Improving LED conversion efficiency using inductive converter design

LEDs have been widely used in LCD backlighting for cell phones for several years. Today, their use is expanding to large-area LCD applications, including pocket PCs , car navigation GPS, digital photo frames, portable DVDs and even notebook computers. LEDs are also beginning to replace traditional incandescent and halogen light sources in homes, cars and other general lighting sources. The driving force behind this trend is the rapid advancement of technology, including brighter, more efficient and more competitively priced LEDs. In fact, the reason for using LEDs is simple, that is, they have high reliability and longer life, providing end users with maintenance-free products where replacement is not required.

To provide uniform backlighting in LCD applications, several white LEDs are usually installed along one edge of the LCD. The number of LEDs is proportional to the size of the LCD. For medium sizes (7-10 inches), a total of 20-40 LEDs are usually used. These LEDs are usually connected in parallel strings of 3 or more LEDs. To reduce the number of connection points, many LCDs only provide a 2-terminal interface. Here, all LED strings must be connected in parallel internally and then connected to a single power supply.

Driver Types

To achieve the desired brightness in medium-sized LCD applications, the driver is required to provide an adjustable current to the LED under all operating conditions. Two LED driving technologies are commonly used: capacitive charge pumps and inductor-based switching regulators. This article will focus on inductive converter LED driver circuits that can provide 1-6W of power to the LED.

Charge pump LED drivers are now popular in mobile phones and other small-sized LCD backlight applications because of their high brightness, low cost and ease of implementation. The only external components required for the charge pump are 3 or 4 capacitors and no inductor. However, its output power is limited.

Although some high-power flash LED charge pumps can provide up to 2W of power, their maximum output voltage is only 6V, so they cannot drive more than two LEDs in series. The number of channels in the charge pump (usually 6) determines the number of LEDs. Since more channels mean more pins and larger packages, charge pumps limit their application to medium-sized panels.

Different combinations of LED forward voltage (VF), LED current, and supply voltage range determine the type of inductive switching LED driver required. LED VF varies with current, temperature, and LED type. The maximum VF at the lowest temperature is the key parameter for selecting the LED driver circuit structure, usually a linear structure, buck or boost structure. In this article, the maximum VF is assumed to be 3.8V.

When selecting an LED driver IC, key parameters include switch current limit, maximum output voltage, and overvoltage protection threshold required to protect the open LED condition. External components such as inductors and capacitors also need to be carefully selected.

Application Example

Take an 8-inch LCD module as an example, including a total of 9 strings (3 per string) of white LEDs as the backlight (Figure 1). The total voltage of the LED string (LED VF is 3.3V) is typically 10V (3×3.3V). The current of each LED is 20mA, so the total drive current is 180mA (9×20), and the total LED power consumption is 1.8W. A 5V power supply is provided by an AC power adapter. An inductor-based LED driver is well suited for this application.

Figure 1: 8-inch LCD module backlight circuit

First, calculate how much switch current is needed to handle a 2W load. Assuming an efficiency of 80%, the input current is equal to Vout×Iout/Vin×efficiency=10×0.18/5×0.8=450mA. The CAT4139 inductive boost LED driver has a drive capability of 750mA (minimum), so it is suitable for this application.

The current rating of the inductor should be able to handle the peak switch current of the LED driver without entering saturation. Once saturation occurs, current surges will occur because the inductor functions as a resistor and the circuit no longer works as expected. A suitable inductor current rating should be greater than or equal to 80mA. The

maximum output voltage of the LED during operation should be lower than the rated maximum output voltage. With three LEDs in series, the total forward voltage can be as high as 11.4V (3×3.8V) at cold temperatures. The 24V open-circuit LED detection threshold is much higher than its limit. If the LED is disconnected, the output voltage will rise and remain at 30V, and the component is in low-power mode, drawing only a few milliamps from the power supply. A capacitor with a 30V rated output voltage is suitable.

Now consider a 6W LED lamp powered by a 12V supply. This can be achieved by connecting 6 high-brightness white LEDs in series, driven by a fixed current of 300mA, with a typical forward voltage of 3.3V.

The voltage of the LED string is usually 20V, which will increase to 23V (6×3.8V) at cold temperatures. This voltage is too high for components such as the CAT4139. A boost LED driver with a higher voltage, such as the CAT4240, is needed to drive the load. The CAT4240 boost LED driver has an overvoltage detection threshold of 40V and is suitable for LED strings with up to 10 lamps in series.

Choose a buck switching power supply

When the supply voltage is higher than the total LED forward voltage, a linear current source or switching buck regulator can be used to provide a constant current to the LED. However, a linear current source has a drawback that the power dissipated in the regulator is proportional to the voltage difference from the supply to the load. The high efficiency of the switching power supply can avoid any large heat dissipation in the IC, and the operating temperature is close to or slightly higher than the ambient temperature.

Figure 2 shows how to use the CAT4201 to drive five 1W LEDs from a 24V power supply. The LED current is set by the external resistor R1. The CAT4201 buck LED driver uses a two-stage switching operation to provide an accurate average current. In the first stage, the internal CAT4201 FET switch connects the SW terminal to ground, allowing the current to rise and charge the inductor.

Figure 2: Using CAT4201 to drive five 1W LEDs

The voltage across the inductor is essentially 24V minus the voltage drop across the LED. Once the current reaches a predetermined peak, the internal switch turns off and current continues to flow through the Schottky diode until the inductor is discharged.

When the inductor current drops to zero, the process repeats, resulting in a triangular current waveform. In this example, the switching frequency is approximately 260kHz. Capacitor C2 across the LED minimizes the current surge in the LED. Using a larger capacitor will reduce the surge even further. The overall converter efficiency (LED power divided by the power from VBAT) in this example is 94%.

The LED current remains intact during regulation as long as VBAT is above the total VF + 3V. Below this level, the LED current decreases linearly. The appropriate switching regulator and external components must be selected for the specific application. The high efficiency of the switching regulator makes heat dissipation in the power management circuit less of an issue, and the user benefits in energy savings.

Linear current regulator ICs offer inherently low noise performance (no switching), but are only suitable for low current applications due to package temperature limitations. When used to drive medium-sized panels and general lighting applications, inductive converter LED drivers are the solution of choice for well-controlled LEDs and optimal overall luminous efficiency. Choosing the right inductive converter can help improve efficiency, and whether it is actually step-up or step-down depends on the application's power supply and LED configuration structure.