Application of LED screen in automobile system

Time:2018-11-28 Views:136

There are two main DC/DC power categories to consider when building led powered electronic drive solutions: linear voltage regulators and switching voltage regulators.Linear voltage stabilizer has the advantages of reducing the number of parts and electromagnetic interference, but it has serious disadvantages in efficiency and heat consumption.Therefore, switching voltage stabilizer is the driving solution of choice for many designers.Dc power supply and the number and type of LED required determine the LED driver topology choice.If the power supply voltage exceeds the total LED voltage, a step-down converter is required.If the LED group voltage exceeds the supply voltage, a booster converter is required.Finally, depending on the specific conditions, the LED voltage may be higher or lower than the power supply voltage, so power topology techniques such as step-down/step-up or single-terminal primary inductor converter should be used.

When designing lighting system, there are many factors to consider besides LED power supply.Another major consideration in LED circuit design is thermal management.One way to improve the thermal performance of LED driver IC is to control the positive current of LED, which is a function of temperature.This can be achieved by using an external current to sense temperature and control the current supply to the LED, but a more efficient solution is to use integrated circuits with the necessary built-in functions.

National semiconductor‘s products include a wide range of switching voltage stabilizers integrated circuits that enable multiple topologies.In addition, national semiconductor has developed a series of integrated circuits specifically for LED applications, many of which have functions suitable for automotive electronic system applications.This article will discuss several application examples of headlamp drive in automotive electronic system.

Example of headlights using booster stabilizers

Leds are increasingly being used in car headlights and other forward-looking lighting systems.Typical headlight application might use in a variety of ways to arrange the 10 white leds. In the case of each LED the largest VF is 4 v, if the designer wants to use a headlamp unit in the topology of the series all LED, will need to set a buy DC/DC level to drive the leds. In this case, the nominal 12 v ~ 14 VDC power supply can be bus using a single pressure switch power level.

National semiconductor has developed a variety of integrated circuits that meet this application requirement, such as LM342x series: LM3421, LM3423, LM3424 and LM3429 components.This series of integrated circuits includes a variety of multipurpose components that can be used as controllers for voltage boost, step-down, step-down/step-up, or low side external MOSFET in the SEPIC topology.LM3421, LM3423, and LM3429 components all use peak current mode controllers and predictive shutdown time designs to regulate LED current.The combination of peak current mode controller and predictive shutdown time design simplifies circuit compensation design and provides intrinsic input voltage feedforward compensation.LM3429 is a basic component in the series and a controller solution that optimizes the cost and size characteristics.The LM3421 adds an integrated driver to control the external dimming FET and the system‘s "zero current" off characteristics.LM3423 further adds LED status output markers, fault markers, programmable fault timers, and logical 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.LM3424 also has the ability to program switching frequencies, or synchronize switching frequencies with external sources through programmable slope compensation, soft start, and LED current heat return.

The LM342x series USES controller integrated circuits to achieve desired functionality and maximum flexibility in overall system design.Figure 1 shows an example of an LM3421 booster configuration that USES the LM342x series to drive LED light sets.One of the main features of the LM342x topology is the current induction on the high side of the LED, allowing the cathode of the last LED in the light set to be locally grounded on the chassis, and allowing the induced voltage to be fed back to the integrated circuit in a differential manner.This is an important advantage because it allows LED light sets and driver integrated circuits to be separated from each other.

LED manufacturers usually include graphs showing the maximum allowable forward current and temperature of LED in the data sheet to ensure the reliability of components, which is also known as safe working area.The maximum current rating of the LED is measured at a lower temperature, but the maximum permissible current value decreases beyond a specified temperature.Since proper cooling and ventilation are the primary design elements of LED systems, many applications need to consider unpredictable conditions that even the best thermal design may not be able to prevent.For example, the headlamp assembly is blocked by sludge or other debris.Since it is critical to the safe operation of the vehicle, it is necessary to maintain the LED at a lower operating point for normal lighting in such cases, while keeping the current in a safe working area to prevent catastrophic failure of the lighting system.

In order to achieve the goal of adjusting LED current according to temperature, various methods can be used.One method is to construct a temperature sensing circuit for driving analog current adjustment pins of LED driver IC.A simpler solution is to use LED driver integrated circuits such as LM3424, which have built-in heat return function.Figure 2 shows an example of the external parts required for the LM3424 heat return function.There are several advantages to using LM3424 to drive leds and perform thermal current control.First, you don‘t need to have most of the complex parts on the outside, because they are integrated in the integrated circuit.In the simplest configuration, only a small number of standard resistors and negative temperature coefficient thermistors are required to achieve heat return.If higher precision is required, designers can replace RBIAS and RNTC with accurate temperature sensors such as LM94022. In addition, LM3424 enables users to set the temperature at which LED current begins to heat return and the slope of current return.This allows designers to use a small number of external components to accurately reproduce the current rating drop curve provided in the manufacturer‘s data sheet, while improving the performance as the temperature changes.

As shown in LM3424, the integrated circuit will send back LED current when it reaches a certain temperature. At this time, LED current is zero.This is different from the LED as the main heat generator in the system.For applications such as headlight components, designers may want to set up a security feature that will always provide light output even if the LED may work outside the safe working area.For such cases, the LED current and temperature curves are shown in the example in figure 4.While the LM3424 does not have this built-in capability, this can be easily achieved using an external clamp circuit and prevents the voltage on the TSENSE pin from falling below the predetermined value.

Although the automotive electrical system usually works under the condition of 12V~14VDC, under special circumstances, the power supply voltage to the system components may exceed or be lower than the normal operating range.For example, in the case of cold start, the system power supply may be 4.5v or lower, and in the case of sudden load drop, the voltage may be between 40V and 60V.If LED work or protection is still required under these special circumstances, the designer may wish to select a power level that provides a constant LED current regardless of the relationship between the power supply voltage and the LED group voltage.A switching voltage regulator with SEPIC can perform both boost and buck operation.

SEPIC converters may not be as efficient as step-down or step-up converters, but the topology has several advantages.In addition to the boost and reduce voltage function, another advantage especially suitable for automotive electronic system applications is that the CSEPIC capacitor provides the isolation between input and output.The disadvantage of SEPIC converters is that they require two inductors, but two inductors can easily be wound around the same core rather than as separate components.Figure 6 shows an example of an application circuit that also USES the LM3421 controller.