Low-voltage battery power supply is still the mainstream application of LED drivers

Introduction
In recent years, white LEDs have become an increasingly important innovation in the lighting field. Their superior lifetime and light efficiency make white LEDs the preferred solution for customers in a rapidly growing number of applications. Integrated circuit manufacturers have developed a variety of solutions to enable engineers to use these new components under optimal working conditions. This article aims to help them choose the best LED driver topology. Although high-voltage AC power can also be used to drive LEDs, low-voltage battery-powered systems remain the mainstream application solution. Therefore, this article will focus on the latter type of driver.

Driver Comparison
Before we get into the driver operation, let's take a quick look at some basic characteristics of LEDs. The light emitted is linearly proportional to the amount of current passing through the light emitting diode. Therefore, tight control of the forward current is extremely important to obtain the desired light emission. Moreover, white LEDs are highly nonlinear compared to other diodes. The only difference in the electrical performance of white LEDs compared to ordinary rectifiers is the forward voltage, which ranges from 3V to 4V. Because the relationship between current and voltage is temperature and process dependent, applying voltage to the LED can cause the current to run slightly out of control. Therefore, the best way to drive an LED is to operate it in current mode.
There is a wide range of LED drivers to choose from, but the number of their target applications is limited. The widespread use of color screen mobile phones has driven the production of white LEDs. White LEDs are seen as the best solution for display backlighting. Therefore

, drivers were first developed to be powered by lithium-ion batteries. The permitted supply voltage range is generally 2.7V to 5.5V. For the same reason, the number of LEDs connected to a driver directly depends on the size of the LCD. Many times, the number of LEDs used is 3 to 5, but for large displays, up to 10 LEDs can be used. The third limiting factor is the size of the solution. Manufacturers are always looking for the smallest and thinnest solution. Since the solution is powered by batteries, efficiency becomes a key factor in differentiating drivers. The second circuit type is the flash, and the main difference between the backlight circuits is the output power. LEDs used for camera phone flash require 100mA to 1.2A current, while ordinary backlights only require 20mA, and the number of LEDs varies from 1 to 4. If no more than 500mW of electrical power is required, there is a great deal of room for choice in display backlight circuits. Generally speaking, the specifications from 500mW to 5W are closer to flash applications.
Now let's look at the different topologies (as shown in Figure 1). The forward voltage is usually higher than the battery voltage, so a capacitive charge pump or an inductive boost device can be used to increase the available voltage. The unique advantage of the inductive boost device is that only one power switch is needed to generate enough voltage to connect the LEDs in series. No other solution can ensure that all LEDs have the same current and brightness as this solution, and the charging and discharging of the inductor can be effectively controlled to ensure a stable current through the sense resistor in series with the LED. The characteristic of this circuit is high efficiency. For example, the NCP5006 can achieve up to 90% efficiency at an output voltage of 22V. Of course, these excellent characteristics also come at a price, that is, the need for inductors. This component is generally more expensive and thicker than capacitors. For applications where thickness and PCB area are very important, the best solution is usually a capacitive charge pump.

A charge pump charges a capacitor with battery voltage and then "stores" the capacitor voltage electrically to provide an output voltage higher than the supply voltage, requiring several switches to properly connect the capacitors. The internal complexity is increased, but the external components can be small. The charge pump is a voltage source whose value depends on the number of capacitors and switches. Therefore, it is more difficult to achieve a higher output voltage without significantly increasing the complexity. In this case, the LEDs are connected in parallel rather than in series. To ensure that the LED current is stable, additional current sources or resistors can be used. The current sources are matched to ensure that the current difference between the LEDs is negligible. When accuracy is secondary, it is better to use resistors to reduce the number of connections and complexity. The main disadvantage of the charge pump is its efficiency. The number of switches and capacitors determines the multiplier N of the charge pump. This ratio is usually 1.5 or 2. The following simple equation gives the algorithm for the ideal efficiency:

In reality, Vout cannot be equal to N.Vin because the minimum voltage drop to ensure the correct biasing of the internal circuits is in the driver itself, but some manufacturers claim efficiencies as high as 95%. When analyzing data sheets, it is important to note that the values ​​in the table are optimal values. Unless the input voltage is completely optimal, it is necessary to check the worst case on the parameter curve. For example, Figure 2 shows the efficiency difference between an inductive boost device (here, the NCP5006) and a charge pump with two multipliers.
As mentioned above, the driver limits the supply voltage to usually 5.5V or 6V, which is a very important limitation in some applications. Fortunately, some inductive boost devices do not need to be connected to the same supply as the inductor, such as the NCP5006-NCP5007. The circuit limits the supply voltage to 5.5V, but the power switch can maintain up to 22V. Due to the different internal architecture of the circuit, it can be powered by a low voltage regulated source in the application, while the inductor needs to be connected to a higher battery voltage.

One important parameter is the noise generated by the LED driver. The charge pump is a source of large current spikes because the capacitors are charged and discharged. To reduce this effect, high-quality input filtering is necessary. Inductive boost devices cause electromagnetic interference (EMI) due to the presence of inductance. Usually, variable frequency interference is less, but the frequency value depends on the operating conditions. System designers usually prefer a slightly higher EMI level, but use a fixed frequency.

Typical parameter values ​​achievable using different LED driver topologies. Obviously, it is possible to find devices that exceed these performances for a certain parameter, but this table is intended to help engineers identify the types of circuits that can meet a range of requirements.