Small and wide input voltage LM2842 LED lighting

Most people believe that high brightness LED (HBLED) will be the ultimate wise choice for lighting. HBLED manufacturers have significantly increased the number of lumens per watt of their products and continuously improved the energy efficiency of their products, which has also made HBLED more and more popular. It is believed that in the near future, the number of lumens per watt of LED will greatly exceed the current level of fluorescent lamps.

LED lighting is primarily used to replace low-voltage spotlights and track lights. This includes replacing the original standard-sized lamps, such as MR16, PAR20, PAR25, PAR30 and PAR38. National Semiconductor (NS) has provided a variety of solutions for beautifying LEDs to make them suitable for these occasions, including the LM342X and LM340X series of LED drivers. But no matter how these LEDs are driven, they all face a common problem: how to dissipate heat. For these small packages, heat sinks that rely solely on the free flow of air are often not enough. This forces designers to consider adding some fixtures that can force air flow. For this size of installation, the most common approach is to use a DC fan, or use Nuventix's Synjet as a more sophisticated and durable solution. Fans are often more cost-effective and can operate on 5V or 12V DC voltages, but they are not easy to embed into lighting fixtures, and the service life of fans is at most tens of thousands of hours. On the other hand, Synjet is specially designed for these lighting fixture combinations, operates on a voltage of 5V DC, and can last up to 100,000 hours.

The standard inputs to these systems can be either DC or AC systems. 12V and 24V AC systems are already widely used in low voltage lighting systems, while 12V and 24V DC systems are gradually being promoted and becoming more popular. Since 12V systems are very common, it is not difficult to design and drive a 5V cooling system. However, because there are so many systems, the distribution of input voltages will be very wide, especially for AC inputs. The ideal situation is to have a solution that can convert the wide range of input voltages to provide 5V voltage while taking up a small space and being able to be easily embedded in the lighting fixture. Fortunately, for small fixtures, the LM2842 can achieve the above requirements well, and the similar model LM2841 can also do the same.

The DC input system can be used as a transformer to convert the wide input voltage directly to 5V, which is also the easiest and simplest solution. Typical solutions include fans and Synjets where the required drive current is between 100 and 600mA. The LM2841 can provide 300mA for small cooling systems, and the LM2842 can provide up to 600mA for larger and more powerful cooling systems. The LM2842 design shown in Figure 1 has an input voltage of 12V DC or 24V DC and can provide the necessary 5V voltage. The entire solution can provide up to 600mA of current, but only occupies about 1/4 of the PCB (printed circuit board) area, making it easy to embed in any retrofit device. The LM2841 is based on the same design, specifically for lower current applications, while also allowing the use of small physical inductors, thus occupying less space.

However, as mentioned above, many 12V AC and 24V AC systems are widely used, and people hope that they can continue to work after improvement rather than completely replace them with more advanced DC systems. This means that a wider input voltage range must be considered in the design. To achieve this goal, the original AC-DC rectifier can be simply composed of a bridge rectifier and a large number of input capacitors. Taking into account the line tolerance, the peak voltage that a 24V AC system can provide will be 40V. But if the input capacitance is minimized, the input voltage will drop to 10~12V. LM2841 and LM2842 allow the input voltage to vary over a wide range, provide sufficient power rejection, and ensure that the output voltage is accurately stabilized at 5V. Figures 2 and 3 show the circuit diagrams of 12V AC and 24V AC systems, respectively.

There are a few things to note here. The 24V AC input has a higher RMS voltage, which makes the current of the LM2842 smaller. It also has a higher peak value. For these reasons, the LM2842 can use a smaller input capacitor to maintain a reasonable operating input voltage. It is also important to note that the circuits in Figures 2 and 3 are designed for the full output voltage of 600mA. Because of this, it is obvious that if the cooling system uses lower current, then the circuit size can be further reduced by using smaller input capacitors. For example, if the LM2842 is replaced with an LM2841, the maximum output current will be 300mA, so the input capacitance can be halved.

Thermal management of LEDs is no longer a problem if you have the right driver tools.