high integration of the LT3599 provides an ideal LED driver solution for automotive backlighting applications.