Design Considerations for Driving LED Backlights on TFT LCDs
An LED (Light Emitting Diode) backlight is an essential component of color TFT (Thin-Film Transistor) LCDs (Liquid Crystal Display), which provides the necessary illumination for the display. Driving an LED backlight can be a challenging task, as it requires a proper understanding of the electrical and electronic principles involved. In this article, we will discuss how to drive an LED backlight for a color LCD.
Before we proceed, it is essential to understand the different types of backlights used in TFT LCDs. The most common types are:
- CCFL (Cold Cathode Fluorescent Lamp) Backlight: This type of backlight uses a tube filled with gas to produce light.
- LED Backlight: This type of backlight uses an array of LEDs to produce light.
CCFL backlights are rarely used nowadays due to their high-power consumption and low efficiency. LED backlights are more energy-efficient, cost-effective, and have a longer lifespan. This article will only discuss driving an LED backlight.
UNDERSTANDING THE BASICS
Before we delve into the details of how to drive an LED backlight for a color LCD, it is essential to understand the basics. An LED is a semiconductor device that emits light when an electric current flows through it. The amount of light emitted by the LED depends on the amount of current flowing through it. The LED has two terminals, namely the anode and the cathode. When a positive voltage is applied to the anode and a negative voltage is applied to the cathode, current flows through the LED, and it emits light.
LED BACKLIGHT DRIVING TECHNIQUES
There are mainly three types of LED backlight driving techniques:
- Constant Current (CC) Driving: In this technique, the current flowing through the LED backlight is kept constant with a resistor. This method is simple and reliable but can require a high voltage to achieve the desired brightness.
- Pulse Width Modulation (PWM) Driving: In this technique, the current flowing through the LED backlight is modulated by varying the pulse width of the voltage applied to the backlight. This method is more energy-efficient and can achieve a higher brightness level compared to CC driving.
- Constant Current Boost Switch Mode Power Supply (SMPS): In this technique, the current flowing through the LED backlight is kept constant by a boost regulator. The boost regulator creates the necessary high voltage of LEDs in series with the energy efficiency of PWM while maintaining a constant current flow.
DESIGNING THE LED BACKLIGHT DRIVER CIRCUIT
The LED backlight driver circuit is responsible for regulating the current flowing through the LED. The driver circuit should be designed to ensure that the LED is not subjected to excessive current, which can damage the LED. The driver circuit should be designed to provide a constant current to the LED, irrespective of the variations in the input voltage and temperature.
The first step is to determine the voltage and current requirements of the LED backlighting. This will depend on the specific LEDs used and the desired brightness.
IMPLEMENTATION OF THE LED BACKLIGHT DRIVER CIRCUIT
The implementation of the LED backlight driver circuit involves the selection of the appropriate components, such as resistors, capacitors, and transistors. The driver circuit can be implemented using a variety of techniques, but for this discussion, it will be limited to SMPS’.
Figure 2: Boost Switch Mode Power Supply (SMPS)
THE INITIAL STEPS INVOLVED INCLUDE:
- Determine the voltage and current requirements of the LED backlighting from the display spec/data sheet.
- Select a driver IC that is compatible with the LED backlight used in the TFT LCD. The driver IC should have built-in protection features like overvoltage protection, overcurrent protection, and over-temperature protection to ensure the backlight’s safety.
- The output voltage of the driver IC should be greater than the maximum voltage of the LED backlight.
- The maximum switch current limit should be greater than the current requirements of the LED backlight.
- The switch current limit can change according to the voltage input of the driver IC.
- Select an appropriate inductor. It plays a critical role in the operation of the boost SMPS. Some parameters to consider:
- Use the switching frequency from the driver IC datasheet.
- Define the input and output voltages.
- Determine the duty cycle.
- Determine maximum load current.
- Assume a ripple current of 10-30%.
- Calculate the inductance value.
- Compute the peak current.
- Calculate the RMS current.
- Finally, select the inductor.
Selecting the right inductor for a constant current boost regulator used on LED backlighting is crucial to ensure the proper functioning and efficiency of the circuit. Consider the following factors when selecting the inductor:
- The inductance value of the inductor is an essential parameter to consider as it affects the current ripple in the circuit. The inductor’s value should be selected to ensure that the current ripple is within an acceptable range, which is typically between 10% to 30% of the maximum output current.
- The inductor’s saturation current rating should be selected based on the maximum current (peak current) that will flow through the inductor. The saturation current rating specifies the maximum current that the inductor can handle before the inductance value starts to decrease. A higher saturation current rating will provide better protection against overcurrent conditions.
- The inductor’s DC resistance affects the efficiency of the circuit as it contributes to power losses in the form of heat. A lower DC resistance will result in lower power losses and better efficiency.
- The physical size of the inductor is an essential factor to consider, especially when designing circuits with limited board space. The inductor’s physical size should be selected to ensure that it fits within the available space on the PCB.
- Price: The cost of the inductor is also an essential factor to consider, as it affects the overall cost of the circuit. As an engineer, consider the trade-off between the inductor’s cost and its performance specifications to select an inductor that meets the design requirements at an affordable price.
THE NEXT STEPS IN THE DESIGN PROCESS ARE:
- Choosing the switching frequency of the boost SMPS affects its efficiency and electromagnetic compatibility (EMC). A higher switching frequency typically results in higher efficiency but may require additional EMC filtering. This is typically limited by the driver IC.
- The switch used in the boost SMPS should be selected based on the maximum input voltage, output current, and switching frequency. Popular options include MOSFETs and IGBTs.
- Design the feedback loop which is used to regulate the output current of the boost SMPS. A current-sensing resistor is typically used to measure the output current, and a control circuit adjusts the duty cycle of the switch to maintain the desired output current. Most of this functionality is internal to the driver IC.
- Choose the input and output capacitors. The capacitors are used in the boost SMPS to store energy and filter noise. The input capacitor should be selected based on the input voltage and ripple current, while the output capacitor should be selected based on the output voltage and ripple voltage.
TESTING THE LED BACKLIGHT DRIVER CIRCUIT
After the implementation of the LED backlight driver circuit, it is essential to test the circuit to ensure that it is functioning correctly. The testing involves the measurement of the current flowing through the LED, the voltage across the LED, and the temperature of the LED. The temperature of the driver circuit should also be evaluated to ensure the temperature does not exceed the design limits. The design may need to be optimized by adjusting component values or changing the feedback loop parameters to improve efficiency, reduce thermal rise, or reduce noise.
Driving an LED backlight for a color LCD requires a proper understanding of the electrical and electronic principles involved. The selection of the LED backlight, designing the LED backlight driver circuit, implementing the driver circuit, and testing the circuit are the essential steps involved in driving an LED backlight for a display. By following these steps, it is possible to design an efficient and reliable LED backlight driver circuit for a color TFT LCD.
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