In many lighting applications, linear LED drivers are the preferred solution because they are relatively simple, easy to design, and enable the LED to be driven with accurate 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 ranging from 10 mA to 1 A, including a novel linear constant current regulator (CCR) solution and many other linear driver solutions.
Linear CCR and Application Example for Low Current LED Drive
In many low-current LED applications with currents below 350 mA, such as automotive combination tail lights, neon replacement, traffic lights, large Display backlighting, architectural lighting, and indicators, common linear regulators or are resistors to provide LED drive solutions. Resistors are used to limit the current in the LED string and 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, and at low voltage conditions, the forward current is lower, resulting in insufficient LED brightness and possible damage to the LED during transient conditions such as load dump. The resistance scheme also has the lowest energy efficiency, which is not conducive to energy saving, which is especially disadvantageous in applications that emphasize high and low energy consumption. In addition, the resistance scheme also has LED thermal runaway and screening problems. The linear regulator solution provides better current regulation accuracy (±2%), supports over-power self-regulation, and has no EMI problems. This solution has low energy efficiency and moderate cost.
Customers need a drive solution that is more economical than ordinary linear regulators, but has much higher performance than resistors. ON Semiconductor uses the patent-pending Self-Biased Transistor (SBT) technology, combined with its own superior process control capabilities, to launch a novel LED drive solution – NSI45 series linear constant current regulator (CCR). Compared to resistors, linear CCRs provide 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 feature, customers can reduce or eliminate the coding cost of different LEDs from different suppliers, reducing the total system cost. The CCR is also EMI free, packaged in a high power density package, and certified to AEC-Q101 for the automotive industry.
ON Semiconductor’s CCRs include two-terminal fixed output and three-terminal adjustable output types, with current levels ranging from 10 to 350 mA and 20 to 160 mA, respectively. The maximum anode-cathode voltage VAK is 50 V and 45 V, respectively. High VAK voltage helps suppress surges and protect LEDs. This series of CCRs has no voltage offset before current flow, and its fast turn-on/off characteristics provide wide-range and precise pulse-width modulation (PWM) dimming capability. 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 will not turn on immediately like CCR (see Figure 1 left). CCR can provide accurate PWM dimming with external bipolar junction transistor (BJT) (see Figure 1 right). Typical PWM dimming frequency is 0.1 to 3 kHz, and there is no color drift during dimming because the LED is always optimized current conducts.
Figure 1: Comparison of Ireg-Vin curves of a CCR at 25 mA versus a competing device (left); an example of a CCR dimming application (right).
The CCR features a negative temperature coefficient (NTC) that protects the LED from thermal runaway at extreme voltages and operating temperatures. CCR is easy to design, suitable for high-side and low-side applications (see Figure 2a), requires no external components, is very simple, and is suitable for a wider range of applications; in comparison, some suppliers provide Similar functionality or performance to ON Semiconductor CCR, but requires additional external components, cannot be configured as high-side or low-side drivers, is packaged differently, or has less thermal reliability.
Figure 2: CCR can be used flexibly for high-side or low-side drive, and can also drive multiple strings of LEDs.
The CCR can also be used to drive multiple strings of LEDs (see Figures 2b and 2c), both on the high side and on the low side. 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 the failure of one string of LEDs increases the current of other strings of LEDs, increasing the risk of failure. . More power is dissipated in a single CCR package in this configuration. Figure 2c shows multiple CCRs driving multiple strings of LEDs. This configuration provides the best protection, with no matching LEDs required, failure of one string has no effect on other strings, and 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 current and drive single or multiple strings of LEDs. Among them, the use of a three-terminal adjustable output CCR helps meet specific current setting requirements, and the adjustable resistor consumes less than 150 mW.
Figure 3: Multiple CCRs in parallel provide higher current to drive multiple strings of LEDs with a single string.
The CCR can be used in applications powered directly from 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 lamps (see Figure 4) results in 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 NSI45090DDT4G CCR in driving LED T8 tube application.
ON Semiconductor’s NSI45 series of CCRs are available in 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 to automotive standards. ON Semiconductor also provides CCR sample kits and evaluation boards for customers to apply for trial.
Linear LED Drivers for Addressable Signs, Architectural Lighting and More
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 applications such as Display driving, addressable signs, and architectural lighting.
LED applications such as billboard signs, rolling banners, smart vehicle signs, and sports scoreboards require multiple LEDs, often multiple strings of LEDs, requiring LED driver solutions that provide constant light output and high current matching accuracy between channels , to provide an easy-to-use interface to control different LED channels, and to have reliable protection functions. ON Semiconductor’s solutions for this type of application include constant current LED sink drivers such as CAT4008 and CAT4106, the former supports 8 channels and the latter supports 16 channels, and the LED current ranges from 2 to 100 mA, which is set by an external resistor. Certainly.
Figure 5: Typical application circuit diagram of CAT4016.
Certain architectural lighting applications designed for high visual impact use high-brightness LEDs, requiring LED drivers capable of driving high-brightness LEDs and a high-speed interface to support high data rates and ensure that high data integrity is maintained, There is also a need for ultra-low pressure drop to provide higher energy efficiency. LED drivers from ON Semiconductor suitable for this type of application include CAT4101, CAT4103 and CAT4109.
Figure 6: Typical application circuit diagram of CAT4103 and CAT4109 RGB pixel LED drivers.
Among them, the 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 design, targeting high-current building LED decorative lighting applications. The CAT4103 is a 3-channel serial programmable constant current RGB LED pixel driver designed for high-end, multi-color, “smart” LED architectural lighting applications, featuring a high-speed serial interface capable of supporting data rates up to 25 MHz, fully buffered , ensuring the highest data integrity is maintained in distributed (long distance), daisy-chained lighting systems. The CAT4109 is also a three-channel RGB LED pixel driver using a parallel interface with dedicated Pulse Width Modulation (PWM) control per channel, ideal for more conventional LED visual effects applications such as color mixing and architectural accent lighting.
In addition, in medium and large size LCD TV applications, LED backlights are also accelerating the replacement of traditional CCFL backlights. ON Semiconductor’s CAT4026 is a 6-channel linear LED driver for large Panel LED backlight applications. Designers can incorporate ON Semiconductor’s NCP1397 half-harmonic dual-inductor-plus-single-capacitor (LLC) resonant controller and CAT4026 edge-lit LED linear driver in the LED backlight portion of an LCD TV to support LED backlights with board heights as low as 8 mm LCD TV design. The CAT4026 supports 6 channels in a single IC, and is easily graded into up to 12 or 18 channels (with 2 or 3 controllers, respectively), with a target energy efficiency higher than 90%, and a typical energy efficiency of 94%. In addition, the driver provides forward voltage monitoring to limit overall power dissipation and protection against various LED string failures such as open LEDs and excessive LED shorts.
Figure 7: Schematic diagram of a 46-inch LED-backlit LCD TV power supply using NCP1397 and CAT4026 in the backlight part.
As the premier supplier of high-performance silicon solutions for high-efficiency electronic products, ON Semiconductor provides high-efficiency driver solutions for various LED lighting/backlight applications, including AC-DC LED solutions, DC-DC LED switching stabilizers. voltage regulator scheme and different linear LED driver schemes, etc. This article focuses on different linear LED driver solutions from ON Semiconductor, including a novel linear CCR solution for low current applications, and a linear LED driver solution for addressable sign and architectural lighting applications. Customers can choose the appropriate ON Semiconductor linear LED driver solution according to the actual application needs, shorten the design cycle, and speed up the time-to-market process.