Overview of linear LED driver solutions and their typical application cases

In many lighting applications, linear LED drivers are the preferred solution because they are relatively simple, easy to design, and enable LEDs to be driven with a precise regulated current regardless of changes in the LED forward voltage drop or input voltage. Since the drivers are linear structures, they must match the power dissipation requirements of the application. ON Semiconductor offers a wide range of linear LED driver solutions with currents ranging from 10 mA to 1 A, including novel linear constant current regulator (CCR) solutions and many other linear driver solutions.

Linear CCR for low current LED driving and application examples

In many low-current LED applications with currents below 350 mA, such as automotive combination taillights, neon replacements, traffic lights , large display backlights, architectural decorative lights and indicators, ordinary linear regulators or resistors can be used to provide LED drive solutions. Resistors are used to limit the current of the LED string. They are the lowest cost solution, easy to design, and have no electromagnetic compatibility issues. However, when using resistors, the LED forward current is determined by the voltage. Under low voltage conditions, the forward current is low, which will lead to insufficient LED brightness and possible damage to the LED under transient conditions such as load dump. The resistor solution also has the lowest energy efficiency, which is not conducive to energy saving, which is particularly disadvantageous in applications that emphasize high and low energy consumption. In addition, the resistor solution also has problems with LED thermal runaway and screening. The linear regulator solution provides better current regulation accuracy (±2%), supports over-power self-regulation, and has no EMI issues. This solution has low energy efficiency and moderate cost.

Customers need a driver solution that is more economical than ordinary linear regulators but has much higher performance than resistors. ON Semiconductor uses patent-pending self-biased transistor (SBT) technology, combined with its own strong process control capabilities, to launch a novel LED driver solution - the NSI45 series linear constant current regulator (CCR). Compared with resistors, linear CCRs have constant brightness over a wide voltage range, protect LEDs from overdrive 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 provided by different suppliers, reducing the total system cost. CCRs are also EMI-free, use high power density packaging, and are AEC-Q101 certified for the automotive industry.

ON Semiconductor's CCRs include two types: two-terminal fixed output and three-terminal adjustable output. The current levels cover 10 to 350 mA and 20 to 160 mA respectively, and the maximum anode-cathode voltage VAK is 50 V and 45 V respectively. The high VAK voltage helps suppress surges and protect the LED. This series of CCRs has no voltage offset before current flows, and its fast turn-on/off characteristics provide wide range and precise pulse width modulation ( PWM ) dimming capabilities. There is no "plug and play" device like CCR on the market. Other devices require a minimum voltage of 0.5 V to turn on, and they will not turn on immediately like CCR (see Figure 1 left). CCR can provide precise PWM dimming with an external bipolar junction transistor (BJT) (see Figure 1 right). The typical PWM dimming frequency is 0.1 to 3 kHz. There is no color drift during dimming because the LED is always turned on at the optimized current.

Figure 1: Comparison of Ireg-Vin curves of CCR and competitive devices at 25 mA (left); example of CCR dimming application (right).

CCR has a negative temperature coefficient (NTC) characteristic, which protects LEDs from thermal runaway at extreme voltages and operating temperatures. CCR is easy to design and suitable for high-side and low-side applications (see Figure 2a). It does not require external components and is very simple, suitable for a wider range of applications. In comparison, some suppliers offer similar functions or performance to ON Semiconductor CCR, but require additional external components, cannot be configured as high-side or low-side drivers, have different packages, or have poorer thermal reliability.

Figure 2: CCRs can be used flexibly for high-side or low-side driving, and can also drive multiple strings of LEDs.

CCR can also be used to drive multiple strings of LEDs (see Figures 2b and 2c), which can also be used for both high and low ends. Figure 2b shows an application example of a single CCR driving multiple strings of LEDs. This configuration has the lowest cost, but the forward voltage drops of different LEDs must be matched, and if one string of LEDs fails, the current of other strings of LEDs will increase, increasing the risk of failure. In this configuration, more power is dissipated in a single CCR package. Figure 2c shows multiple CCRs driving multiple strings of LEDs. This configuration has the best protection performance, does not require matching LEDs, and the failure of one string of LEDs has no effect on other strings, and the power is dissipated in multiple CCR packages.

In addition to driving a single or multiple strings of LEDs with a single CCR, multiple CCRs can be connected in parallel to provide higher currents to drive single or multiple strings of LEDs. Using a three-terminal adjustable output CCR helps meet specific current setting requirements, and the adjustable resistor consumes less than 150 mW.

Figure 3: Connecting multiple CCRs in parallel provides higher current to drive a single or multiple strings of LEDs.

CCR can be used in applications powered directly by AC power. After the AC mains input is bridge rectified, it is only necessary to ensure that the remaining voltage after subtracting the total voltage of the LED string from the input voltage does not exceed the VAK of the CCR. CCR can also be used in T8 fluorescent tube LED replacement applications. Using CCR to drive LED T8 tubes (see Figure 4) has lower input power, higher power factor, lower total harmonic distortion, and higher light output than fluorescent lamps using electronic ballasts.

Figure 4: Circuit diagram of NSI45090DDT 4G CCR driving LED T8 tube application.

ON Semiconductor's NSI45 series CCR includes 10, 15, 20, 25 or 30 mA fixed output versions, 60 to 160 mA adjustable output versions, and 20 to 160 mA adjustable output versions certified by automotive standards. ON Semiconductor also provides CCR sample kits and evaluation boards for customers to apply for trials.

Linear LED drivers for addressable signs, architectural decorative lighting, etc.

In addition to providing low-current LED drivers such as the new linear CCR, ON Semiconductor also provides a variety of other linear drivers for display drivers, addressable signs, building decorative lights and other applications.

LED applications such as billboard signs, scrolling banners, smart vehicle signs and sports scoreboards require multiple LEDs, usually multiple strings of LEDs, and require LED driver solutions to provide constant light output, high current matching accuracy between different channels, easy-to-use interfaces to control different LED channels, and reliable protection functions. ON Semiconductor's solutions for such applications include constant current LED sink drivers such as CAT4008 and CAT4106, the former supports 8 channels and the latter supports 16 channels, with LED current ranges of 2 to 100 mA, which is set by external resistors.

Figure 5: CAT4016 typical application circuit diagram.

Certain architectural decorative lighting applications designed to create high visual impact use high-brightness LEDs, requiring LED drivers to be able to drive high-brightness LEDs and have high-speed interfaces to support high data rates and ensure high data integrity, as well as ultra-low voltage drops to provide higher energy efficiency. ON Semiconductor's LED drivers for such applications include CAT4101, CAT4103, and CAT4109.

Figure 6: Typical application circuit diagram of CAT4103 and CAT4109 RGB pixel LED drivers.

Among them, CAT4101 is a 1 A high brightness linear LED driver that does not require an inductor , eliminates switching noise, minimizes the number of components, and simplifies the design for architectural LED decorative lighting applications that require high current. CAT4103 is a 3-channel serial programmable constant current RGB LED pixel driver designed for high-end, multi-color, "intelligent" LED architectural lighting applications. It has a high-speed serial interface that can support data rates up to 25 MHz and provides fully buffered data outputs to ensure the highest data integrity in distributed (long-distance), daisy-chain lighting systems. CAT4109 is also a three-channel RGB LED pixel driver that uses a parallel interface and has dedicated pulse width modulation (PWM) control for each channel, making it ideal for more conventional LED visual effects applications such as color mixing and architectural accent lighting.

In addition, LED backlights are also accelerating the replacement of traditional CCFL backlights in medium and large-sized LCD TV applications . ON Semiconductor's CAT4026 is a 6-channel linear LED driver for large- panel LED backlight applications. Designers can combine ON Semiconductor's NCP1397 semi-harmonic dual inductor plus single capacitor (LLC) resonant controller and CAT4026 edge-lit LED linear driver in the LED backlight section of LCD TVs to support LED backlight LCD TV designs with board heights as low as 8 mm. The CAT4026 supports 6 channels with a single IC and is easily graded into up to 12 or 18 channels (using 2 or 3 controllers accordingly), with a target efficiency of more than 90% and a typical efficiency of 94%. In addition, the driver provides forward voltage monitoring to limit overall power consumption; it can also provide protection for different LED string faults such as LED open circuits and excessive LED short circuits.

Figure 7: Schematic diagram of a 46-inch LED-backlit LCD TV power supply using NCP1397 and CAT4026 for the backlight.

Summarize:

As a leading supplier of high-performance silicon solutions for energy-efficient electronic products, ON Semiconductor provides energy-efficient driver solutions for various LED lighting /backlighting applications, including AC-DC LED solutions, DC-DC LED switching regulator solutions, and different linear LED driver solutions. This article focuses on different linear LED driver solutions from ON Semiconductor, including novel linear CCR solutions for low-current applications, and linear LED driver solutions for addressable signs and architectural decorative lighting applications. Customers can choose the appropriate ON Semiconductor linear LED driver solution based on actual application requirements, shorten the design cycle, and speed up the product launch process.