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Considerations for Driving Led Backlights on TFT LCDs in Design

Views: 233     Author: Reshine Display     Publish Time: 2023-12-07      Origin: Site

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Considerations for Driving Led Backlights on TFT LCDs in Design

An LED (light-emitting diode) backlight is a necessary component of color TFT (thin-film transistor) LCDs (liquid crystal displays) that provide the necessary illumination. Driving an LED backlight can be a difficult task because it necessitates a thorough understanding of the electrical and electronic principles involved. This article will go over how to power an LED backlight for a color LCD. Before we continue, it is critical to understand the various types of backlights used in TFT LCDs.


1. Various types of backlights:

Cold cathode fluorescent lamp (CCFL) backlight: To produce light, this type of backlight uses a tube filled with gas.

LED backlight: An array of LEDs is used to produce light in this type of backlight.

Because of their high power consumption and low efficiency, CCFL backlights are rarely used nowadays. LED backlights use less energy, are less expensive, and have a longer lifespan. Only driving with an LED backlight will be covered in this article.


2. Understanding the fundamentals

Before we get into the specifics of driving an LED backlight for a color LCD, it's important to understand the fundamentals. A light-emitting diode (LED) is a semiconductor device that emits light when an electric current passes through it. The amount of light emitted by the LED is determined by the current flowing through it. The anode and cathode terminals are found on the LED. Current flows through the LED and it emits light when a positive voltage is applied to the anode and a negative voltage is applied to the cathode.


3. Techniques for driving led backlights

LED backlight driving techniques are classified into three types:

Constant current (cc) driving: A resistor is used to keep the current flowing through the LED backlight constant. This method is straightforward and dependable, but it may necessitate a high voltage to achieve the desired brightness.

Driven by pulse width modulation (PWM), the current flowing through the LED backlight is modulated by varying the pulse width of the voltage applied to the backlight. When compared to CC driving, this method is more energy-efficient and can achieve a higher brightness level.

Constant current boost switch mode power supply (smpsLED): A boost regulator keeps the current flowing through the LED backlight constant in this technique. The boost regulator generates the necessary high voltage of LEDs in series while maintaining a constant current flow using the energy efficiency of PWM.


4. Development of the LED backlight driver circuit

The LED backlight driver circuit is in charge of controlling the current flowing through the LED. The driver circuit should be designed to protect the LED from excessive current, which can damage it. The driver circuit should be designed to provide a constant current to the LED regardless of input voltage or temperature variations.


The first step is to figure out what voltage and current the LED backlighting requires. This will be determined by the type of LEDs used and the desired brightness. Click here for Ips Tft Lcd Display.


5. LED backlight driver circuit implementation

The selection of appropriate components, such as resistors, capacitors, and transistors, is required for the implementation of the LED backlight driver circuit. The driver circuit can be implemented using a variety of techniques, but for this discussion, SMPs' will be used.

Boost Switch Mode Power Supply (SMPS)

5.1 The initial steps are as follows:

Determine the LED backlighting voltage and current requirements using the display specification or data sheet.

Choose a driver IC that is compatible with the TFT LCD's LED backlight.

To ensure the safety of the backlight, the driver IC should include built-in protection features such as overvoltage protection, overcurrent protection, and over-temperature protection.

The driver IC's output voltage should be greater than the maximum voltage of the LED backlight.

The maximum switch current limit should be greater than the LED backlight's current requirements.

The switch current limit can vary depending on the driver's voltage input.

Choose the proper inductor. It is crucial to the operation of the boost smps. Consider the following parameters:

Use the switching frequency from the datasheet of the driver IC.

Define the voltages at the input and output.

Establish the duty cycle.

Calculate the 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.


Choosing the right inductor for a constant current boost regulator used in LED backlighting is critical to ensuring the circuit's proper operation and efficiency. When choosing an inductor, keep the following factors in mind:

The inductor's inductance value is an important parameter to consider because it affects the current ripple in the circuit. The value of the inductor should be chosen to keep the current ripple within an acceptable range, which is typically between 10% and 30% of the maximum output current.

The saturation current rating of the inductor should be chosen based on the maximum current (peak current) that will flow through the inductor. The saturation current rating specifies the maximum current that an inductor can handle before its inductance value begins to fall. A higher saturation current rating will provide better overcurrent protection.

The inductor's DC resistance affects the circuit's efficiency because it contributes to power losses in the form of heat. Lower DC resistance equals lower power losses and higher efficiency.

The physical size of the inductor is an important consideration, particularly when designing circuits with limited board space. The physical size of the inductor should be chosen so that it fits within the available space on the PCB.

Price: The cost of the inductor is also an important consideration because it affects the overall cost of the circuit. Consider the trade-off between inductor cost and performance specifications as an engineer to select an inductor that meets the design requirements at an affordable price.


5.2 The following steps in the design process are:

The switching frequency of the boost smps is important for its efficiency and electromagnetic compatibility (EMC). Higher switching frequencies usually result in higher efficiency, but they may necessitate additional EMI filtering. Typically, the driver limits this.

The boost SMPS switch should be chosen based on the maximum input voltage, output current, and switching frequency. MOSFETs and IGBTs are popular choices.

Create the feedback loop that will be used to control the output current of the boost smps. To measure the output current, a current-sensing resistor is typically used, and a control circuit adjusts the duty cycle of the switch to maintain the desired output current. The driver handles the majority of this functionality internally.

Select the capacitors for the input and output. The capacitors in the boost smps are used to store energy and filter noise. The input capacitor should be chosen based on the input voltage and ripple current, and the output capacitor on the output voltage and ripple voltage.


6. Putting the led backlight driver circuit to the test

It is critical to test the LED backlight driver circuit after it has been implemented to ensure that it is working properly. The current flowing through the LED, the voltage across the LED, and the temperature of the LED are all measured during testing. The temperature of the driver circuit should be evaluated as well to ensure that it does not exceed the design limits. To improve efficiency, reduce thermal rise, or reduce noise, the design may need to be optimized by adjusting component values or changing feedback loop parameters.


7. Conclusion

A thorough understanding of the electrical and electronic principles involved in driving an LED backlight for a color LCD is required. The steps involved in driving an LED backlight for a display include selecting the LED backlight, designing the LED backlight driver circuit, implementing the driver circuit, and testing the circuit. Following these steps will allow you to create an efficient and dependable LED backlight driver circuit for a color TFT LCD.

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