Design Analysis and examples of high-power LED constant current drive circuit LM2734 is a 1A buck regulator. Based on constant current drive circuit LM2734 (Figure 3) using the LM321 op amp to obtain the sampling voltage resistance on the Rset, in combination with other resistors and capacitors can form a complete, high-efficiency power LED constant current drive circuit. In actual use, some LED constant current drive circuit can be directly obtained feedback voltage from the sampling resistor, as shown in figure 4.
Figure 3 sampling resistance Rset determines the constant current drive circuit design, and has a significant impact on the efficiency of the entire system, so careful design the Rset to energy savings is critical. Figure detailed design documents 3 and 4 of the National Semiconductor please ask your local authorized distributors. n general, if the LED drive current changes require no more than 5-10% of the nominal value, then use the precision of 2% resistor is enough. A typical LED drive current fluctuation range of plus or minus 10%. Due to the large sampling resistor power consumption, avoid using smaller power chip resistors. In addition, LM3478 scheme suitable for multiple high-power LED constant current driver, and based on LM5021 constant current driver design is targeted at 220V AC / DC converter applications.Constant current driver and thermal considerations On electronic circuit design, the engineers in the design of LED constant current driver circuit must first understand the LED constant current parameters. At present a lot of LED chip manufacturer, the difference of LED at home and abroad mainly lies in the same electrical parameters, number of lumens may be different, so the design engineer must clearly recognize the power is not the only parameter for LED luminous efficiency. For example, the same 1W of LED, some LED can reach 40 lumens of brightness, while others can only achieve 20 lumens of brightness, this is because the LED optical efficiency also depends on the materials and the production process and many other aspects. Some designers through increase the driving current of led to improve the luminous efficiency.for example, the same 1W LED, the drive current is increased as before the brightness can be increased from 20 lumens to 40 lumens, but the LED working temperature also increased.. Once the temperature exceeds the limit LED temperature point, it will affect LED life and reliability, which is an important issue in the design process of constant current driver need to pay attention.In addition, the optical efficiency of LED lighting systems depends not only on LED constant current driver program, is also closely related to the thermal design of the whole system. To reduce the volume, some LED constant current drive system make the LED drive circuit and the heat dispose part close to the design, so it is easy to affect the reliability.In general, the heat source of LED lighting system form the LED the lamp itself, the heat source is too concentrated will produce heat loss, so the LED driving circuit and temperature dissipation system can not close together. Recommends the following measures: LED lamp heat dissipation by cooling of the aluminum plate;
the power device is evenly distributed; as far as possible to avoid the LED drive circuit and the heat radiating part close to the design; thermal impedance restrain package to a printed circuit board; improve the heat flow LED chip to reduce the thermal impedance.
Figure4