LEDs are well known for use in a variety of automotive lighting components such as headlights, daytime running lights, fog lights, turn signals, interior lighting, infotainment backlighting, and combined taillights (RCL) and high level brake lights (CHMSL).
There are two main DC/DC power supply categories to consider when building an LED-powered electronic drive solution, a linear regulator and a switching regulator. Linear regulators have the advantage of reducing component count and reducing electromagnetic interference (EMI), but have serious drawbacks in terms of efficiency and heat dissipation. Therefore, switching regulators are the first choice for many designers' drive solutions. The DC power supply and the number and type of LEDs required determine the topology choice of the LED driver . If the supply voltage exceeds the total LED voltage, a buck converter is required. If the voltage of the LED group exceeds the supply voltage, a boost converter is required. Finally, depending on the specific conditions, the LED voltage may be higher or lower than the supply voltage, so power topology techniques such as buck/boost or single-ended primary inductor converter (SEPIC) should be used.
In the design of lighting systems, in addition to LED power supply, there are many factors to consider. Another major consideration in LED circuit design is thermal management. One way LED driver integrated circuits improve thermal performance is by controlling the LED forward current as a function of temperature. This can be achieved by using an external current to sense the temperature and control the current supply to the LED, but a more efficient solution is to utilize an integrated circuit with the necessary built-in functionality.
National's products include a wide range of switching regulator ICs that enable multiple topologies. In addition, National Semiconductor has developed a range of integrated circuits for LED applications, many of which have features suitable for automotive electronics applications. This article will explore several application examples of automotive electronics system headlight drivers.
Example of a headlight using a boost regulator
LEDs are increasingly being used in automotive headlights and other front-view lighting systems. Typical headlight applications may use 10 white LEDs arranged in a variety of ways. For the case where each LED has a maximum VF of 4V, if the designer wishes to use a topology in which all LEDs are connected in series in one lamp group, a DC/DC stage will need to be set to drive the LEDs. In this case, a single boost switching power stage can be used for the nominal 12V~14VDC power bus.
A variety of integrated circuits developed by National Semiconductor meet this application requirements, such as the LM342x family: the LM3421, LM3423, LM3424, and LM3429 components. This family of integrated circuits includes a variety of versatile components that can be used as controllers for low side external MOSFETs in boost, buck, buck/boost or SEPIC topologies. The LM3421, LM3423, and LM3429 components both use a peak current mode controller and a predictive off-time design to regulate the LED current. The combination of a peak current mode controller and a predictive off-time design simplifies the loop compensation design while providing inherent input voltage feedforward compensation. The LM3429 is a basic component in the series and is a controller solution that optimizes cost and size characteristics. The LM3421 adds an integrated driver for controlling the external dimming FET and the system's “zero current†shutdown feature. The LM3423 further adds LED status output flags, fault flags, programmable fault timers, and logic pins to control the polarity of the dimming driver. Finally, the LM3424 is similar to the LM3421 but uses a standard peak current mode controller. The LM3424 also has the ability to program the switching frequency or synchronize the switching frequency to an external source through programmable slope compensation, soft start and LED current hot return.
The LM342x family uses controller ICs to achieve the maximum flexibility required for the required functionality and overall system design. Figure 1 shows an example of using the LM342x series to drive LEDs in a boost configuration, using the LM3421 as an example. One of the main features of the LM342x topology is current sensing on the high side of the LED, allowing the cathode of the last LED in the group to be locally grounded on the chassis and allowing the induced voltage to be differentially fed back into the integrated circuit. This is an important advantage because the LED lamp sets and driver integrated circuits can be separated from one another.
Figure 1 Using the LM3421's boost regulator to drive 10 LEDs
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