How to Implement LCD Screen PMW Dimming on the Motherboard

      PWM (Pulse Width Modulation) dimming for LCD screens is a common solution for brightness control by adjusting the duty cycle of the backlight source. The core of this method is to generate a PWM signal with a specific frequency and duty cycle on the motherboard, driving the backlight circuit (usually an LED backlight) to turn on and off according to this signal cycle. This method utilizes the human eye's persistence of vision to achieve smooth brightness adjustment. Shenzhen Hongjia Technology engineers, drawing on years of R&D experience, offer the following design ideas for your engineers' reference:

A. Basic Principles of PMW Dimming

Liquid crystal displays (LCDs) do not emit light themselves; instead, they rely on a backlight source (mostly LEDs) to provide light. PWM dimming adjusts average brightness by controlling the ratio of the backlight's on and off times (duty cycle):

· When the PWM duty cycle is 100%, the backlight is always on, achieving maximum brightness.

· As the duty cycle decreases, the backlight is on for only part of the cycle, resulting in a corresponding decrease in average brightness.

· The human eye cannot perceive high-frequency flicker (typically >200Hz) due to its persistence of vision (approximately 24 frames per second), thus achieving smooth dimming.

B. Motherboard PWM Signal Generation

The motherboard must provide a stable and adjustable PWM signal, typically implemented by a main control chip (such as an MCU, SoC, or dedicated backlight controller). The specific steps are as follows:

1. Main Control Chip Selection

· MCUs/SoCs with integrated PWM controllers: These include the ARM Cortex-M series (STM32), mobile phone access points (such as Qualcomm Snapdragon), and TV controllers (such as MediaTek MTK). These MCUs/SoCs have an integrated PWM generator module, allowing software configuration of parameters such as frequency, duty cycle, and polarity.

Dedicated Backlight Driver IC: If the motherboard only needs to drive the backlight, a backlight driver IC with integrated PWM control (such as TI's TPS61088 or NXP's PCA9685) can simplify circuit design.

2. PWM Parameter Configuration

Frequency Selection: A balance must be struck between brightness smoothness and interference. A frequency that is too low (<200Hz) can cause visible flicker; a frequency that is too high (>20kHz) can increase switching losses and potentially interfere with audio circuits. Common range: 100Hz-20kHz (TVs/monitors typically use 1-10kHz, and mobile phones typically use 200Hz-1kHz).

Duty Cycle Range: Typically, 0%-100% corresponds to brightness from 0 to maximum. This must match the linearity of the backlight driver circuit to avoid brightness nonlinearity at low duty cycles.

Resolution: The degree of duty cycle adjustment accuracy (e.g., 8-bit → 256 levels, 10-bit → 1024 levels), which determines the fineness of brightness adjustment (mobile phones/TVs typically require ≥8 bits).

C. PWM Signal Transmission to the Backlight Driver Circuit

The PWM signal generated by the motherboard needs to be transmitted to the backlight driver board (or the backlight circuit directly integrated on the motherboard) via a wired connection. The following issues should be considered:

1. Signal Integrity

· Short-distance transmission (e.g., the motherboard and backlight board are adjacent): Use standard PCB traces (impedance control is not required), but avoid running parallel to high-frequency signals (such as LVDS and eDP) to prevent crosstalk.

· Long-distance transmission (e.g., laptop screen cables): Use shielded cables (such as coaxial cables) or differential cables (such as LVDS), and minimize trace lengths to avoid PWM signal attenuation or distortion.

2. Level Matching

The PWM level output by the motherboard (e.g., 3.3V CMOS) must be compatible with the input level of the backlight driver IC. If the driver IC is powered by 5V, level conversion may be required (e.g., using a 74LVC series logic chip).

3. Isolation Protection

If there is a high voltage between the backlight circuit and the motherboard (for example, cold cathode fluorescent lamps (CCFLs) require high voltage, while LED backlights are mostly low voltage), an optocoupler (such as the HCPL-0723) must be used to isolate the ground loop and prevent the high voltage from interfering with the PWM signal.

D. Backlight Driver Circuit Design (PWM Signal Output Stage)

The PWM signal must be converted to the current/voltage required by the LED by the driver circuit. The core function is to convert the PWM duty cycle into the average current of the LED. A typical circuit includes a constant current drive + PWM modulation architecture, with the following components:

1. Boost/Buck Converter

LED backlights require a stable current (e.g., 350mA to 2A per string). The motherboard power supply is typically 5V/12V, requiring a DC-DC converter to adjust the voltage:

Boost Circuit: When the total voltage of the LEDs in series exceeds the motherboard power supply voltage (e.g., when multiple LEDs are connected in series), a boost chip (such as the TI TPS61020) is used.

Buck Circuit: When the LED voltage is less than the motherboard supply voltage (e.g., a single string of low-voltage LEDs), a step-down IC (such as the TI TPS5430) is used.

2. PWM Modulation Module

The driver IC needs to receive the motherboard PWM signal and adjust the output current based on its duty cycle. Common solutions:

Driver ICs with integrated PWM control: Such as the Maxim MAX16834 and NXP PCA9685, which have an integrated PWM comparator that directly converts the motherboard PWM signal into LED current modulation (no additional components required).

Discrete Solution: Build a PWM switching circuit using transistors/MOSFETs (e.g., using a MOSFET as an electronic switch, with the PWM signal controlling its on/off, thereby adjusting the average LED current).

3. Current Sampling and Feedback (Constant Current Control)

To ensure stable brightness, the LED current must be sampled and fed back to the driver IC for constant current regulation:

A sampling resistor (e.g., 0.1Ω to 1Ω) is connected in series with the LED circuit, and the sampled voltage is amplified by an op amp (e.g., the TI INA219).

The driver IC compares the sampled voltage with a reference voltage and adjusts the PWM duty cycle or the on-resistance of the switch to maintain a constant current (to prevent LED brightness drift due to temperature fluctuations).

F. Key Parameters and Considerations

1. Frequency Interference: The PWM frequency must avoid the pixel clock of the display signal (e.g., LVDS's 75MHz-150MHz) to prevent electromagnetic interference (EMI) and screen distortion. High-frequency noise can be filtered out by adding an RC filter (e.g., connecting a 100pF capacitor in parallel with the PWM signal line).

2. Multi-Channel Dimming: If independent RGB dimming is required (e.g., for high-end monitors), the motherboard must output three independent PWM signals to control the red, green, and blue LED backlights, respectively, to achieve a wider color gamut and greater color accuracy.

3. Soft Start and Protection: The driver circuit must have a soft start function (to avoid instantaneous current surge during startup) and integrated overcurrent protection (OCP), overvoltage protection (OVP), and overtemperature protection (OTP) to prevent damage to the LEDs or driver IC.

      Shenzhen Hongjia Technology specializes in the research and development, production and sales of 1.14-inch to 12.1-inch displays and supporting touch screens. We have a 3,000-square-meter clean room, three fully automatic production lines, and 12 years of industry experience. We have a team of experienced engineers who can provide advice to customers when designing products, thereby simplifying the design of the motherboard PWM. We have displays of various sizes that all support PWM dimming. Customers are welcome to email us for inquiries.