ON Semiconductor's NUD4700 device in Powermite package provides a compact shunt protection scheme for high-brightness LEDs (HBLEDs) and medium-power LEDs (MPLEDs) for applications where continuous light output is a key requirement. These applications include high-reliability lighting for transportation systems such as aviation, railways, and automobiles. LEDs sometimes have short-circuit faults, but more often open-circuit (non-conductive) faults. In the case of an open LED, if there is no shunt mechanism, the entire LED string will be darkened, thus greatly reducing the light output and lamp life. Street lighting is another application where continuous light output is critical (even if some LEDs fail) because it is a labor-intensive application, so the cost of replacing broken street lights is high. When each LED is equipped with a shunt protector connected in parallel, the LED failure causes the shunt protector to conduct a predetermined level of direct current, thus causing the remaining LED protection in the LED string to emit light. The forward voltage range of HBLED is between 3 V and 4 V, depending on the technology used, the power level it is driven and the light output (lumens / watt, lm / W) obtained from it. Generally speaking, HBLED works with a minimum current of 350 mA. In some cases, a single HBLED emitter can withstand a DC current of 1 A. ON Semiconductor ’s NUD4700 utilizes a unique technology with snapback function to provide low-concentration current, breakdown voltage of approximately 6.5 V, and high DC capability of more than 1 A during normal LED operation. To keep the heat dissipation at a minimum level, its forward voltage is maintained below 1.2 V. The NUD4700 is compatible with Pulse Width Modulation (PWM), which is a key dimming function, and the duty cycle is usually in the range of 10% to 50%. When the LED fault is open, the shunt protector is in its low leakage current off state during the off period of the PWM pulse. When the current pulse rises, a voltage is generated across the shunt protector because the entire LED string is initially in a non-luminous state. Once this voltage exceeds the breakdown voltage (VBR), the shunt protector enters the low voltage conduction mode. When the current pulse disappears, the shunt protector enters a non-conductive state again. The NUD4700 can switch on and off in less than 40 ns, making it easy to withstand a PWM frequency of 200 kHz. ANODE anode CATHODE cathode CONTROL CIRCUIT control circuit SHUNT CURRENT FLOW
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