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AN2754 Datasheet(PDF) 4 Page - STMicroelectronics |
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AN2754 Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 20 page Background AN2754 4/20 1 Background When designing a power supply for a white high-brightness LED, the main requirements are efficiency, size and cost of the complete solution. A standard buck converter is the best choice for providing a constant current because only the buck converter among the switching topologies has an average inductor current that is equal to the average load current. For this reason, the conversion of a constant voltage into constant current is much easier. LEDs are current-driven devices whose brightness is proportional to their forward current. Forward current can be controlled in two ways: voltage mode and current mode. The first method uses the LED V-I curve to determine what voltage has to be applied to the LED in order to generate the desired forward current. This is typically accomplished by applying a voltage source and using a ballast resistor as shown in Figure 1. This method has two serious drawbacks. The first is that every change in LED forward voltage creates a change in LED current. The second problem is the power lost across the ballast resistor which reduces the efficiency. Figure 1. Constant voltage control Equation 1 LEDs are PN junction devices with a steep I - V curve. For this reason, driving an LED with a voltage source can lead to large swings of forward current in response to even a small change in forward voltage. In general, to meet the needs of a driver for an HBLED, the current output must be in the ±5% to ±20% range. The best way to drive the LEDs is to control the forward current so that it eliminates changes in current due to variations in forward voltage, which translates into a constant LED brightness. Figure 2 illustrates the configuration of a typical buck converter driver circuit. The value of current-sense resistor (RSENSE) depends on the desired LED current and the feedback voltage that the buck converter requires. Multiple LEDs should be connected in a series configuration to keep an identical current flowing in each LED. FB F MAX _ F L H FB OUT R I V n R R 1 V V × + × = ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + = |
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