White light-emitting diodes ( LEDs ) are steadily growing in a variety of lighting applications, such as flashlights in handheld products, display backlighting, and the like. They provide true white light and consume less power than incandescent lamps. They are easier to use than fluorescent lamps. When used in batches, white LEDs are less consistent, and low-cost linear regulators (LDOs) can be used to improve matching, even with different LEDs.
Since the invention of white LEDs a few years ago, the application of white LEDs has developed steadily. Compared with fluorescent lamps, it has the advantages of simple use and low cost. Compared with incandescent lamps, it can provide light closer to white. Therefore, its dosage has been steadily increasing in recent years, and it is widely used in handheld products. For flash, display backlight, etc. Of course, there are some technical problems to be solved for white LEDs in practical applications.
The most obvious problem with white LEDs is the poor matching of the products. According to the typical specifications of white LEDs, the minimum forward voltage is 3.0V, the typical value is 3.5V, and the maximum is 4.0V when the current is 20mA. Obviously, the regulated source is not a reasonable solution. Using the same current to drive each LED gives uniform brightness, but at a high cost. In many applications, the LED is simply driven with a fixed bias voltage and ballast resistor, as shown in Figure 1.
Figure 1: Most applications simply use a fixed bias voltage (5V in this example) and a ballast resistor (75 ohms in this example) to achieve an approximately matched white LED brightness.
A circuit designed in accordance with Figure 1 typically achieves a fairly good brightness match. However, for mass-produced users, the same batch of products have fairly consistent characteristics, but the consistency between batches is poor. This requires testing each batch of new LEDs and configuring them with ballast resistors. If there are multiple LED suppliers, the problem will be more serious. In order to describe the above problems in detail, we selected three manufacturers' LEDs (represented by A, B, and C respectively) to perform the following tests according to the circuit shown in Figure 1: The LED labeled A is selected from the first-level white LED supplier. The LED labeled B is selected from the supplier of a secondary white LED, and the LED labeled C is a low cost LED purchased from the electronics market. The test results are shown in Table 1.
Table 1: In order to evaluate the production capacity of the circuit of Figure 1, we tested white LEDs from three different manufacturers. From the test results, the LEDs of different trademarks have poor consistency and are 3.27 mA away from the standard average current.
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