Innovative two-terminal linear constant current regulator can replace resistor-type drivers

　　As one of the fastest growing applications in the semiconductor field, the LED market has achieved a compound annual growth rate (CAGR) of 20% in recent years. It is expected that the global LED market will reach $11.4 billion in 2012. The DC voltage range of LED applications is usually in three intervals: 0.9V~7V is mainly used in portable devices; 7V~60V is used for billboards, decorations, car taillights, notebooks and LCDTV display backlight applications; 60V~700V is used for lighting, street lights, traffic lights and other applications. Three

　　 major mainstream

　　drivers The three major mainstream drivers currently used in the market are switching regulators, linear regulators and resistor drivers. Among them, switching regulators are used for applications with currents greater than 500 mA, because linear drivers are limited by their own structure and cannot provide such a large current; in low-current applications with currents less than 200 mA, linear regulators and resistor drivers are usually used; in medium-current applications with currents of 200 mA to 500 mA, both linear regulators and switching regulators can be used.　
Asif Jakwani, director of new product marketing and application at the Small Signal Division of ON Semiconductor's Standard Products Division, believes that the three types have their own characteristics: switching regulators are highly efficient and can provide good brightness control; linear regulators have a simpler structure, are easy to design, provide current regulation and overcurrent protection, have external current set points, and have no electromagnetic compatibility (EMC) issues; resistive drivers use simple discrete devices such as resistors to limit the LED string current. It is an economical LED driving solution that is also easy to design and has no EMC issues.

　　Innovative features: simple, economical, and robust

　　Despite this, customers still need a driving solution that is more economical than switching regulators and ordinary linear regulators, but has much higher performance than resistive drivers. Based on this consideration, ON Semiconductor recently launched a double-ended linear constant current regulator series NSI45. This constant current regulator solution does not require external components and can reduce user inventory costs.

　　The NSI45 series uses ON Semiconductor's self-biased transistor technology (SBT), which is currently pending patents and is suitable for LED applications with different input voltages. It stabilizes voltage over a wide range and ensures constant brightness within the operating voltage range. The design has a negative temperature coefficient, which protects the LED from thermal runaway at extreme voltages and operating temperatures.  

　　Simplicity, economy and ruggedness are the most critical features of the NSI45 series. The NSI45 series offers different options such as 20mA, 25 mA and 30 mA devices, using SOD-123 and SOT223 packages. This series of devices provides a steady-state current (Ireg) tolerance of ±10% and ±15%, a rated voltage of 45V, an operating temperature range of -40°C~85°C, and a maximum junction temperature of 150°C. The SOD-123 package is a 3.7 mm x 1.6 mm x 1.2 mm surface mount package that can dissipate 230 mW of power at 85°C on a 500 mm long copper sheet weighing less than 29 g on a circuit board. The SOT-223 is a 7.0 mm x 6.5 mm x 1.6 mm surface mount package that can dissipate 630 mW of power at 85°C on a 500 mm long copper sheet weighing less than 29 g on a circuit board, making it ideal for extreme thermal operating environments. These devices are certified to the stringent AEC101 automotive standard and meet the requirements of the lead-free RoHS directive and use halogen-free mold compounds.

　　The NSI45 series is mainly aimed at low-current LED applications from 20 mA to 200 mA, and supports 120/220 V power supply. It can be used in commercial and industrial applications such as automotive combination taillights, billboard text circuits, backlight display boards, buildings, and general lighting applications. In addition, LEDs are also used in household appliances such as refrigerators and washing machines to backlight small LCD displays or replace lighting bulbs. Such applications also require low-current LED drivers.

　　Jakwani said that in low-current LED applications, although the resistor-type driver is low-cost and simple in structure, the forward current of this driver is low under low voltage conditions, which will lead to insufficient LED brightness, and the LED may be damaged under transient conditions such as load dump. Moreover, this solution requires proper LED coding (binning), large resistor inventory, and low energy efficiency. The linear constant current regulator provides stable current, constant brightness over a wide voltage range, protects the LED from overdriving at high input voltage, and provides higher brightness at low input voltage. Thanks to its constant current characteristics, customers can reduce or eliminate the coding costs of different LEDs from different suppliers, making the total system cost lower (see Figure 1).
 

Figure 1. Comparison of VI curves of a resistive driver and a linear constant current regulator with a regulated current of 25 mA.

　　"The double-ended structure of the NSI45 series is easy to design and suitable for high-end and low-end applications," said Jakwani. "Its economic benefits are mainly reflected in the elimination of LED coding, reducing inventory costs, and protecting LEDs, reducing replacement and maintenance costs." It is reported that ON Semiconductor plans to launch a three-terminal adjustable output linear constant current regulator based on the same patented technology.