Author: Piero Bianco
SPM Business Unit Business Manager
Maxim Integrated Products
Currently, automakers are gradually replacing automotive lighting systems from incandescent and cold cathode fluorescent lamps to high-brightness LEDs . These HB LEDs are widely used for backlighting of navigation and entertainment equipment displays and for interior and exterior lighting of automobiles, such as daytime running lights, taillights, etc. Some new applications, such as head-up displays, are also starting to use HB LEDs.
However, when integrating HB LEDs into various systems, designers face many challenges in order to increase productivity, reduce cost, achieve wide-range dimming, and other advantages. Due to the limitations of first-generation drivers, designers are not optimized for efficiency, minimum external component count, minimum EMI, and wide-range PWM dimming. The latest multi-string LED drivers (such as the MAX16814) are the most ingenious way to solve these technical bottlenecks, and the two-way communication between the switches and linear controllers of these drives.
Why choose HB LED?
HB LEDs are becoming more and more popular in the automotive field, and they bring many advantages to automotive designers. Compared to other lighting technologies, HB LEDs are the most environmentally friendly solution with excellent energy efficiency, no mercury, and very few hazardous substances when recycled. In addition, HB LEDs help to improve the safety of cars, thanks to their fast opening and closing speeds (far higher than incandescent lamps). It is also in consideration of this factor that brake lamps are widely used for high-brightness LEDs.
In addition, it provides designers with more room to play in the car's personalized style design, because the LED size is small, almost no space on the back of the panel, designers can arrange LED lights in any shape; Small spotlights are ideal for use as indicator lights; in the end, LEDs have a longer life than other lighting solutions and have a lifetime of 50,000 hours or more, making them ideal for long-term lighting conditions, such as daytime running lights.
LED applications in automobiles have expanded from brake lights and taillights to front lights (daytime running lights, position indicators for mid- to high-end cars, and low-end and low-beam lights for high-end cars), and internal lighting (RGB LEDs can be controlled) Light color) adds a unique touch to the car. In addition, LEDs are becoming mainstream in navigation, entertainment and dashboard backlighting applications (Figure 1).
LED technology also plays an important role in some new applications, such as automotive head-up displays. LEDs have a wide dimming range and are PWM dimmable, making them ideal for applications that require a very wide range of light levels to be adjusted based on ambient light intensity.
HB LED design challenges
Of course, there are many challenges when integrating LEDs into automotive applications, for example, the lowest possible cost. As far as the components themselves are concerned, the price of LED lamps is usually higher than other lighting solutions (incandescent, halogen, CCFL). Therefore, the system-level cost of the LED solution must be reduced to increase the market potential of the technology. One way to reduce the cost of the solution is to minimize the number of components in the drive, which also helps to improve system reliability, because each component on the PCB can be a failure point of the system.
Another challenge is efficiency. Energy efficiency is becoming more and more important in automobiles, especially for hybrid vehicles, where efficiency is maximized to minimize power consumption (heat generation). Auto parts generally work in high temperature environments, the ambient temperature around the engine may reach 105 ° C, and the temperature in other parts will reach 85 ° C. LEDs generate a lot of heat. They don't radiate energy in the IR or UV bands. Their power consumption increases the temperature in the surrounding environment. This requires reducing the power consumption of the driver and avoiding overheating of the driver IC or other components in the driver module.
Of course, the EMI problem is also faced in the automotive environment. No lighting subsystem can interfere with other subsystems in the car, and AM radio is usually one of the most sensitive components.
In automotive applications, LEDs are arranged in multiple strings (each string is defined as a series LED group with the same current). LEDs can be conveniently arranged according to the size of the display. Aligning the LEDs into multiple strings helps to increase the fault tolerance (if one LED is open, only the LEDs in series with the LED are turned off, not all LEDs). Another reason to use multiple strings of LEDs is to limit the voltage of each string of LEDs and improve system security. For example, after a string of LEDs with a total voltage of 80V is divided into two strings, the voltage reaches 40V, which avoids the high voltage that poses a threat to the human body when it comes into contact with the LED or its connection.
Thus, a single IC capable of driving multiple strings of LEDs has significant advantages. Multi-string driver architectures typically include an LED string, a boost converter (which converts the input battery voltage to the high voltage required for each string of LEDs), and a multi-route current sink regulator (used to establish the drive current for each string), as shown in Figure 2. Shown.
Compared to a solution with a multiplexer, this solution has fewer components and lower cost (only one inductor and a few bypass capacitors are needed). The advantage of this approach is that the current can be equalized between each string compared to a single string driver that directly drives the parallel LEDs. If multiple strings of LEDs are directly connected in parallel, because some of the LEDs have a high forward voltage, the current cannot be equally divided between each string. In addition, the forward voltage of the LED will decrease as the temperature rises, and the imbalance of the current will cause the heat to be out of control, because the LED string with a large current generates more heat, and the forward conduction voltage is reduced, thereby absorbing A larger current causes the temperature of the string of LEDs to increase further. As the current difference increases, one or more strings of high current LEDs may fail. Finally, if the LED strings are simply connected in parallel, since the driver can only control the total current, the failed LED current will increase to other LED strings, causing other LED strings to fail due to overdriving. This situation can be avoided by using the scheme of Figure 2.
This architecture uses MOSFETs to regulate the current of the LED strings. To keep the temperature of these MOSFETs as low as possible, ensure that the voltage across the tube is as low as possible, but with sufficient voltage to keep the tube in saturation. Under ideal conditions, the output voltage of the boost converter is:
Vboost=max(Vstring,i)+ Vsat
Where Vstring,i is the forward conduction total voltage of the ith string LED, and Vsat is the VDS when the MOSFET is saturated. A driver capable of setting this voltage to an ideal value is called an adaptive voltage optimization (AVO) function.
Most applications require LEDs to adjust the brightness of the LEDs, such as on/off control at a certain duty cycle, turning the linear current sink regulator on and off, making the AVO design more complex. When all LED strings are off, the operating state of the boost converter has several options and some limitations, which are discussed in detail later.
Traditional multi-string driver
The traditional LED driver scheme uses the topology shown in Figure 2, including a boost switching converter and multiple independently operating current regulators that have some problems when implementing AVO functions with external components.
The external circuit must detect which string of LEDs has the highest forward voltage (or lowest cathode voltage); this can be achieved with several diodes marked in red in Figure 2. This approach takes up more board space and increases system cost.
This solution has another potential problem in the event of LED failure. If an LED is opened, the string of cathode voltage will drop to zero. The diode detection circuit will judge that the string has the highest forward voltage and start to rise. The boosted output voltage attempts to provide enough drive voltage for this string of LEDs. Eventually the voltage on the current sink MOSFET of the other LED string rises, possibly causing the tube to fail or triggering the output overvoltage protection of the boost converter (if this feature is enabled), turning off the converter and all LEDs.
The third problem with this architecture is the PWM dimming of the LED. When the LED is off, the diode circuit has no voltage reference point to set the boost output voltage. One possible solution is to add another diode that is connected to the boost output through a voltage divider circuit, such as the red marker circuit in Figure 2. When the LED is off, the diode turns on and sets the boost output to a preset voltage. A significant problem with this approach is that the boost converter output has a higher ripple when PWM dimming is performed, as shown in Figure 3a. This produces EMI noise, a key issue in automotive design, and it produces audible noise on the output capacitor Cout.
New generation multi-string drive
The new generation of multi-string LED driver's step-up switching converter and linear current sink regulator can perform two-way communication (rather than working independently), solving (or partially solving) the above three problems and greatly improving system performance. These new drivers detect the voltage of the LED string inside the IC (such as the drain voltage of each current sinking MOSFET) and select the lowest voltage using an internal diode or analog switching circuit (Figure 4). This solution greatly reduces the number of external components and the cost of the solution.
In addition, the two-way communication function also solves the above-mentioned problem caused by one LED failure or open circuit. Once this happens, the boost converter output voltage begins to rise. When the overvoltage protection threshold is reached, the faulty LED string can be identified, the corresponding AVO control loop of the string is disabled or removed, and the other LED strings remain in normal operation. In addition to reducing the brightness of the illumination (rather than turning off the LEDs all at all), the failed LED does not have other effects on the user.
When the new generation of drivers adjusts the brightness of the LEDs, the internal switches and linear regulation loops use a different approach than Figure 2, with lower noise. The boost converter may be disabled when the LED is turned off, as shown in Figure 3b. In other words, the converter stops switching during this time, the power switch MOSFET remains off, and the compensation circuit is also open. The compensation capacitor maintains its charge amount (compensating the state when the loop is operating). The boost output voltage is maintained by the output capacitor Cout. Since the LED off capacitor does not discharge, the discharge current is only leakage current. When the LED is turned back on, the converter restarts the switching operation with minimal ripple. In this scheme, the boost converter output voltage remains nearly constant during PWM dimming, greatly reducing EMI noise and audible noise on the output capacitor.
The only limitation of this approach is that the PWM dimming on-time requires more than a few (three or four) switching cycles so that the boost converter recharges the output capacitor, compensating for leakage current losses during turn-off. This limits the minimum duty cycle that can be achieved.
Application solutions for next-generation drives
The car's daytime running lights and heads-up displays have the same performance requirements, that is, they remain open during driving, requiring a high reliability/redundancy design to ensure proper operation under all conditions. The use of next-generation LED drivers such as the MAX16814 ensures that the running lights and head-up displays are highly reliable, while also reducing the number of external components, helping to reduce system cost and increase reliability. These applications also require a wide input voltage range to withstand peak voltages (load throwing) of automotive batteries up to 40V and very low EMI.
Fault tolerance is important for long-life applications where the LED light is not allowed to be turned off even in the event of a fault. The MAX16814 utilizes a multi-string driver architecture to turn off only faulty LED strings in the event of an LED open or short circuit. The other LEDs remain active. In addition, the MAX16814's fault indication output provides an LED failure alarm (Figure 5).
Head-up displays also require a wide (1000:1 or greater) PWM dimming range, and the MAX16814 integrates a unique PWM dimming circuit that effectively rejects the ripple of the boost output voltage (frequency is the dimming frequency), Reduced EMI and audible noise. This scheme is similar to the scheme used in Figure 3b, but is capable of providing a 5000:1 dimming range at 200 Hz (higher than other products), overcoming the above-mentioned minimum on-time limit.
The chip can drive four strings of LEDs, and two-way communication between the switch and the linear regulator greatly reduces the number of external components. In addition, the MAX16814 has complete fault protection and detection functions. Any string of open or shorted LEDs will turn off this string of operation and send a fault alarm output to the system. Meet all automotive product design requirements, such as 40V load dump, operating in the -40 ° C to 125 ° C temperature range.
When designing an HB LED system, a number of factors need to be compromised, including component count, efficiency, and reliability. Table 1 compares and summarizes the various LED drive schemes to help designers choose the best solution for their specific application.
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