Multi-string LED lighting system and four major linear regulator issues

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

• Use a switching regulator to regulate each string individually; 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, providing a switching regulator for each string can be costly and unnecessary when the forward voltage matching between LED strings is relatively good. In addition, placing LED strings in a series-parallel configuration also requires better matching of the forward voltage and dynamic impedance of each string, otherwise it will cause severe string current mismatch problems and lower reliability. In addition, this requirement will increase the cost of the LED because the LEDs need to be binned for forward voltage.

At Texas Instruments (TI), we are always looking for ways to maintain or improve the performance of LED lighting systems while reducing costs. Our family of high power linear regulator devices (  LM3463 , LM3464  and  LM3466 ) provide a low-cost, high-efficiency way to ensure current matching between strings, solving the challenges of designing multi-string LED applications.

Another approach is to use a linear regulator to provide both a weighting function to force each string to achieve current matching and better fault protection. If the input power supply is AC/DC or DC/DC and provides a constant voltage, then  the LM3463  and  LM3464  LED drivers will be the best choice. However, if the input power supply provides a constant current, the LM3466  is the best choice. The typical application schematic is shown below:

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

Question  1 - Aren't linear regulators inefficient though?

The best answer is "sometimes, but not always." 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 members of this family, including the  LM3466  , work together to ensure very high efficiency. In fact, the conversion efficiency of a single linear stage is higher than that of most switching regulator-based conversions!

Question  2 - What about failsafe?

Although LED reliability is improving, we still need to pay attention to fault protection to ensure good system operation in the event of a fault. All of our linear regulator-based solutions include open-circuit and short-circuit LED protection schemes to ensure reliable and safe operation. The data sheet for each device provides complete information on the relevant fault handling functions.

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

As long as the forward voltages of the strings are close enough that they do not stress the linear regulators through excessive power dissipation, this is fine. For  the LM3466  -based system, each circuit uses programming resistors to determine the ratio between the strings . Therefore, the currents do not have to be all equal as in the examples discussed above. If the forward voltages of the LED strings differ too much, a switching regulator must be used.

Question  4 - I have multiple strings of two different types of LEDs that require different currents, does this mean I can't use  the LM3466 ?

It can be used, but there are some limitations. For example, the circuit shown below supports one high-brightness white LED string with a current of 500mA and five red LED strings that each require 100mA.

In this case, if the constant current source provides 1A, then each of the five color strings must equal 100mA, and the total of the five strings must equal 500mA, equal to the high-power white string.

The LM3466  provides a convenient, high-efficiency, low-noise, low-cost method of driving a variety of multi-string LED configurations. Try the device and let me know how well it performs. You can view TI's complete LED lighting portfolio here .