Improve the efficiency of LED lighting with inductive converters
Author: Fabien Franc
Application manager
Catalyst
LEDs have been widely used in LCD backlights for mobile phones for several years. Today's applications are expanding into 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 sources for home, automotive and other general purpose light sources. The driving force behind this trend is the rapid advancement of technology, including brighter LEDs, higher efficiency, and more competitive prices. The reason for actually using LEDs is simply that they have high reliability and long life, providing end-users with maintenance-free products where they do not need to be replaced.
To provide a uniform backlight in LCD applications, several white LEDs are typically mounted along one side of the LCD. The number of LEDs is proportional to the size of the LCD. For medium size (7-10 inches), a total of 20-40 LEDs are typically used. These LEDs are typically connected in parallel with 3 or more LED strings. In order to reduce the connection point, 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.
Drive type
In order to achieve the desired brightness in medium LCD size applications, the driver is required to provide an adjustable current to the LED under all operating conditions. Typically, two LED drive technologies are used: a capacitive charge pump and an inductor-based switching regulator. This article will focus on the inductive converter LED driver circuit that can provide 1-6W power to the LED.
Charge pump LED drivers are now commonly used in cell phones and other small-sized LCD backlight applications because of their high brightness, low cost, and ease of implementation. The external components required for the charge pump have only three or four capacitors and no inductance. However, its output power is limited.
Although some high-power flash LED charge pumps can deliver up to 2W of power, their output voltage can only be up to 6V, so it is impossible to drive more than 2 LEDs in series. The number of channels in the charge pump (usually six) determines the number of LEDs. Charge pumps limit the use in mid-size panels because more channels mean more pins and larger packages.
The different combinations of LED forward voltage (VF), LED current, and supply voltage range determine the type of inductive conversion LED driver required . LED VF varies with current, temperature, and LED model. The maximum VF that occurs at the lowest temperature is a key parameter in selecting the structure of the LED driver circuit, usually in a linear structure, buck or boost structure. The maximum VF is assumed to be 3.8V in this paper.
When selecting an LED driver IC, key parameters include the switch current limit, the maximum output voltage, and the overvoltage protection threshold required to protect the open LED condition. External components such as inductors and capacitors also require careful selection.
Applications
Take the 8-inch LCD module as an example, including a total of 9 strings (3 per string) of white LEDs (Figure 1). The total voltage of the LED string (the VF of the LED is 3.3V) is typically 10V (3 x 3.3V). The current per LED is 20mA, the total drive current is 180mA (9 × 20), and the total LED power consumption is 1.8W. Provide a 5V power supply with an AC power adapter. An inductor-based LED driver is ideal for this application.
Figure 1: 8 inch LCD module backlight circuit.
First calculate how much switching current is needed to process the 2W load. Assuming an efficiency of 80%, the input current is equal to Vout × Iout / Vin × efficiency = 10 × 0.18 / 5 × 0.8 = 450 mA. The CAT4139 inductive boost LED driver has a drive capability of 750mA (minimum), making it ideal for this application.
The current rating of the inductor should be able to handle the LED drive peak switching current without entering saturation. Once saturation occurs, a current surge occurs because the function of the inductor becomes a resistor and the circuit no longer works as expected. A suitable inductor rated current should be greater than or equal to 80 mA.
The maximum output voltage of the LED during operation should be lower than the rated maximum output voltage. Since three LEDs are connected in series, the total forward voltage may be as high as 11.4V (3 x 3.8V) at cold temperatures. The open-circuit LED detection threshold of 24V is much higher than its limit value. If the LED is off, the output voltage will rise and remain at 30V, at which point the component is in low power mode and the current drawn from the supply is only a few milliamps. A capacitor with a rated output voltage of 30V is suitable.
Now consider a 6W LED lamp powered by a 12V power supply. It can be implemented by connecting six high-brightness white LEDs in series, driven by a fixed current of 300mA, with a typical forward voltage of 3.3V.
The LED string voltage is typically 20V and will increase to 23V (6 x 3.8V) at cold temperatures. This voltage is too high for components such as CAT4139. A boosted LED driver with a higher voltage, such as the CAT4240, is required to drive the load. The overvoltage detection threshold of the CAT4240 boost LED driver is 40V, which is suitable for LED strings with up to 10 lamps in series.
Use step-down switching power supply
When the supply voltage is higher than the total LED forward voltage, a linear current source or a switching buck regulator can be used to provide constant current to the LED. However, linear current sources have a drawback in that the power dissipation in the regulator is proportional to the voltage difference from the power supply to the load. The switching power supply is more efficient and can avoid any large heat dissipation on the IC and the operating temperature is close to or slightly above ambient.
Figure 2 illustrates how the CAT4201 can be used to drive five 1W LEDs from a single 24V supply. The LED current is set by the external resistor R1. The CAT4201 buck LED driver utilizes a two-stage switching operation to provide accurate average current. In the first phase, the internal CAT4201 FET switch connects the SW terminal to ground, causing the current to rise and charge the inductor.
Figure 2: Using a CAT4201 to drive five 1W LEDs.
The voltage across the inductor is substantially equal to 24V minus the voltage drop across the LED. Once the current reaches a predetermined peak, the internal switch is turned off and the current continues to flow through the Schottky diode until the inductor discharges.
When the current of the inductor drops to zero, the above process is repeated, and thus the waveform of the inductor current is triangular. In this example, the switching frequency is approximately 260 kHz. Capacitor C2 across the LED minimizes the LED current spike. Using a large capacitor will reduce the glitch to a smaller size. The total converter efficiency (LED power divided by the power from VBAT) in this example is up to 94%.
The LED current remains intact during the regulation as long as VBAT is higher than the total VF + 3V. Below this level, the LED current will decrease linearly. For a specific application, a suitable switching regulator and external components must be selected. The high efficiency of the switching regulator makes heat dissipation in the power management circuit no longer an issue, and the benefit to the user is energy savings.
Linear current regulator ICs offer inherent low noise performance (no switching), but are limited to low current applications due to package temperature limitations. Inductive converter LED drivers are an optional solution for driving medium-sized panels and general-purpose lighting applications, enabling well-controlled LEDs and optimum total luminous efficiency. Choosing the right inductive converter helps increase efficiency, but actually boosting or stepping down depends on the power supply and LED configuration of the application.
(Editor: Yu Zhantao)
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