Simple and robust linear constant current regulator for low current LED applications

With advantages such as energy saving, long service life and rich color combinations, LED has become one of the fastest growing semiconductor fields. In recent years, the compound annual growth rate (CAGR) has reached 20%. It is expected that the global LED market will reach 11.4 billion US dollars in 2012, and the prospects are very promising.

There are two types of typical LED drivers on the market, namely linear drivers and switching drivers; further subdivided, there are three types, namely switching regulators, linear regulators and resistor drivers. These three types of drivers are suitable for different levels of current applications, as shown in Figure 1. For example, switching regulators are used for high current 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; and in medium current applications between 200 and 500 mA, both linear regulators and switching regulators can be used.

Figure 1: LED drivers for different current ranges.

Switching regulators are energy efficient and provide excellent brightness control. Linear regulators are relatively simple in structure, 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. They are an economical LED driving solution that is also easy to design and has no EMC issues.

As a leading global supplier of high-performance, energy-efficient silicon solutions, ON Semiconductor provides a wide range of switching regulators and linear regulators for different LED applications. At the same time, customers in the market also need a driving solution that is more economical than switching regulators and ordinary linear regulators, but has much higher performance than resistive drivers. In this case, ON Semiconductor uses the patent-pending self-biased transistor (SBT) technology, combined with its own strong process control capabilities, to launch a new LED driving solution - discrete linear constant current regulator (CCR), which fills this market gap and further enriches ON Semiconductor's broad LED driver product lineup.

Comparison of low-current LED applications and drivers

The new linear constant current regulator is suitable for low-current LED applications from 20 mA to 200 mA. For example, in automotive applications, the current requirement of the combination taillight is usually only tens of milliamps (mA). In addition, in commercial and industrial applications such as billboard text circuits and backlight display boards, the input voltage is usually as high as 30 Vdc, and the LED current is between 20 mA and 150 mA. In architectural and general lighting applications such as decorative lighting, work lights, landscape lighting, counter lighting, etc., the input voltage is usually 12 to 48 Vac or 120/220 Vac, and the LED current is only between 20 and 100 mA. 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.

In low-current LED applications, although the resistor-type driver is low in cost and simple in structure, the low forward current of this driver under low voltage conditions will result in 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.

Your browser may not support the display of this image. Compared with resistor-type drivers, linear constant current regulators provide stable current, constant brightness over a wide voltage range, protect LEDs from overdriving at high input voltages, and provide higher brightness at low input voltages. 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.

Figure 2: CCRs fill a gap in the market by combining the strengths of resistive drivers and linear regulators.

We can verify the performance comparison of the two drivers through actual tests, as shown in Figure 3. The test compares the combined taillights implemented using the two drivers, where the CCR is a device with a stable current of 25 mA. As can be seen from the figure, at the same input voltage of 9 V, the CCR has a higher current than the resistor-type driver, making the LED brighter. At the same voltage input of 13 V, the brightness of the two is the same. After the voltage is higher than 13 V, the current of the CCR gradually stabilizes to 25 mA, while the current of the resistor-type driver keeps rising, which may cause the LED to be overdriven and damaged. In addition, the current accuracy provided by the CCR is ±10%, which fully meets the application requirements, while the current accuracy of the resistor type is only ±50%, which is far from the same.

Figure 3: Comparison of the VI curves of a resistive driver and a linear constant current regulator with a regulated current of 25 mA.

Simple, economical and robust linear constant current regulator based on patented SBT technology

ON Semiconductor's linear constant current regulator is based on ON Semiconductor's patent-pending self-biased transistor (SBT) technology, providing a certain degree of integration, and combined with ON Semiconductor's superb process control capabilities, it provides a simple, economical and robust LED driver solution for low-current applications.

Specifically, the dual-end of this device is very easy to design, suitable for high-side and low-side applications, and does not require external components, which is very simple. Its economy is mainly reflected in the elimination of LED coding, reducing inventory costs, and protecting LEDs, reducing replacement and maintenance costs. ON Semiconductor also plans to launch three-terminal adjustable output linear constant current regulators, and their structural schematics are shown in Table 1.

It is worth mentioning that ON Semiconductor's linear constant current regulator supports 120/220 Vac power supply. For example, in buildings and general lighting applications, after the 120/220 Vac AC mains input is bridge rectified, it is only necessary to ensure that the remaining voltage after the input voltage minus the total voltage of the LED string does not exceed the anode-cathode maximum voltage VAK of 45 V of this linear constant current regulator. This also reflects its easy design characteristics.

In terms of robustness, this series of devices from ON Semiconductor performs very well. For example, its design has a negative temperature coefficient (NTC) to protect LEDs from thermal runaway at extreme voltages and operating temperatures. For example, the device size of the SOT-223 surface mount package is 7.0 mm x 6.5 mm x 1.6 mm, and it can dissipate 630 mW of power on a copper sheet 500 mm long/less than 29 grams on a circuit board at 85°C, which is very suitable for extreme thermal working environments. In addition, these devices have passed the strict AEC101 automotive standard certification and are suitable for demanding automotive application requirements. These devices also withstand switching transients and voltage spikes, and have a good electrostatic discharge (ESD) pulse rating, measured at 1.8 kV. In addition, these devices support environmental protection, use halogen-free mold compounds, and comply with the lead-free RoHS directive.

ON Semiconductor's first batch of 10 NSI45 series double-ended linear constant current regulators were launched in May 2009. These devices provide ±10% and ±15% steady-state current (Ireg) tolerances, rated voltage of 45 V, operating temperature range of -40℃ to 85℃, maximum junction temperature of 150℃, and different options such as 20 mA, 25 mA and 30 mA devices are available. They are packaged in SOD-123 and SOT223. The target markets include interior and exterior LED lighting in automotive applications, neon replacement in commercial sign applications, traffic signals, and display backlights, flat panel AC lighting, work lights and decorative lights, landscape and outdoor lighting in architectural and general lighting applications.

Table 1: Two-terminal and three-terminal linear constant current regulators that ON Semiconductor has launched and plans to launch.

Table 1 lists the main parameters of ON Semiconductor's NSI450 series linear constant current regulators, including two-terminal devices such as NSI450XXT1G (SOT123 package, 20/25/30 mA) and NSI450XXZT1G (SOT223 package, 25/30 mA), which are currently available for sampling and in mass production; and three-terminal devices such as NSI450XXZT1G (SOT223 package, 30 to 80 mA adjustable) and NSI450XXT1G (D-PAK package, 80 to 150 mA adjustable), which will be available for sampling in the third quarter of 2009 and in mass production in the fourth quarter.

Summary:

ON Semiconductor is a leading global supplier of high-performance, energy-efficient silicon solutions with excellent operating performance, more than 32 billion devices shipped annually from manufacturing facilities around the world, and advanced process and technology capabilities. ON Semiconductor is also familiar with its customers, has established extensive and in-depth relationships with many international and local customers, and serves its customers with design support centers around the world. ON Semiconductor's rich product and solution lineup covers different fields such as standard products, proprietary solutions, application-specific standard products and customized ASIC solutions, as well as application markets such as automobiles, computers and game consoles, portable consumer products, media and entertainment, industrial, medical, high-voltage communications, military and aviation.

The above-mentioned application markets served by ON Semiconductor will involve LED lighting/backlight. ON Semiconductor uses a wide range of technologies to provide solutions for a wide range of LED lighting applications, giving full play to the core advantages of high-efficiency power solutions. ON Semiconductor's new simple, economical and robust linear constant current regulator solutions for low-current LED lighting applications have broadened ON Semiconductor's existing LED driver product lineup including switching regulators and ordinary linear regulators, which is very suitable for automotive interior/exterior lighting, commercial signage and building LED lighting application needs.