When using a white LED as a backlight for a display or other lighting device, it needs to be driven by constant current. The main reasons are:
1. Avoid driving current exceeding the maximum rating and affect its reliability. 2. Obtain the expected brightness requirements and ensure the consistency of brightness and chromaticity of each LED.
This article describes the parameter range of a typical LED and the constant current drive circuit. Figure 1 shows the forward current versus forward voltage for six randomly selected white LEDs (three of which are from two top manufacturers). In this case, if the six LEDs are driven with 3.4V. The corresponding forward current difference is large: 10mA to 44mA.
Figure 1. Correspondence between forward current and forward conduction voltage for six randomly selected white LEDs (three of which are from two top manufacturers). Note that for any given voltage, the forward current varies over a wide range—10mA to 44mA (at 3.4V).
To ensure reliability, the current driving the LED must be lower than the LED rating. The typical maximum is typically 30mA. However, as can be seen from Figure 2, the allowable current is reduced when the ambient temperature rises. When the temperature reaches 50 ° C, the current should be limited to 20 mA. It is not difficult to draw the conclusion by observing Fig. 1 and Fig. 2: the scheme of driving the white LED by the constant voltage method is not reliable.
Figure 2. In general, the maximum absolute value of the forward current of a white LED decreases with increasing ambient temperature (Courtesy Nichia Corporation).
In addition, brightness and chromaticity uniformity can be obtained by driving a white LED with a constant current. Figure 3 shows several general white LED driver circuits.
Figure 3. For a typical white LED, its electrical characteristics are typically tested at IF = 20mA. Therefore, in order to obtain the predicted and matched brightness and chromaticity, it is recommended to use constant current drive (Courtesy Nichia Corporation).
Figure 4 shows four commonly used power circuits for driving LEDs. Fig. 5 is a diagram showing the current adjustment accuracy obtained when the above six LEDs are adjusted. The output load line of the regulator in Figure 5 is drawn on the Vf plot of the LED, and the intersection of the two curves is the adjustment point of each LED.
The circuit shown in Figure 4a uses a voltage regulator source with a ballast resistor to control the current of the LED. The advantage of this structure is that there is a large room for selecting the voltage source. Only one connection terminal is needed between the regulator and the LED; the disadvantage is that the efficiency is low. This is mainly caused by the loss of the ballast resistor. In addition, its control of the LED forward current is not very accurate. From the test curve of Figure 5a, it can be seen that the current variation range of 6 different LEDs is: 14.2 mA to 18.4 mA. The average brightness of the LED provided by the manufacturer A is higher than that of the manufacturer B, and the operating current is 2 mA higher.
The circuit shown in Figure 4b is used to regulate the total current of the LEDs, and the ballast resistors are used to achieve matching between the LEDs. This configuration is used in the MAX1910, which provides better results when driving the same batch of products from the same manufacturer. Under the same current supply as the above circuit, the ballast resistance can be reduced to reduce power consumption by half. Figure 5b shows the variation range of six different LED drive currents: 15.4 mA to 19.6 mA, the LED current variation provided by Manufacturer A is smaller, and the average LED control current from Manufacturer A and Manufacturer B is the same: 17.5 mA. The drawback of this structure is that the ballast resistance still consumes a large amount of power, and the matching of the LED currents is not very good. But this circuit tradeoff takes into account the performance and ease of the circuit.
Figure 4c adjusts the current of each LED separately, eliminating the need for ballast resistors. Current regulation accuracy and matching are dependent on each individual current regulator. The MAX1570 uses this current source architecture with a current accuracy of 2% and a matching ratio of 0.3%. Higher efficiency is achieved because the current regulator allows for a lower differential pressure. Figure 5c shows that all of the six white LEDs tested are maintained at a steady 17.5 mA, which saves board area due to the elimination of ballast resistors, but requires four connections between the regulator and the LED. This circuit provides a high performance specification and is a competitive solution based on the inductor structure.
Figure 4d is an inductor-based boost circuit configured as a current regulator with high conversion efficiency. The lower feedback threshold further reduces the power consumption of the current-sense resistor. In addition, because the LEDs are connected in series, the brightness of the LEDs can be kept consistent under any operating conditions. The accuracy of the current depends on the accuracy of the regulator feedback threshold and is not affected by changes in the LED forward voltage. The MAX1848 and MAX1561 are two typical examples of such current-regulating circuits with conversion efficiency (PLED/PIN) of 87% (3 LEDs in series) or 84% (6 LEDs in series). Another advantage of this circuit is that only two connection terminals are required between the regulator and the LED, providing some flexibility for the user's design. However, due to the use of inductance in the circuit, the size is larger, the cost is higher, and the EMI radiation is larger than the above scheme.
Figure 4. White LEDs typically have four different drive circuits: (a) voltage source and ballast resistor, (b) current source and ballast resistor, (c) multiple current sources, (d) one current source drive in series LED.
Figure 5. The forward voltage (Vf) of each white LED has a different effect on the regulation current accuracy, depending on the structure of the regulation circuit: (a) voltage source and ballast resistor, (b) current source and ballast resistor, (c A multi-channel current source or a current source drives the series LEDs. The Vf curves of the six LEDs (three from manufacturer A and manufacturer B) are shown in the figure. The intersection of the output load curve of the regulator and the LED Vf curve is a stable adjustment operating point.
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