It was a dark and stormy night. With an expression that matched the storm outside, a caped crusader flexed against his five point harness and tapped a gloved hand impatiently on the wheel of his customized electric vehicle while waiting for the light to change. He was late for a special combat training session with the local police department. If only he hadn’t taken a nap, he wouldn’t have been late. Alfred, the knight’s friend, philosopher, confidante, and personal applications engineer, had decided not to wake him after the non-stop 72 hour spree of crime-fighting he had been dragged into earlier in the week.
The bat-EV was equipped with radar-evasion and collision-avoidance measures – a tribute to the flying mammal he had styled himself after - but on this day of cooperating with Chief Gordon’s officers the dark knight had promised himself he wouldn’t take liberties with the laws of the city. He had vowed to serve and protect after all. A fraction of a second before the light turned green, he gunned the motor and felt the vehicle surge forward with the power of 830 horses. The adrenaline rush never failed to rejuvenate frayed nerves, and aching muscles.
Suddenly, he felt a shuddering impact on his vehicle’s right side. Two-Face had rammed the pointy end of a drill-shaped wheel hub into the front of the bat-EV, denting the Kevlar-covered steel fender and cracking the headlamp cover that Alfred had so lovingly polished just the other day. In the process one LED was wrenched out of its solder joint and rendered inoperable, while another suffered a direct hit. It flickered for a second and proceeded to fail as a short circuit. Fortunately the bat-EV’s lamps were equipped with LED shunts from ON Semiconductor, which bypassed the missing LED within eight microseconds and maintained current-continuity for the other LEDs in the string. The shorted LED stopped emitting light but continued to carry current.
All was seemingly well with the headlamp… for now. However lurking in the shorted LED was a mechanism for catastrophic failure as power continued to dissipate and cause thermal stress on solder joints and eutectic bumps.
Whether one is building a customized crime-fighting vehicle or a less expensive kids-mobile, component failure can lead to enormous costs. In some cases, the cost is nothing more than a simple inconvenience to the end user. In other cases it can mean recalls, legal liabilities, and safety hazards. Although LEDs are inherently reliable, designers may wish to implement additional measures to increase reliability. One way to do this is by using ON Semiconductor LED shunts.
As in the winged-avenger’s car, a failed LED may appear as an open or a short circuit. If the state of the LED is to be electronically monitored, this creates a tricky challenge. The monitoring circuitry must contend with multiple failure signatures.
As a fan of simplicity over complicated solutions, Alfred had naturally decided on the use of LED shunts to solve this problem, essentially converting an open circuit into a short circuit. If an LED has a shunt across it and it fails either as a short or an open, its voltage drop will be low. To monitoring circuitry, a functional LED will have a high logic level and a failed LED will have a low logic level. As the table below illustrates, there are only two possible LED voltage levels when the shunt is used.

With the help of additional logic circuitry, this binary high or low signal was exploited by Alfred for a wide range of purposes. For example, a new design note demonstrates how the lifetime of an LED lamp can effectively be doubled by using an LED shunt to automatically switch to a secondary LED if the primary LED fails in a transition that takes less than 50 microseconds.
Following the impact, the masked manhunter grimly smiled to himself, appreciating the irony of being side-swiped even as he was trying to follow the rules. Sensors from ON Semiconductor in his seat indicated a drop in body temperature as he coolly ran through his options in his mind. Proximity sensors sent data to Alfred through a one-way channel as he shifted in his seat and turned the joystick a hard right in pursuit of Two-Face. An ON Semiconductor pressure sensor in his right tire indicated that a leak caused by the impact had self-sealed back to normal levels… but we digress. A more immediate concern was the fact that as he swung the wheel and diligently switched on his turn signal, the damage from the impact had rendered it inoperable.
Reliability and safety are paramount in vehicles. If the turn signal lights fail, signaling capability is extremely diminished. Due to this safety hazard, the turn signal lights need to be replaced immediately after failure.
To mitigate this problem ON Semiconductor has created intelligent headlight circuitry. In some headlight modules there is a daytime running light LED string next to the turn signal LEDs. Thanks to a novel scheme, this daytime running light string will blink instead of the turn signal, if the latter fails. This is all made possible by using the HBL5006 LED shunt in conjunction with the LEDs and monitoring circuitry.

The HBL5006 is used in this headlight along with monitoring circuitry to blink the daytime running lights when a turn signal LED fails.
ON Semiconductor’s LED shunt portfolio is expanding to meet the needs of nearly any LED lighting application. The NUD4700 features a proprietary technology which allows for extremely efficient operation at high currents. The ultra-compact SOD-923 package for the HBL5006 is only 0.65 mm2 in size and is rated for 350 mA operation. LED shunts are simple to add in for enhanced reliability of any LED module, and thus manage to keep our most famous customer safe on Gotham’s streets.
On this occasion as the bat-EV turned to give chase to the arch-villain, Two-Face’s turbo-charged dragster accelerated off into the distance. The bat-EV could easily have followed and caught up but the caped one decided to fire one last volley and turn back into traffic, en route to his appointment. Promises must be kept, even in the face of a mocking enemy. Back at home, Albert heaved a sigh of relief and silently thanked ON Semiconductor for LED bypass shunts.

Here the NUD4700 is running at 1.3 A with a case temperature measured by a FLIR camera of only 73.3 °C. The device is mounted on a test PCB and the crosshair of the IR camera is on the top of the device.