LCD TV backlight driver circuit design based on DS3984/DS3988

In LCD TV applications, a variety of architectures can be used to generate the AC waveform required to drive CCFL. The three key design challenges faced when driving multiple CCFLs are selecting the best drive architecture, multi-lamp driving, lamp frequency, and pulse dimming frequency control. This article compares and analyzes four commonly used drive architectures, and proposes methods to solve problems such as uneven brightness and possible interference of the drive frequency in multi-lamp design, and proposes a circuit solution based on DS3984/DS3988.

Figure 1: Royer drives are simple, but not very precise

Liquid crystal display (LCD) is becoming the mainstream display technology for television. LCD panels are actually electronically controlled light valves that need a backlight to produce visible images. LCD TVs usually use cold cathode fluorescent lamps as the light source. Other backlight technologies, such as light-emitting diodes, have also received some attention, but their application is limited due to their high cost.

Since LCD TVs are consumer products, the overriding design consideration is cost—of course, minimum performance requirements must be met. The CCFL converter driving the backlight must not significantly shorten the life of the lamp. In addition, safety is also a factor that must be considered due to the high voltage drive. In LCD TV applications, the three key design challenges faced when driving multiple CCFLs are: selecting the best driver architecture; driving multiple lamps; and strict control of lamp frequency and pulse dimming frequency.

Figure 2: Full-bridge drivers are well suited for a wide range of DC supplies.

1. Choose the best driver architecture

A variety of architectures can be used to generate the AC waveform required to drive CCFLs, including Royer (self-oscillating), half-bridge, full-bridge, and push-pull. Table 1 summarizes the advantages and disadvantages of each of these four architectures.

1.1 Royer Architecture

The best application of the Royer architecture (Figure 1) is in designs that do not require strict control of lamp frequency and brightness. Because the Royer architecture is a self-oscillating design, it is affected by component parameter deviations and it is difficult to strictly control the lamp frequency and lamp current, both of which directly affect the brightness of the lamp. Therefore, the Royer architecture is rarely used in LCD TVs, even though it is the cheapest of the four architectures described in this article.

1.2 Full-bridge architecture

The full-bridge architecture is best suited for applications with a very wide DC supply voltage (Figure 2), which is why almost all notebook PCs use a full-bridge approach. In a notebook, the converter's DC power comes directly from the system's main DC supply, which typically ranges from 7V (low battery voltage) to 21V (AC adapter). Some full-bridge solutions require p-channel MOSFETs, which are more expensive than n-channel MOSFETs. In addition, p-channel MOSFETs are less efficient due to their inherent high on-resistance.

1.3 Half-bridge architecture

Figure 3: A half-bridge driver uses two fewer MOSFETs than a full-bridge driver

The biggest benefit of the half-bridge architecture over the full-bridge is that it uses two fewer MOSFETs per channel (Figure 3). However, it requires a transformer with a higher turns ratio, which increases the cost of the transformer. Also, like the full-bridge architecture, the half-bridge architecture may also use p-channel MOSFETs.

1.4 Push-Pull Architecture

There are many benefits to the push-pull driver: This architecture uses only n-channel MOSFETs (Figure 4), which helps reduce cost and increase converter efficiency; it is easily adaptable to higher converter DC supply voltages; when using higher converter DC supply voltages, it is only necessary to select MOSFETs with appropriate drain-source breakdown voltages. The same CCFL controller can be used regardless of the converter DC supply voltage. This is not possible with full-bridge and half-bridge architectures using n-channel MOSFETs.

The biggest disadvantage of the push-pull topology is that it requires the converter DC supply voltage to have a range of less than 2:1. Otherwise, when the DC supply voltage is at high levels, the system efficiency will be reduced due to the high crest factor of the AC waveform. This makes the push-pull topology unsuitable for notebook computers, but it is ideal for LCD TVs because the converter DC supply voltage is usually stable within ±20%.

Figure 4: Push-pull drivers are very simple and can be precisely controlled

2. Multi-lamp driving

CCFLs have been used for many years in notebook computers, digital cameras, navigation systems, and other devices with small LCD screens. These types of devices usually use only one CCFL, so traditional designs use only one CCFL controller. With the emergence of large-size LCD panels, the need for multiple CCFLs has necessitated a new approach to address this new need. One possible approach is to use a single-channel CCFL controller to drive multiple lamps (Figure 5). In this approach, the CCFL controller monitors the lamp current through only one of the lamps and drives all parallel lamps simultaneously with almost the same AC waveform. However, this approach has several drawbacks.

Figure 5: Using a single-channel CCFL controller to control multiple lamps is not ideal due to uneven brightness and other considerations.

This prevents the display from having noticeable bright and dark areas. Driving all lamps with the same waveform will cause uneven brightness due to differences in lamp impedance. Also, CCFL brightness varies with temperature. Because heat rises, lamps at the top of the panel will be hotter than those at the bottom of the panel, which can also cause uneven brightness.

The second disadvantage of driving multiple lamps with a single-channel CCFL controller is that the failure of a single lamp (e.g. breakage) will cause all lamps to shut down. The third disadvantage is that since all lamps are driven in parallel and turned on and off at the same time, the converter DC power supply must use larger capacitors for enhanced decoupling, which increases the cost and size of the converter.

One way to solve the above problems is to use a separate CCFL controller for each lamp. However, the main disadvantage of this approach is that the additional CCFL controller brings additional costs.

The ideal solution for backlighting LCD panels is a multi-channel CCFL controller, where each channel drives and monitors each lamp independently. This multi-channel CCFL controller not only solves the problem of uneven brightness and single lamp failure, but also reduces decoupling requirements and is cost-effective.

Figure 6: The DS3984/DS3988 drives and monitors each lamp individually to provide uniform brightness for LCD TVs and PC monitors.

3. Tight control of lamps and pulse dimming

Since LCD TVs need to display dynamic and continuously moving images, they have some special requirements that are not present in static display applications (such as computer monitors and notebook computers). The driving frequency of CCFLs may interfere with the image displayed on the LCD screen. If the lamp frequency is close to a certain multiple of the video refresh rate, slow-moving lines or bands will appear on the screen. This problem can be eliminated by strictly controlling the lamp frequency within ±5%.

The pulse dimming frequency used to adjust the brightness of the lamp also requires the same strict control. This dimming method usually uses a pulse width modulation (PWM) signal with a frequency range of 30Hz to 200Hz to turn off the lamp for a short period of time to achieve the purpose of dimming. Because the off time is very short, it is not enough to eliminate the ionization state. If the pulse dimming frequency is close to the multiple of the vertical synchronization frequency, rolling lines will also be generated. Again, strictly controlling the pulse dimming frequency within ±5% can eliminate this problem. In addition, in some LCD TVs, in order to improve the image response of the LCD screen, the slow CCFL pulse dimming frequency is also required to be synchronized with the video vertical synchronization frequency.

Figure 7: Each channel of the DS3984/DS3988 can also drive multiple lamps

4 Solutions to Solve LCD TV Backlight Challenges

The DS3984 (quad-channel) and DS3988 (octal-channel) CCFL controllers address all of these design challenges. These devices can be configured to drive one lamp per channel (Figure 6) or multiple lamps per channel (Figure 7), giving users the flexibility to tailor their designs to meet their price/performance goals. Multiple DS3984/DS3988s can be easily cascaded to support any number of lamps to backlight LCD TV screens.

The DS3984/DS3988 uses a push-pull drive architecture that allows the use of lower-cost, more efficient n-channel MOSFETs. The converter DC supply voltage can also use a higher voltage. Individual lamp control and monitoring can provide uniform brightness and reduce the total number of components in the converter. When using individual lamp control, if a lamp fails, only the failed lamp will stop working, and other lamps will continue to work and will not be affected. The lamp frequency and pulse dimming frequency generated by the on-chip oscillator are strictly limited to the ±5% level, eliminating the impact on the displayed image, and can also be synchronized to an external clock source.

5 Cold cathode fluorescent lamp

A cold cathode fluorescent lamp (CCFL) is a long, thin, sealed glass tube filled with an inert gas. When a high voltage is applied to the tube, the gas is ionized, generating ultraviolet (UV) light. The UV light hits the fluorescent material coated on the inner wall, exciting it and emitting visible light.

CCFL has many advantages, including: it is an excellent white light source; low cost; high efficiency (ratio of light output to input electrical power); long life (>25k hours); stable, deterministic working state; easy to adjust brightness; light weight.

CCFLs have some special properties that must be carefully considered to maximize their efficiency, life, and usefulness. However, these characteristics bring some special design challenges. For example, to maximize the life of the lamp, CCFLs need to be driven with an AC waveform. Any DC component will cause a portion of the gas to accumulate at one end of the lamp, causing an irreversible light gradient that makes one end of the lamp brighter than the other. In addition, to maximize its efficiency (the ratio of light output to input electrical power), the lamp needs to be driven with a waveform that is close to sinusoidal. Therefore, CCFLs usually require a DC-AC converter to convert the DC supply voltage into an AC waveform of 40kHz to 80kHz, and the operating voltage is usually 500VRMS to 1000VRMS.

The lamps in an LCD TV are evenly spaced across the entire LCD backplane to provide optimal light distribution. It is important that all lamps operate at the same brightness. Although diffusers are placed between the CCFL lamps and the LCD panel to help evenly distribute the backlight, uneven lamp brightness is still easily noticeable and affects the TV's image quality. Depending on the size of the LCD panel, the number of CCFL lamps used may be as many as 30 or even 40.