There are two main options for LED electronic product designers to configure multiple LED strings

“Using a switching regulator to regulate each LED string (usually in a buck configuration) provides the highest level of flexibility for power supply fluctuations, LED stack voltage fluctuations, and fault protection. However, it may be costly to provide a switching regulator for each string, and it is not necessary when the forward voltage matching between the LED strings is relatively good. In addition, placing LED strings in a series-parallel configuration requires better matching between the forward voltage of each string and the dynamic impedance, otherwise it will cause serious string current mismatch problems and lower reliability. In addition, this need will increase the cost of LEDs because of the need to perform forward voltage classification processing on the LEDs.

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Before the invention of dynamic regulation of high-power linear regulators, LED electronics designers had two main options for configuring multiple LED strings:

Use a switching regulator to adjust each string separately; or try your luck with a series-parallel configuration.

Using a switching regulator to regulate each LED string (usually in a buck configuration) provides the highest level of flexibility for power supply fluctuations, LED stack voltage fluctuations, and fault protection. However, it may be costly to provide a switching regulator for each string, and it is not necessary when the forward voltage matching between the LED strings is relatively good. In addition, placing LED strings in a series-parallel configuration requires better matching between the forward voltage of each string and the dynamic impedance, otherwise it will cause serious string current mismatch problems and lower reliability. In addition, this need will increase the cost of LEDs because of the need to perform forward voltage classification processing on the LEDs.

As employees of Texas Instruments (TI), we are always looking for ways to reduce costs while maintaining or improving the performance of LED lighting systems. Our series of high-power linear regulator devices (ie, LM3463, LM3464, and LM3466) provide a low-cost and high-efficiency way to ensure current matching between strings, which can solve various challenges in designing multi-string LED applications.

In this application, the LM3466 integrated circuits can communicate with each other to ensure that the input current imposed by the constant current power supply can be evenly distributed among the strings. The linear regulator will absorb any difference in the forward voltage of the LED stack.

Question 1-But isn’t the linear regulator inefficient?

The best answer is “sometimes, but not often.” Linear current regulators are only inefficient when the input and output voltages do not match, but if the two voltages are close, they are actually very efficient. All products in this series, including LM3466, can work normally, ensuring extremely high efficiency. In fact, the conversion efficiency of a single linear stage is higher than most conversions based on switching regulators!

Question 2-How about fault protection?

Although LED reliability is improving, we still need to pay attention to fault protection to ensure that the system runs well in the event of a fault. All of our solutions based on linear regulators include open circuit and short circuit LED protection schemes to ensure reliable and safe work. The product manual of each device provides complete information on related troubleshooting functions.

Question 3-What if you want to provide different currents in different strings?

As long as the forward voltages of the strings are sufficiently close that they will not stress the linear regulator through excessive power consumption, it is fine. For LM3466-based systems, each circuit uses programming resistors to determine the ratio between strings. Therefore, the currents do not have to be all equal as in the example discussed above. If the forward voltage difference of the LED string is too large, a switching regulator should be used.

Question 4 ― I have multiple strings of two different types of LEDs, which require different currents. Does this mean that I cannot use LM3466?

It can be used, but it has some limitations. For example, the circuit shown in the figure below supports one high-brightness white LED string with a current of 500mA and five red LED strings each requiring 100mA.

In this case, if the constant current power supply provides 1A current, then the current of each of the five color strings must be equal to 100mA, and the sum of the currents of the five strings must be equal to 500mA, which is equal to the high-power white light string.

LM3466 provides a high-efficiency, low-noise, low-cost convenient method to drive various multi-string LED configurations. Try this device and tell me how good it is. You can click here to view TI’s complete LED lighting product portfolio.