Overview of Methods for Driving LED Backlight
1. Operating Principle of LEDs
Before designing a driver circuit, it is important to understand how an LED operates. The brightness of an LED is primarily determined by its forward voltage (VF) and forward current (IF). The current–voltage characteristic curve is shown in Figure 1. Here, VF represents the forward voltage drop, while IF is the forward current.
Once the applied forward voltage exceeds the threshold level (also known as the turn-on voltage, approximately 1.7 V in this case), IF can be considered nearly proportional to VF. As illustrated in the figure, the maximum forward current of an LED can reach up to 1 A, while the typical forward voltage range is about 2 V to 4 V.
Figure 1. Relationship between VF and IF
The forward voltage drop of an LED can vary over a relatively wide range (more than 1 V). From the VF–IF curve shown above, it is clear that even a small change in VF can cause a large variation in IF, which in turn leads to significant fluctuations in brightness. For this reason, the luminous characteristics of LEDs are generally described as a function of current rather than voltage.
However, in typical rectifier circuits, the output voltage fluctuates with changes in the mains supply voltage. This means that using a constant-voltage source cannot ensure consistent LED brightness and may negatively affect LED performance. Therefore, LED drivers are usually designed to operate as constant-current sources.
2. LED Driving Techniques
From the operating principle of LEDs, it is clear that to maintain optimal brightness, an LED must be driven by a constant-current source. The driver’s role is not only to ensure this constant-current characteristic but also to achieve low power consumption.
To meet these requirements, the commonly used methods of current control include:
- Adjusting the value of a current-limiting resistor to regulate the current.
- Varying the reference voltage across the current-limiting resistor to control the current.
- Using PWM (Pulse Width Modulation) to achieve current regulation.
The techniques employed in LED drivers are very similar to those used in switch-mode power supplies. In essence, an LED driver is a type of power conversion circuit, but its output is a constant current rather than a constant voltage. Under all conditions, the circuit must deliver a stable, average current, with ripple current kept within a specified range.
(1) Current-Limiting Method
Figure 2 shows the simplest circuit that uses the current-limiting method.
Figure 2. Simplest Circuit of the Current-Limiting Method
As shown in Figure 3, this is the traditional circuit configuration. The mains voltage is stepped down, rectified, and filtered, then a series resistor is used for current limiting to keep the LED operating stably and to provide basic protection.
However, the fatal drawback of this approach is that the power dissipated in the resistor R directly reduces system efficiency. Combined with transformer losses, the overall system efficiency is only about 50%. Moreover, when the supply voltage fluctuates within ±10%, the current through the LED can vary by 25% or more, and the power delivered to the LED may change by over 30%.
The main advantage of resistor current limiting is its simplicity, low cost, and lack of electromagnetic interference (EMI). Nevertheless, its disadvantages are significant: the LED brightness changes with variations in VF, the efficiency is very low, and heat dissipation becomes a serious issue.
Figure 3. Traditional Resistor Current-Limiting Circuit
There is also a straightforward article online about the current-limiting method that can be referenced: https://www.ourpcb.com/current-limiting-resistor.html
For more information on constant-current LED backlight driving, see: https://www.orientdisplay.com/wp-content/uploads/2018/07/OrientDisplay-Backlight-Constant-Current-Driver.pdf
(2) Voltage Regulation Method
As shown in Figure 4, this circuit is based on Figure 3, with the addition of an integrated voltage regulator (MC7809). This keeps the output voltage essentially stable at 9 V, allowing the current-limiting resistor R to be made very small, which prevents voltage instability across the LED.
However, the efficiency of this circuit remains low. Since the voltage drop across both the MC7809 and resistor R1 is still significant, the overall efficiency is only about 40%. To achieve both stable LED operation and higher efficiency, low-power current-limiting components and circuits should be used to improve system performance.
The linear voltage regulation method has the advantages of simple structure, few external components, medium efficiency, and relatively low cost.
Figure 4 Voltage Regulation Method
(3) PWM Method
PWM (Pulse Width Modulation) controls LED brightness by adjusting the duty cycle of the driving current pulses. This dimming technique repeatedly switches the LED driver on and off using simple digital pulses. By supplying digital pulses of varying width, the output current can be modulated, thereby changing the brightness of a white LED.
The distinctive feature of this driving circuit is that energy is transferred to the load through an inductor. Typically, a PWM control signal is used to switch a MOSFET transistor on and off. By varying the duty cycle of the PWM signal and the charging/discharging time of the inductor, the ratio between input and output voltage can be regulated.
Common circuit topologies of this type include buck, boost, and buck–boost converters. The advantages of the PWM method are high efficiency and stable performance, but its drawbacks include audible noise, higher cost, and more complex design.
Figure 5. LED Driving Circuit Using PWM Method
As shown in Figure 5, the PWM signal is connected through the base of transistor VQ1 to the gate of a P-channel MOSFET. The gate of the P-channel MOSFET is driven by a simple NPN transistor amplification circuit, which improves the MOSFET’s conduction process and reduces the power consumed by the driver circuit.
If the MOSFET is driven directly by the circuit, the rapid switching on and off of the MOSFET can cause oscillations in the drain–source voltage. This may lead to radio frequency interference (RFI) and, in some cases, expose the MOSFET to excessively high voltages, resulting in breakdown and damage.
To address this problem, a non-inductive resistor is inserted in series between the gate of the driven MOSFET and the driver circuit output. When the PWM signal is at a high level, transistor VQ1 conducts, pulling the MOSFET gate voltage below the source voltage. As a result, the MOSFET turns on, and the LED lights up. Conversely, when the PWM signal is at a low level, VQ1 is cut off, the MOSFET turns off, and the LED is extinguished.
3. LED Driver IC Solutions
LED backlight driver ICs are primarily used in LCD displays (televisions, laptops, mobile phones, automotive screens, etc.) to provide a constant current or constant voltage to the LED modules. Their purpose is to ensure uniform brightness, high efficiency, and long lifetime. Common driver topologies include boost (step-up), buck (step-down), buck–boost, and multi-channel constant-current drivers. Below are several representative categories of LED backlight driver ICs:
(1). Texas Instruments (TI)
- TPS61169: Single-channel boost constant-current driver, suitable for small-size LCDs (e.g., mobile phones).
- LP8556: Supports I²C control, multi-channel output (up to 6 channels), and both PWM/analog dimming. Widely used in laptops and automotive displays.
(2). ON Semiconductor (now onsemi)
- NCP3170 / NCP3170B: High-efficiency buck drivers, suited for small- to medium-size screens.
- NCV7685: 16-channel constant-current driver, often used in automotive backlighting and dashboards, featuring high reliability and diagnostic functions.
(3). STMicroelectronics (ST)
- STLED524: Multi-channel LED backlight driver with I²C interface.
- L5973D: Boost DC-DC converter for medium-power LED backlight systems.
(4). Renesas Electronics
- ISL98611: Integrates boost and positive/negative charge pump outputs, designed for smartphone power and backlight driving.
- ISL97900: Multi-channel LED backlight driver with high-precision current matching.
(5). China Manufacturers
- Macroblock (MBI series): e.g., MBI5030, focused on large-display and backlight drivers, widely used in TVs and advertising panels.
- Solomon Systech: Has released LED backlight driver solutions for mobile phones and small-to-medium displays.
- Summary
- Small-size screens (phones, tablets): TI TPS/LP series, Renesas ISL series.
- Medium-to-large screens (laptops, monitors, TVs): Multi-channel constant-current drivers such as TI LP8556, ST STLED524, Macroblock MBI series.
- Automotive and industrial applications: Require reliability and multi-channel control, typically using onsemi NCV series.
4. Comparison Table of LED Backlight Driver ICs
Manufacturer | Model | Channels | Driving Method | Control Interface | Typical Applications |
TI (Texas Instruments) | TPS61169 | Single-channel | Boost constant-current | PWM / Analog | Mobile phones, small displays |
TI | LP8556 | 6 channels | Multi-channel constant-current with boost | I²C + PWM | Laptops, automotive displays |
onsemi (formerly ON Semiconductor) | NCP3170 | Single-channel | Buck constant-current | PWM | Small- to medium-size screens |
onsemi | NCV7685 | 16 channels | Constant-current | SPI / I²C | Automotive backlight, dashboards |
ST (STMicroelectronics) | STLED524 | 6 channels | Multi-channel constant-current | I²C | Monitors, televisions |
ST | L5973D | Single-channel | Boost DC-DC constant-current | PWM / Analog | Medium-power backlight |
Renesas | ISL98611 | 3 channels + power outputs | Boost + charge pump | I²C | Smartphones, tablets |
Renesas | ISL97900 | Multi-channel | Constant-current | I²C | Laptops, tablets |
Macroblock (明微电子) | MBI5030 | 16 channels | Constant-current | SPI | TVs, large advertising displays |
Solomon Systech (晶门科技) | SSD series (e.g., SSD2805) | 6–8 channels | Multi-channel constant-current | I²C | Mobile phones, small-to-medium displays |
5. Key Parameter Comparison of LED Backlight Driver ICs
Manufacturer | Model | Input Voltage Range | Output Channels | Max Current (per channel) | Efficiency | Package | Typical Applications |
TI | TPS61169 | 2.7–18 V | 1 | 1.2 A | ~90% | SOT-23 | Mobile phones, small displays |
TI | LP8556 | 2.7–5.5 V | 6 | 30 mA | ~90% | WQFN | Laptops, automotive displays |
onsemi | NCP3170 | 4.5–18 V | 1 | 3 A | ~90% | SOIC-8 | Small- to medium-size screens |
onsemi | NCV7685 | 6–40 V | 16 | 75 mA | ~85% | TSSOP | Automotive backlight, dashboards |
ST | STLED524 | 2.7–5.5 V | 6 | 30 mA | ~85–90% | QFN | Laptops, monitors |
ST | L5973D | 4–36 V | 1 | 2 A | ~90% | HSOP-8 | Industrial / medium-power backlight |
Renesas | ISL98611 | 2.5–5.5 V | 3 + power rails | 30 mA | ~90% | WLCSP | Smartphones, tablets |
Renesas | ISL97900 | 2.5–5.5 V | 6 | 25 mA | ~90% | QFN | Laptops, tablets |
Macroblock | MBI5030 | 3–5.5 V | 16 | 80 mA | ~85% | SSOP/QFN | Large TVs, advertising panels |
Solomon Systech | SSD2805 | 2.7–5.5 V | 6–8 | 25 mA | ~85% | QFN | Mobile phones, small-to-medium displays |
Key Comparison Points
1. Number of Channels
o Small screens → Single-channel (e.g., TPS61169)
o Medium screens / Automotive → 6-channel (e.g., LP8556, STLED524)
o Large screens / Televisions → 16 channels or more (e.g., NCV7685, MBI5030)
2. Driving Method
o Boost (step-up) → Common in smartphones and tablets, used to raise low supply voltages to higher levels for driving multiple LEDs in series.
o Buck (step-down) → Better suited for high-voltage power sources driving fewer LEDs.
o Multi-channel constant-current → Ensures brightness uniformity, ideal for large-screen backlighting.
3. Control Interface
o PWM → Simple, widely used in mobile devices.
o I²C → More flexible, allows adjustable current, voltage, and dimming curves.
o SPI → High-speed and multi-channel, well-suited for TVs and advertising displays.
6. Recommended Application Scenarios for LED Backlight Driver ICs
- Small-size screens (smartphones / tablets) → Single-channel boost drivers, e.g., TI TPS61169, Renesas ISL98611
- Medium-size screens (laptops / automotive displays) → 6-channel multi-channel constant-current drivers, e.g., TI LP8556, ST STLED524, Renesas ISL97900
- Large-size screens (monitors / TVs) → 16-channel or higher constant-current drivers, e.g., Macroblock MBI5030
- Special scenarios (automotive / advertising displays) → High-reliability multi-channel drivers, e.g., onsemi NCV7685, Macroblock MBI series