Automotive LED

   Silicon-based Application Specific Standard Products (ASSP) have the opportunity to string based on the series combination of LED taillights provide steady flow function. This article will discuss an instance of an ASSP, and show how the combination taillight design of the specific application of this ASSP. In addition to explain the function of this device will also display relevant application circuit, introducing an LED open circuit in the case of how to provide LED array automatically latch. This imitation of the latch circuits required by incandescent fault "is all the fault of a failure" (one out all out) basis for the work.

Background knowledge and application of

    Slim design with high reliability and the advantages for making LED lights into the appropriate technology choices. Incandescent lights with respect to the main trade-off advantage of LED is that the initial cost and replacement cost.

    Therefore, based on a combination of LED taillights steady flow requirements, the 9.0 V to 16 V continuous voltage rating of vehicles to provide a stable under the conditions or restrictions on LED array voltage. In addition, compared to incandescent and LED technology, the light conditions to open the body control module (BCM) problems caused by very different.

    Figure 1 shows the car's body control module and the combination of rear interface. Body control module contains multiple high-end drives (High Side Driver) channel, for a number of different ground vehicles load switch power. Each high-end drives are usually limited flow, and the standard failure mode for at least a subset of a diagnostic signal, such as short-circuited, the battery short circuit, load the device turn on / off open and so on. If these high-end drive is perfect for the incandescent lightbulb, then it must adapt to the high limit input current surge and high steady-state current, and to drive high-current-limit threshold is very high. However, incandescent bulbs burning, the high-side driver can easily determine the open-circuit fault, open circuit failure because incandescent bulbs, body control module is easy to detect high-end drives to zero current. LED taillights on the appropriate combination of different open-circuit condition, body control module combination of high-end drive and rear of the steady flow devices need to be special consideration.

Figure 1. Motor vehicle body modules and combination taillights interface.
LED string theory

     Based on a combination of LED taillights in the arrangement of the various LED There are two common topologies: cross-connect (cross coupled) or strings connected in series (series strings). Figure 2a and Figure 2b depicts in detail the two topologies, provided to the LED array to limit the current through the power resistor. Most LED does not allow parallel connection, because the LED forward voltage drop may not match, resulting in both unbalanced current flow (current sharing). Taillight mode current of all current more complex, because the current requirements are lower. Since the rated LED current (and "parking" current) to measure the characteristics and size of the current functional requirements usually higher than the current tail 10 times, as shown in Figure 2b more popular LED topology. Figure 2a in the single power resistor in Figure 2b is divided into several different power resistors, respectively, devoted to each LED string. In the tail model, a balanced distribution of the total current to all the current series, and "parking" feeder lines with blocking diodes.

            
Figure 2a (above). Cross-connect topology; Figure 2b (bottom). Series N-1 serial connection and fault conditions

     Combination taillights manufacturer of electrical design to solve the problem, must meet the government requirements (such as the U.S. Federal Motor Vehicle Safety Standard FMVSS 108). This specification requires that the United States in the case of an LED open circuit still maintain the required light output. This is the N-1 rule, may be forced to mix with the extra taillight LED. In Figure 2, removed from the LED array, at least one LED does not reduce the supply current; the contrary, the total current supply switch to the rest of the LED, led the rest of the LED current increases, the service life shortened. In order to determine which LED will be any open, each LED requires a large number of circuits and wiring harness, the result may make the cost worthwhile.

Figure 3. LED strings arranged together (each string of bias can use 3 LED). Each LED string can be easily carried out on the open circuit measurements.

 Figure 2b, application of the N-1 rule, remove from the LED array, 3 LED, the total current decreased slightly, but the body control module can not detect this change. However, LED open detection circuit to reduce the number of required LED current source serial number or quantity. Although the open-circuit fault in Figure 3 has three LED, compared with Figure 2a, only one, if the diagnostic signal to ensure that the words, the series string topology is the right choice. This is directly applicable to the turn-based LED indicator, which lights to use diagnostic instructions to latch the LED array, the body control module to make a real open-load detection and alert driver. Mandatory in some regional markets with a stop lamp function that latch feature.

     The existence of the market to provide matching drain current, multi-string LED-powered-specific standard products (ASSP) business opportunities linear integrated circuits. This device should provide diagnostic signal, which signal any LED in the LED string N-1 rule applies when the use of mark (flag). The warning tag can be used to latch the combination taillights, forcing the body control module detects an open condition. In addition, this device can also be added, such as over-temperature shut down and power features such as anti-take to make it in the car needs to withstand a variety of load and transient electrical short circuit is more robust under the conditions.

     ON Semiconductor developed to meet the combination of taillights NCV7680 engineers routinely applied in any topology, the requirements for a variety of LED arrays. Because of cost and electromagnetic interference (EMI) problems, this device is designed not for the constant current switching power supply linear regulator.

    Figure 3a shows the block diagram of NCV7680. Each output steady flow of current through the single resistor Rstop pin configuration. This resistor provides specific small current for each output copy. Output current can be set for each string of less than the minimum 10 mA, maximum 100 mA, so that the total steady flow of current 800 mA. LED string current higher output can be shorted together. Each output to match the accuracy of the full automotive temperature range (-40 ℃ to +125 ℃) to less than 5% lower.

    Triggered the "stop" input, all outputs turn on, and to set the value of the steady flow Rstop. Output has remained on-until the "stop" input cycle. If the "stop" to the low pin loop, and the body control module is still in the diode Or (see Figure 3b) to the LED array feed, NCV7680 to return to the internal pulse width modulation (PWM) mode, the output / string to The average value of current intensity dimming. This is the "tail" mode, the duty cycle can Rtail value to select. The resistance of the grounding tail PWM duty cycle can be set from 0 to 80%. Internal oscillator frequency is internally set to 1 kHz, most importantly, the current conversion rate (slew rate) is limited to low levels of 6 mA / μs. For the sake of PWM current waveform, which ensure that no radiation. Diagnostic signal is derived from the eight current output of the internal diodes Or. Each "stop" input rose high, diagnostic signal is pulled low. How any LED string open, "stop" function is activated, causing no diagnostic activation. The simple pull-up resistor, you can complete the circuit to provide a simple diagnostic functions.

   Figure 3a. To provide the necessary development of LED combination taillights block diagram of all the features of the NCV7680.

    NCV7680 is a linear regulator, in which case thermal management is quickly becoming a big problem. Although this device has integrated into the SOIC 16 package exposed thermal pad, but in some cases, need extra help to manage the car voltage drop. This device is used to drive high-end P-channel MOSFET's external gate or the base of PNP bipolar transistors. This device with the feedback pin, the internal 1 V reference to control. This feature is not to be used, but, LED heat problem always exists; in the case of an external transistor can be in the printed circuit board copper layers, or TO-220 and other cooling configurations directly on the design of the power channel.

   Figure 3b is NCV7680 application circuit. This combination of taillights with "parking" current level to modulate, and in the "tail" mode, the current is precisely adjusted to Rstop 15% of current value. This design with integrated ballast transistor Q1 and resistors R11 and R12, as a linear regulator to limit the pressure drop NCV7680. If any use of the N-1 LED rules will make Q2, Q3 and diagnostic output action, leading to the LED array latch. Once the LED array, latch, "parking" need to pull to the low line and then pulled high. This is to ensure that no chatter (no chatter free) features to reduce the current will be reduced to less than 6 mA, and can easily be sensed body control module.

Conclusion

    LED combination taillights If you need to target the global market, you need special attention. Some combination of taillights with standardized application of force under the N-1 rule to provide diagnostic functions or latch function. Semiconductor NCV7680 can be used to control any combination of applications and taillight configurations, including parking cars, turning cars, taillights and outline marker lamps. The IC program supports built-in protection against over voltage, overload and over-temperature shutdown adverse effects.

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