One of the biggest challenges for lighting system designers today is how to take advantage of all the benefits of the latest generation of LEDs . Because they typically require an accurate DC current source and require dimming, the LED driver IC must be designed to meet the needs of a variety of applications. Power solutions must be efficient, reliable, and as always, very compact and economical. There is reason to believe that one of the most demanding LED driver applications is in automotive applications, including LED-based interior lighting, LCD display backlighting, brake/turn signal indicators, and headlights. This article will analyze the automotive headlamp application design techniques for Linear Technology's LT3755 LED driver .
LT3755 technical characteristics
3000:1 True Color PWM dimming
Wide input voltage range: 4.5V to 40V
Output voltage up to 75V Constant current and constant voltage regulation
100mV High Side Current Sense LEDs can be driven in boost, buck, buck-boost, SEPIC or flyback topologies
Adjustable frequency range: 100kHz to 1MHz
Open circuit LED protection
Programmable Undervoltage Lockout with Hysteresis Open LED Status Pin (LT3755)
Frequency synchronization (LT3755-1)
PWM Disconnect Switch Driver CTRL pin provides analog dimming function Low shutdown current: <1μA
Programmable Soft Start Thermally Enhanced 16-Pin QFN (3mm x 3mm) and MSOP Package
Automotive LED lighting design parameters
Table 1: Forward voltage drop and drive current for high current white LEDs.
To ensure optimum performance and long life, LEDs require an efficient drive circuit. These drive circuits must be able to adapt to the most demanding automotive power bus and be both economical and space efficient. To ensure a longer life, the first thing is to not exceed the current and temperature limits of the LED. Table 1 shows the typical forward voltage and drive current for high current white LEDs.
Most headlamp applications require approximately 50W of LED current. Linear Technology's LT3755 is designed to serve this type of application. It boosts the car's bus voltage (nominally 12V) to 60V to drive up to 14 1A LEDs connected in series, as shown in Figure 1.
Figure 1: 50W headlight circuit with LT3755
Figure 2: Efficiency of the LT3755 circuit shown in Figure 1.
Figure 2 illustrates that the LT3755 can be as efficient as 93%. This is very important because it eliminates the heat dissipation requirements of all power components and can result in a very compact solution footprint. Although the circuit shown in Figure 1 is a boost topology, the LT3755's unique high-side current sense design allows it to be configured for boost mode, buck mode, buck-boost mode, or vice versa, depending on the application requirements. Excited topology. The LT3755 drives an external low-side N-channel MOSFET with a stable internal 7V supply. The fixed frequency, current mode architecture allows for stable and accurate operation over a wide range of supply and output voltages. The LT3755 provides a constant current source that is important for LED driver ICs to achieve constant LED brightness when the input voltage is unstable, and is especially important in automotive applications because of transients in the event of cold start and load dump. The input voltage may have a large swing. With a maximum input voltage of 40V, the LT3755 is capable of stabilizing LED current and voltage when the main automotive bus encounters a 40V transient. 40V transients are common in load dump situations.
The ground-referenced voltage feedback pin acts as an input to many LED protection circuits, such as LED open-circuit protection, and also makes it possible to use the converter as a constant voltage source. The frequency adjustment pin allows the user to program the frequency from 100kHz to 1MHz to optimize efficiency and performance while minimizing external component size. If external synchronization is required, the LT3755-1 version can be synchronized to an external clock over the entire frequency range of 100kHz to 1MHz.
The true color PWM dimming of the LT3755 achieves a dimming ratio of up to 3000:1, and the color that is illuminated during dimming does not change, allowing continuous adjustment of the LED headlights to suit a variety of environmental conditions. Because Linear Technology's high current LED drivers are current mode regulators, they do not directly modulate the duty cycle of the power switch. Instead, the feedback loop controls the peak switching current for each cycle. Compared to voltage mode control, current mode control improves loop dynamics and achieves cycle-by-cycle current limit.
Conclusion In current and future vehicles, LED lighting applications will accelerate at an unprecedented rate, which has led to many very specific performance requirements for LED driver ICs in high current LED automotive applications. These applications range from headlights to interior lighting. In addition, there are many LED applications in a large number of commercial and industrial environments outside the automotive market, and most of the high performance requirements in automotive applications are also required in these applications. These LED drivers must provide a constant current to maintain consistent brightness as the input voltage or LED forward voltage changes, and must operate at high efficiency. They must also be able to withstand the harsh electrical characteristics of the automotive power bus. These applications also require very compact, high thermal efficiency solutions. Linear Technology has noted these automotive design needs and has developed a range of high current LED driver products to meet the needs of automotive and a variety of general purpose LED lighting applications. Lighting system designers now have a very convenient source of LED drivers for challenging lighting designs.
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