Technical Challenges and Solutions for LED Lighting and Solar Charging

As an environmentally friendly and energy-saving solution, LED lighting has found its place in a wide range of applications such as cars, homes, office buildings, hotels, airports and street lamps. However, its large-scale commercial use not only needs to overcome cost barriers, but also needs to solve technical problems such as dimming flicker, heat dissipation, and color uniformity. In addition, the focus on clean energy and the decline in the cost of solar panels have also driven the current solar energy commercial boom in the industry. In order to help readers grasp this business opportunity faster and better, this magazine specially invited Linear power expert Tony Armstrong to share his unique insights.

Q: How do I eliminate LED light flicker when using PWM or analog dimming?

A: Faced with the increasing popularity of high-power, high-brightness LEDs, electronic lighting designers must provide efficient, accurate and simple LED driving solutions. This task is made more difficult by the interchangeability of commercial series LED arrays in high-power lighting (such as automotive headlamps or large LCD display backlights).

Traditionally, driving high-power LED strings with accurate currents has been at odds with simplicity and efficiency, typically requiring the use of some inefficient linear regulator solution or a more elaborate multi-IC switching regulator configuration. In addition, ensuring that each LED has uniform brightness and does not produce any flicker has also become a major design hurdle.

There are two generally accepted methods for controlling LED brightness: analog dimming and PWM digital dimming. When using analog dimming, the LED current is adjusted from a maximum value to about 10% of that maximum value (10:1 dimming range). Because the color spectrum of LEDs is current-dependent, this method is not suitable for some applications. However, PWM digital dimming switches between zero current and maximum LED current at a rate fast enough to mask visual flicker (usually above 100kHz). This duty cycle changes the effective average current, allowing dimming ranges as high as 3000:1 (limited only by the minimum duty cycle). Because the LED current is either at maximum or off, this method also has the advantage of avoiding LED color shifts, which are common with analog dimming.

Q: How should the heat dissipation problem of high-power LED lighting be solved?

A: The two largest, highest-power LED lighting applications are backlighting for large-screen LCD TV displays and automotive headlamps. Take a look at the standard LED automotive headlamps used by Lexus, Audi, and even GM's Cadillac Escalade. The overall lighting architecture of all these cars is very similar. Each automotive headlamp includes five LED-powered beams optimized for various lighting requirements, including: low beam, high beam, turn assist, daytime running lights, and turn signal indicators.

A standard LED lighting beam will typically require 35W to 50W of power. This may not seem like a lot of power; however, LEDs provide 10 times the brightness of HID halogen lamps, so the light output of LEDs is equivalent to that of a 500W halogen lamp. High beams generally require the same or slightly more power than standard lighting beams, while cornering lights, daytime running lights and turn signals require less power. However, the total automotive headlamp will consume more than 200W of power, which can cause significant thermal power dissipation issues. This is really not a good thing, because the light output and operating life of LEDs will quickly decrease as the operating temperature increases.

There are many ways to deal with this thermal problem. One is to add a large heat sink to move the heat away from the lamp. However, this creates another set of problems, including the increase in cost and weight due to the use of heat sink materials. The most effective way to solve this problem is to use a very high efficiency driver (efficiency>93%) to minimize the heat dissipation of the LED driver circuit. This is not as difficult as it sounds, because a 50W high beam lamp can typically consist of 14 1A LEDs in series. Since the forward voltage drop over temperature is about 4V per LED, the boost converter LED driver topology can boost the nominal battery voltage of 12V to just over 56V with 93% efficiency. This results in only 3.5W of power dissipation, a power dissipation value that can be easily met by a low-grade copper heat sink routed within the printed circuit board where the LED automotive headlamp is installed.

Q: What are the key design challenges when charging batteries using electricity harvested from solar panels?

A: Solar panels have become widely accepted as a practical method of generating electricity in both commercial and residential settings. However, despite advances in technology, solar panels remain expensive. A large part of this high cost comes from the panels themselves, where the size (and therefore cost) of the panels increases as the required output power increases. Therefore, it is important to maximize panel performance in order to create the smallest, most cost-effective solution.

Generally speaking, energy harvested from a solar panel is used to charge a battery, which in turn provides power for the operation of the end application circuitry when there is no sunlight. To achieve the best design for a solar battery charger, it is necessary to understand the characteristics of the solar panel. First, due to the large bonding area, solar panels will leak, and the battery will discharge through the panel in dark conditions. Also, each solar panel has a characteristic IV curve with a maximum power point, so when the load characteristics do not match the panel characteristics, energy extraction will be reduced. Ideally, the panel will be continuously loaded at the maximum power point to fully utilize the available solar energy and thereby minimize the cost of the panel.

Typically, the panel leakage problem is solved by placing a Schottky diode in series with the panel. Reverse leakage is reduced to a very low value; however, the forward voltage drop of the Schottky diode (which dissipates a lot of power under high current conditions) still causes energy losses. Therefore, expensive heat sinks and careful layout are required to keep the Schottky diode cool. A more effective way to solve this power dissipation problem is to replace the Schottky diode with a MOSFET-based ideal diode. This will reduce the forward voltage drop to as low as 20mV, significantly reducing power dissipation while reducing the complexity, size and cost of the heat sink layout. Fortunately, this goal is easily achieved because some IC suppliers already have ideal diodes with such specifications (for example: the LTC4412 from Linear Technology).

However, two issues remain: “float voltage control to a fully charged battery” and “loading the panel at the optimum power generation point.” These issues can often be solved by using a switch-mode charger and a high-efficiency buck regulator.

Linear Technology has developed such a circuit, which consists of the LTC1625 No RESNSE (no sense resistor) synchronous buck controller, the LTC1541 micropower operational amplifier, comparator and reference, and the LTC4412 ideal diode. The circuit is given below for reference:

The circuit in Figure 1 is placed between the solar panel and the battery to regulate the battery float voltage. An additional control loop based on the LTC1541 forces the charger to operate at the maximum panel power point. This increase in efficiency reduces the required panel size, thereby reducing the cost of the overall solution. This circuit has important advantages that are particularly evident when there is a mismatch between the peak panel supply voltage and the battery voltage.

Figure 1: Peak power tracking buck charger maximizes efficiency

Q: What unique solutions does Linear provide to solve the above design challenges?

A: To meet the design needs of LED drivers and solar panel battery chargers, Linear Technology offers a variety of products. The LT3595, LT3518 and LT3755 are some of them.

An example of such a product and LED driver IC is Linear Technology's LT3595 buck mode LED driver, which has 16 separate channels, each capable of driving a string of up to 10 50mA LEDs from an input of up to 45V. Each channel can be used to drive 10 series LEDs to provide local dimming. Thus, each LT3595 can drive up to 160 50mA white LEDs. A 46-inch LCD TV would require approximately 10 LT3595s for each HDTV. Each of its 16 channels can be independently controlled and has a separate PWM input capable of providing up to 5000:1 PWM dimming ratios.

Each channel requires only a tiny chip inductor and an even smaller ceramic output capacitor. The only other components required are a single input capacitor and current setting resistor (Figure 2). The clamping diodes, power switches, and control logic with compensation for all 16 channels are squeezed into the LT3595's relatively small 56-pin, 5mm x 9mm QFN package.

Figure 2: A 16-channel LED driver driving 160 white LEDs from a 45V input. The PWM dimming ratio is 5000:1.

Most battery-powered portable products have one or more displays that convey graphical information to the user. However, powering TFT-LCD displays (and even OLED screens) requires special attention from the system designer. To properly power a TFT-LCD screen, a DC/DC converter must be able to provide three independent output voltages: AVDD, VON, and VOFF with the correct power-up and power-down sequencing. Linear Technology recognized this and developed dedicated monolithic DC/DC converters specifically for this purpose. The latest device is our LT3513. This converter features 5 independently controlled regulators to provide all the necessary power rails inside a TFT-LCD screen.

Its buck regulator can deliver up to 1.2A of continuous output current for the logic rail. A lower voltage auxiliary logic supply can be generated using an LDO controller and an external NPN MOSFET. A high power boost converter (ISW = 1.5A), a lower power boost converter (ISW = 250mA) and an inverting converter (ISW = 250mA) provide three independent output voltages: AVDD, VON and VOFF, which are commonly required by LCD panels. An integrated high-side PNP provides a delayed turn-on of the VON signal, while the display protection circuitry protects the TFT-LCD panel by disabling VON when any of the four outputs falls more than 10% below its programmed output voltage. Other features include an integrated Schottky diode, a PGOOD pin for the AVDD pin, output disconnect, and an inductor current sense function for the buck regulator.

The LT3755/-1 is a 60V, high side current sensing DC/DC controller designed to drive high current LEDs from an input voltage range of 4.5V to 40V. The LT3756/-1 uses the same design but provides an output of 100V from an input of 6V to 100V. The "-1" versions of both devices have external synchronization capability, while the standard device version replaces the pin's functionality with an open circuit LED status indicator. Both devices are well suited for a wide range of applications, including automotive, industrial and architectural lighting.

For applications that require input voltages greater than 40V (e.g., 48V rails), the LT3756/-1 is the preferred solution. Both the LT3755/-1 and LT3756/-1 use an external N-channel MOSFET and can drive up to 14 1A white LEDs from a 12V (nominal) input, delivering more than 50W of power. They have a built-in high-side current sensing circuit, enabling them to be used in boost, buck, buck-boost or SEPIC and flyback topologies. The LT3755/-1 and LT3756/-1 offer efficiencies of more than 94% in boost mode, eliminating the need for any external heat sink. A frequency adjustment pin allows the user to set the frequency from 100kHz to 1MHz, optimizing efficiency and minimizing the size and cost of external components. Combined with the 3mm × 3mm QFN package or thermally enhanced MSOP-16E package, the LT3755/-1 and LT3756/-1 provide a very compact high power LED driver solution.

Both the LT3755/-1 and LT3756/-1 feature True Color PWM dimming, which provides constant LED color and a dimming range of up to 3000:1. For less demanding dimming requirements, the CTRL pin can be used to provide a 10:1 analog dimming range. Its fixed frequency, current mode architecture achieves stable operation over a wide range of supply and output voltages. A ground-referenced FB pin serves as the input for multiple LED protection features, allowing the converter to function as a constant voltage source.