The forward voltage of LEDs depends on several factors. Most prominent are the manufacturing tolerances which can be mitigated by a selection process — binning — based on testing that increases the cost of the LEDs dependent on the accepted tolerances. Another effect that leads to a drift of the LED forward voltage is the ambient temperature. Fig. 2 gives an idea of a typical LED in terms of forward voltage change relative to temperature. For this LED, the Vf change over the whole temperature range is more than 10%. Using a resistor-based design with such an LED would reduce or increase its brightness proportionally over the temperature range. A string of several LEDs in series can compensate or increase this effect. A linear regulator guarantees constant current output within narrow tolerances despite any Vf variations.

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Voltage drop and power factor

As mentioned, LED strips are popular applications for linear control of LEDs. Typical LED strips support lengths of 10m (30 ft), and some range even longer. The electrical current needs to travel the whole length and back again. The conductor in a strip has a specific inherent resistance that leads to a voltage drop depending on the length and the resistance of the conductor.

A typical 10m strip with 25 mA required per LED/segment and 10 segments per meter can lead to a voltage drop of approximately 1.5V. With a resistor-based solution, this generates a difference in brightness of almost 10% in a 12V system between the beginning and the end of the strip.

Light fluctuations can also result from the AC-based supply of the DC voltage. For structures with more than 5W power consumption, a lighting system needs to have a power factor correction (PFC) circuit. A cost-effective topology is a single-stage topology that combines the PFC with the generation of the constant voltage. The difficulty comes from the PFC that needs some ripple on the output voltage (5–10%) to function, and only a rather big and costly capacitor can compensate for this requirement.

A linear regulator smooths the current through the LEDs despite the voltage fluctuations due to PFC. This avoids the visible flicker otherwise caused by the current ripple without adding additional components. The same principle confines the visible effects of other voltage perturbations.

Lifetime and temperature

Another factor with all SSL products is that the system design must match or support the long-life benefits inherent in LED sources. The lifetime of LED components correlates with the operating time and operating temperatures. Above a device-specific temperature threshold, the remaining lifetime deteriorates severely. A system design that limits the operating temperature of a luminaire considerably also extends the usability of that luminaire.

There are a number of ways to protect against system operation at temperatures above a specified limit. In the simplest case, you can use a positive temperature coefficient (PTC) resistor, either replacing or supplementing the current limiting resistor. A closer look at the functionality of a PTC thermistor, however, reveals a temperature characteristic that makes the use for this purpose unattractive, as the resistance increases exponentially after a certain threshold temperature. Therefore, this approach does not work for most use cases.

Some linear ICs provide temperature protection with a more seamless transition property. This operational functionality is maintained during low-temperature incidents, providing at least some partial light output. The use of the feature is easy, as it does not require any special effort compared to the calculation, selection, and purchase of the right PTC.

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Operating temperature can further impact cost and efficacy of the LED array in a product. With a linear LED driver IC, the current through the LEDs is independent
of the supply voltage as long as it is higher than the sum of the forward voltages of the LEDs and the voltage drop of the IC itself. But with temperature extremes, a potential problem arises.

In a 12V system based on the LED documented in Fig. 2, a linear design would only work above a temperature of approximately -30°C with a low-drop (~ 0.15V) controller and four LEDs in series. With many legacy linear driver ICs, the voltage drop is above 1V. With the rest of the setup being the same, such a legacy IC would work at temperatures higher than 40°C only, which essentially requires a reduction of the string length to three LEDs. A linear IC with low voltage drop provides more design freedom — in this case, four instead of three white LEDs — and leads to improved efficiency.

Dimming and control

Next, let’s consider that most modern illumination systems usually require a dimming capability. For a smart building environment, dimming is mandatory. In resistor-based systems, the only option to enable dimming is by pulsewidth modulation (PWM) of the DC supply voltage. The resulting average current is determined by the duty ratio (on-time/cycle length). This brings additional challenges to the system design as the PWM switch needs to carry the load of the whole chain — in a 10m strip system, such as discussed earlier, this requirement amounts to a 2.5A load.

In order to reduce the total voltage loss below 0.1V in the PWM switch, an expensive switch with a maximum resistance of 40 mΩ is needed. Many applications require stable light output without variations; therefore, the PWM frequency needs to be high enough to avoid flicker (according to the IEEE- recommendation, the frequency should be higher than 3.0 kHz). Switching a current of 2.5A at this frequency that flows through a 10m cable back and forth creates problematic radiation.

Many linear LED driver ICs are dimmable in the same way. To avoid the EMC problems, a dedicated signal is guided to every regulator and reduces the transferred charges by 1–2 orders of magnitude, and scales the EMC problem by the same amount.

Smart and connected

Advanced SSL applications also bring special considerations. The emerging trend of smart lighting or smart buildings requires many additional features that can be realized via a multi-channel architecture. For example, so-called human-centric lighting (HCL) changes the correlated color temperature (CCT) of the emitted light. Typically, such a tunable light source relies on two channels with different CCT LEDs, each channel independently dimmed to achieve a blended CCT. The mixture of the two intensities produces a resulting color temperature between the two extremes. Therefore, the light can be adjusted to the target CCT — for example, K in the morning and K at night. This scheme follows and supports the natural biological rhythm of the human body. The full system design is straightforward and cost-effective as only one AC circuit is required and the electronics dedicated to the different channels are low-cost, small, and of little weight. Fig. 3 shows an example system.

What is the benefit of low voltage drop linear LED drivers (BCR43X family)?

What is the benefit of low voltage drop linear LED drivers (BCR43X family)?

Benefits of LITIX™ LED Drivers

1. Cost reduction: Enables a higher number of LEDs in a string, higher system efficiency and reduces the number of ICs in the system.
2. Increases LED tape length while using the same number of LEDs in the string.

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