Automobile LED combination taillights and their practical applications

The automotive design community has become increasingly enthusiastic about using LEDs in recent years due to their inherent long life and advantages in novel styling. Unlike incandescent bulbs that emit a single point of light, multiple LEDs are combined in automotive applications to provide the light intensity required for the stop and tail light combination function. The tail light function requires a lower light intensity, so the average regulated power must be lower than that of the stop light function. The electrical connection of the LED array is either a parallel-series arrangement or a number of series LED strings (usually 3 per string), each with its parallel bias connected power resistor and diode to form the combination tail light. The number of LED strings in each combination tail light can be as low as 2 strings or as high as more than 8 strings. When it comes to large quantities of LEDs and their inherent light intensity distribution, resistor-based current sources have certain manufacturing and cost limitations, and a large number of resistors of different resistance values ​​need to be on hand to provide a standard light intensity for the combination tail light to meet the light output requirements set by customers and governments.

Silicon-based application-specific standard products (ASSPs) have the opportunity to provide current regulation for combination taillights based on series-connected LED strings. This article will discuss an ASSP example and show how this ASSP can be specifically applied in a combination taillight design. In addition to explaining the functionality of this device, an application circuit will be shown to explain how to provide automatic latching of the LED array in the event that one LED is open. This latching circuit forms the basis for the "one out all out" operation required to mimic the failure of an incandescent lamp.

Background knowledge and application

High reliability and the ability to fit into slim designs make LED lamps a suitable technology choice. The main trade-offs of incandescent lamps over LED lamps are initial cost and replacement cost.

To this end, LED-based combination taillights require current regulation to provide a stable or limited LED array voltage at the nominal automotive continuous voltage of 9.0 V to 16 V. In addition, the open-lamp condition presents a very different problem to the body control module (BCM) than incandescent and LED technologies.

Figure 1 shows the BCM and taillight combination interface of an automobile. The BCM contains multiple high-side driver (HSD) channels that provide switching power to multiple different grounded loads in the vehicle. Each high-side driver usually has current limiting and at least a subset of standard failure modes as diagnostic signals, such as short circuit to ground, short circuit to battery, load device on/off open circuit, etc. If these high-side drivers are suitable for powering incandescent bulbs, the current limiting must accommodate high inrush input currents and high steady-state currents, and make the high-side driver current limit threshold very high. However, when an incandescent bulb burns out, the high-side driver can easily determine the open circuit fault because the incandescent bulb fails open circuit and the high-side driver of the BCM easily detects zero current. The corresponding open circuit condition of LED-based taillight combination lamps is different, and the high-side driver of the BCM and the current regulator of the taillight combination lamp need special consideration.

Figure 1. Automobile body module and combination taillight interface.

LED string theory

There are two common topologies for LEDs in LED-based combination taillights: cross coupled or series strings. Figures 2a and 2b show these two topologies in detail. The current supplied to the LED array is limited by power resistors. Most LEDs cannot be connected in parallel because the LED forward voltage drops may not match, resulting in unbalanced current sharing. Current sharing is more complicated in taillight mode because the current requirements are lower. Since LEDs are characterized by rated operating current (and "parking" current), which is usually 10 times greater than the current required for the taillight function, the LED topology shown in Figure 2b is preferred. The single power resistor in Figure 2a is divided into several different power resistors in Figure 2b, one for each LED string. In taillight mode, the total current is evenly distributed to all current strings, with blocking diodes in the "parking" feed line.

Figure 2a (top). Cross-connect topology; Figure 2b (bottom). Series connection and N-1 fault condition

To address electrical design issues, manufacturers of combination tail lamps must comply with government requirements (such as the Federal Motor Vehicle Safety Standard FMVSS 108 in the United States). This US specification requires that the required light output be maintained even if one LED is open-circuited. This is the N-1 rule, which may force the combination tail lamp to be equipped with additional LEDs. In Figure 2, removing at least one LED from the LED array does not reduce the supply current; instead, the total current is transferred to the remaining LEDs, resulting in increased current for the remaining LEDs and shortened service life. In order to determine which LED will open, each LED requires a lot of circuitry and wiring harnesses, which may make the cost uneconomical.

Figure 3. LED strings are arranged in series (each string can be biased to use 3 LEDs). Open-circuit measurements can be easily made on each string.

In Figure 2b, the N-1 rule is applied, and removing three LEDs from the LED array will slightly reduce the total current, but the BCM will not detect this change. However, the number of LED open circuit detection circuits is reduced to the number of current sources or LED strings required. Although there are three LEDs that fail open in Figure 3, compared to only one in Figure 2a, the series string topology is the appropriate choice if the diagnostic signal is guaranteed. This directly applies to LED-based turn indicators, which use diagnostic indications to latch the entire LED array, allowing the BCM to make a true load open circuit detection and alert the driver. Some regional markets require this latching feature for parking light functions.

There is an opportunity in the market to provide an ASSP linear integrated circuit that matches the current draw to power multiple strings of LEDs. Such a device should provide a diagnostic signal that flags when any LED in the string is subject to the N-1 rule. This warning flag can be used to latch the entire tail light combination, forcing the body control module to detect an open circuit condition. In addition, features such as over-temperature shutdown and power reversal can be added to such a device to make it more robust to the various short circuit and transient electrical conditions that the automotive load needs to withstand.

ON Semiconductor developed the NCV7680 to meet the requirements of combination tail lamp engineers for routine application of various LED arrays in any topology. Due to cost and electromagnetic interference (EMI) issues, this device is specifically designed as a constant current linear regulator rather than a switching power supply.

Figure 3a shows the block diagram of the NCV7680. The regulated current for each output is set by a single resistor at the Rstop pin. This resistor provides a specific small current for replication at each output. The output current can be set to a minimum of less than 10 mA per string and a maximum of 100 mA, resulting in a total regulated current of 800 mA. The outputs can be shorted together for higher LED string currents. The matching accuracy of each output is less than 5% over the full automotive ambient temperature range (-40°C to +125°C).

When the Stop input is triggered, all outputs turn on and regulate current at the programmed Rstop value. The outputs remain on until the Stop input cycle ends. If the Stop pin is cycled low and the BCM is still feeding the LED array with diodes Or (see Figure 3b), the NCV7680 reverts to internal pulse width modulation (PWM) mode and the output/string is dimmed at the average current strength value. This is the Taillight mode and the duty cycle can be selected by the Rtail value. This resistor to ground sets the Taillight PWM duty cycle between 0 and 80%. The internal oscillator frequency is internally set to 1 kHz and, most importantly, the current slew rate is limited to a low level of 6 mA/μs. This ensures that there will be no radiation issues due to the PWM current waveform. The diagnostic signals are derived from the internal diodes Or of the eight current outputs. Each time the Stop input goes high, the diagnostic signal is pulled low. If any LED string is open circuit, the Park function will be active, resulting in no diagnostic activation. Using a simple pull-up resistor, the circuit can be completed to provide simple diagnostic functions.

Figure 3a. Block diagram of the NCV7680 that provides all the functions needed to develop an LED combination taillight.

The NCV7680 is a linear regulator, and thermal management quickly becomes a big issue in this case. While the device has an exposed thermal pad integrated into the SOIC 16 package, additional help is needed in some cases to manage automotive voltage drops. The device is used to drive the external gate of a high-side P-channel MOSFET or the base of a PNP bipolar transistor. The device has a feedback pin that is controlled by an internal 1 V reference. This feature is not required, however, LEDs always have thermal issues; in the case of an external transistor, a direct power path can be designed on a pc board copper layer or other thermal configurations such as a TO-220.

Figure 3b is the application circuit diagram of NCV7680. This combination tail light is modulated with a "stop" current level, and in the "tail light" mode, the current is precisely regulated to 15% of the Rstop current value. This design integrates the ballast transistor Q1 and resistors R11 and R12, which act as a linear regulator to limit the voltage drop of NCV7680. If any LED applies the N-1 rule, it will cause Q2, Q3 and the diagnostic output to operate, causing the entire LED array to latch. Once the LED array latches, the "stop" line needs to be pulled low and then pulled high. This ensures a chatter free characteristic, and the reduced current is reduced to less than 6 mA and can be easily sensed by the body control module.

LED combination taillights require special attention if they are targeted at the global market. There are regulations that force certain combination taillight applications to provide diagnostic functions or latch functions according to the N-1 rule. ON Semiconductor's NCV7680 can be used to apply and control any combination taillight configuration, including parking, turn, tail and clearance lights. This integrated circuit solution supports built-in protection against the adverse effects of overvoltage, overload and over-temperature shutdown.