The LED is preferably driven by a constant current source. For LED applications that require less current, the linear drive scheme works well. However, when the LED requires more current, the switching power supply design is required. If there is only one LED string in the application, the control is straightforward. But if there are multiple strings of LEDs, the control becomes complicated. For example, one factor to consider is what to do if a string of LEDs in a multi-string LED is open. If current mode control is used, then in general, the current of the string of LEDs that are originally flowing through the open circuit is now forced to shunt into the remaining LED strings, increasing the current of these LED strings and shortening their lifetime. This situation is not a good thing. This design note describes a method for automatically detecting LED string open and adjusting accordingly.
Central issue:
*Automatically detect LED string open circuit and adjust accordingly
solution:
* Add a sense resistor to the bottom of each string of LEDs, using only one control loop
*PNP and NPN transistors for open-circuit LED strings are automatically turned off, and the feedback voltage of the LEDs is removed from the loop
When a switching regulator is normally used in constant current mode, a small sense resistor at the bottom of the LED string measures the current, and then the voltage of the sense resistor is fed back to the regulator's control loop. But if there are multiple strings of LEDs, how do designers control the loop? Some designers use a sense resistor at the bottom of all LED strings, but this creates the aforementioned LED string open problem. Other designers have Zener clamps for each LED. This method is certainly effective but costly because each LED requires a Zener device. There are also designers who use separate switching regulators and control loops for each string of LEDs, which also makes the cost prohibitive.
Instead of using multiple control loops for each string of LEDs, we can add only one control loop by adding a sense resistor to the bottom of each string of LEDs. Then, some PNP transistors are added to the sense resistors, and the collectors of these PNP transistors are connected together and transmitted to the control loop. A separate NPN transistor is also added in series with the base of the PNP transistor to open the current path when the PNP transistor is in reverse bias. This reverse biasing occurs when the LED string is open; when the feedback voltage is applied to the collector of the PNP transistor, the emitter junction is pulled to ground (Figure 1).
Figure 1 NCP3063 block diagram
When current flows through the LED string, the NPN transistor turns on, correspondingly pulling the base resistance of the PNP transistor to ground level, turning it on. The outputs of these PNP transistors are tied together and passed to the feedback of the switching regulator. If both PNP transistors are turned on, the two sense resistors are essentially parallel, suggesting that the loop requires enough current to drive the two LED strings (Figure 2).
Figure 2 is based on the NCP3063 single current loop to drive multiple strings of LEDs
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