Two pin crowbar protects from open circuit strings Driver Led Luminaries
2013/07/31
For protecting drivers when an LED goes open circuit, NUD4700 from On Semi is a 2x4mm electronic shunt, claming to clamp to 1V in what looks like less than 100ns, according to the data sheet.
As well as protecting the driver, it will keep the rest of a string of LEDs functioning.
“It provides a current bypass in the case of a single LED going into open circuit. LEDs are by nature quite fragile when subjected to transients and surge conditions. There are also many cases where high reliability of the LED lighting must be maintained such as headlights, lighthouses, bridges, aircraft, runways and so forth. In these cases the low cost addition of the NUD4700 will provide full assurance that an entire string of LEDs will not extinguish should one LED fail,” said the firm.
This device is designed to be used with 1W LEDs (nominally 350mA 3V) and has a breakdown voltage of between 5.5 and 7.5V.
On a 26x26mm copper pad, which can be shared with the LED as only one of them will be dissipating at any moment, the chip is rated up to 1.3A – or 376mA on a minimal copper site.
“Since the NUD4700 has a voltage that is nominally 30% of the LED, then the power dissipation would be easily handled by the same heat sink as the LED,” said On.
Also according to the data sheet, the chip “automatically resets itself if the LED heals itself or is replaced”, but exactly how this works is not obvious from the various hold current and hold voltage figures provided.
Parasitic current leakage when the LED is working properly is 250µA.
The firm also expects the chip to be used as a crowbar to protect sensitive circuits.
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