New driver technology helps reduce power consumption of LCD TVs

The results of the SEAD project show that TVs account for about 3% to 8% of global household electricity consumption. Analysis by the Lawrence Berkeley National Laboratory in the United States shows that in the next few years, advances in TV technology (such as improved LED drive efficiency) will help significantly reduce TV power consumption.

There seems to be no doubt that LCD technology with LED backlighting is the only viable way to achieve the energy efficiency goals advocated by the government. Plasma TV technology has the following disadvantages: each pixel is an active light emitter, and the number of pixels on the TV screen directly determines the power consumption of the TV. With the same screen resolution and brightness, a high-definition plasma TV consumes 2 to 3 times more power than an LCD TV.

According to recent reports, the previously highly touted OLED (organic light-emitting diode) technology is unlikely to be rapidly adopted. This is because this cutting-edge technology for large-screen TVs requires huge investments. However, if large display panels use the most advanced TFT-LCD technology available today and "intelligent" direct LED backlighting with local dimming, the cost will be much lower than OLED, while the power consumption and image quality are comparable. The LCD TVs currently produced (even with LED backlighting) will still have difficulty meeting the energy-saving requirements set in the next few years. However, new design techniques for LED driver circuits are expected to significantly reduce power consumption, allowing TV manufacturers to meet stringent power consumption requirements.

TV power consumption standards continue to improve

TV standards (such as Energy Star) were first developed in 2008, and the power consumption of TVs in the standards has been reduced year by year since then. The current standard stipulates that the maximum power consumption of any size screen is now 85W. This standard has brought greater challenges to the design of large-screen TVs.

Energy Star is an international standard for energy-efficient consumer products that has great influence. Manufacturers can decide whether to comply with the standard. In addition, there are many similar standards. For example, the California Energy Commission in the United States has set its own power consumption standards, which came into effect in 2011. The energy consumption standards set by the California Energy Commission are stricter than the Energy Star standards. The standards stipulate that products that do not meet the energy consumption standards are prohibited from being sold in California.

In Europe, regulations have for many years allowed for direct comparisons of power consumption of white goods (EU power consumption labelling), which consumers can use to base their purchasing decisions. These regulations are now mandatory for TVs, cars and home appliances.

LED backlight

In the total power consumption of LCD TVs, the power consumption of LED backlight components accounts for 30% to 70%. Therefore, to reduce the power consumption of the entire TV, we must first try to reduce the power consumption of LED backlight components. Sometimes, as long as the operating efficiency of certain components is slightly improved, the total energy consumption of the equipment will be greatly reduced. This situation is very common in the field of power system design.

There are two ways to implement LED backlighting (as shown in Figure 1): indirect backlighting and direct backlighting. If indirect backlighting technology (also called edge-lit backlighting) is used, the LEDs are arranged at the edge of the screen. A light guide distributes the light evenly to the entire screen. This structure has good lighting consistency for screens within 60 inches and can achieve a backlight unit with a thickness of only 5mm~10mm.

Figure 1: LED backlighting can be implemented in two ways in LCD TVs: indirect backlighting (also called edge-lit backlighting, as shown in Figure a) and direct backlighting (as shown in Figure b).

If direct backlighting is used, the LEDs are placed directly behind the LCD screen. This approach not only consumes less power and is easy to dissipate, but also has good scalability, especially with no restrictions on screen size. Compared with edge-lit backlighting, LCD screens with direct backlighting are generally thicker. But with advances in light distribution technology, some screens are now as thin as 8mm. A prominent advantage of direct backlighting is that it allows for perfect local dimming. Local dimming can both reduce the TV's power consumption and improve dynamic contrast, making the latest TV designs comparable to OLED TVs.

System Architecture

The choice of an LED backlight drive system architecture can maximize energy savings and greatly improve image quality. Designers need to find an optimal balance between local control of the LED string and a minimal bill of materials (BOM).

In a single-string, single DC/DC converter backlight system, a switching power supply (SMPS) provides power to the backlight LEDs arranged in a string. A current sink is used to regulate the current through the LED string. To minimize power consumption, the voltage of the ILED sink must be slightly higher than the required voltage to ensure that the LED receives its specified current (as shown in Figure 2).

Figure 2: To minimize power consumption in a single-string, single DC/DC converter backlight system, the voltage at the ILED well must be slightly higher than required to ensure the LEDs receive the specified current.

In order to achieve the above requirements, it is usually necessary to place a feedback circuit between the ILED sink and the SMPS to adjust the voltage output by the SMPS. This feedback circuit must be able to withstand the fluctuation of the forward voltage (VF) between the LEDs. Generally, the rated forward voltage flowing to the white light LED is about 3.2V, with a floating range of ±200mV. Therefore, if a string of LEDs contains 10 LEDs, the total rated voltage of the LED string (VLED total) is 30V~40V.

The required voltage of the DC/DC converter can be expressed as: VDC-DC = VLED + VSINK, where VLED = n × VF (LED). Assuming VSINK is 0.5V, the ILED well needs to adjust the VDC-DC range within 30.5V~34.5V, depending on the actual LED forward voltage.

Single string LEDs are generally not suitable because as the number of LEDs in the string increases, the required output voltage also increases. When the VOUT/VIN ratio exceeds a certain value, the efficiency of the SMPS will drop significantly. In order to prevent the SMPS output voltage from being too high, LED backlight system designers generally use multiple LED strings.

The simplest approach is to completely repeat this single-string, single DC/DC converter structure for each string (Figure 3). The advantage of this approach is efficiency, because the voltage of each string of LEDs can be adjusted individually. The disadvantage is higher cost, because each string of LEDs requires its own DC/DC converter, MOSFET, coil, diode, and output capacitor. To save BOM cost, designers can reduce the number of LED channels and use long strings of multiple LEDs on a single string. However, this approach will also affect the local dimming function of the system, which is another important technology for reducing TV power consumption. Therefore, the trade-offs of this structure are not very attractive.

Figure 3: Using a separate DC/DC converter for each LED string is a more expensive option.

A more effective way to reduce BOM cost is to use a multi-LED string + single DC/DC converter structure (Figure 4). However, this method also has its own disadvantages, that is, the voltage output of the SMPS must be regulated at a voltage higher than the maximum forward voltage of the LED string, which means that its operating voltage is higher than the voltage requirements of those strings with lower forward voltages. As a result, the ILED well must consume the excess voltage of the LED string with a lower rated voltage, and the heat generated must be conducted away from the circuit board, which reduces power efficiency.

Figure 4: With one DC/DC converter for multiple LED strings, the SMPS voltage is not optimal.

The architecture that strikes the best balance between efficiency and BOM cost is one that combines elements of the aforementioned multi-string and multi-DC/DC converter architectures. This hybrid architecture (Figure 5) has multiple DC/DC converters powering groups of LED strings.

Figure 5: The hybrid architecture approach achieves the best balance between BOM and power efficiency.

This multi-string hybrid architecture solution provides the best overall energy efficiency because it combines the advantages of local dimming of direct-lit backlight systems with good DC/DC output voltage regulation. In addition, its BOM cost is much lower than the equivalent multi-string and multi-DC/DC converter architecture.

Current Regulation

Due to the production process, the brightness and color temperature of each LED produced by the manufacturer vary greatly. For the convenience of users, white light LED manufacturers classify LEDs with similar performance (such as color, brightness and forward voltage) into groups (or "batches", bins). However, the manufacturer divides the brightness and color temperature of each LED according to specific nominal operating conditions. This means that in order for the LED to emit a specific brightness and color, the current passing through the LED must be set to the nominal current indicated in the data sheet.

Therefore, dimming and brightness control can only be achieved by switching the current sent to any individual LED on (with nominal current) or off (zero current) through a digital PWM control signal. If analog dimming is used, the LED will operate beyond its specified nominal current range, which will produce unacceptable color temperature changes and poor brightness matching between individual LEDs (Figure 6).

Figure 6: The brightness of LEDs from the same batch is only guaranteed to match at the nominal current (in this case, 20mA.)

Current Sink Performance

LEDs require a completely stable constant current power supply when they are working, so the main function of the LED driver is to adjust the current to the nominal value when the LED is on and to adjust the current to 0 A when the LED is off. Therefore, the feedback loop used to control the regulation accuracy requires a high-precision current sink (Figure 7).

Because there are many current sink design styles, according to the accuracy requirements of TV backlight, a precision op amp is used to independently set the ILED current regardless of the ILED voltage. However, the task of backlight driver applications is more difficult because the accuracy of current regulation must be maintained even when the current in the current sink becomes extremely low.

This requirement is not easy to achieve, but the four generations of precision current sink LED drivers designed by AMS are specifically designed for these applications. The AS369x, AS381x, AS382x, and AS385x devices all have an offset compensation operational amplifier. The current sink driver requires a minimum drain voltage (VDS(SAT)) to ensure full accuracy and proper operation of the current sink transistor in the saturation region. In the saturation region, the output current is mainly controlled by the gate-source voltage.

If the current sink is to operate efficiently, it is important that the voltage drop between VSET and VDS is small. LED drivers with built-in offset suppression op amps can keep VSET as low as 125mV~250mV. To allow VDS to have an additional margin of 150mV above VDS (SAT), the current sink must have a total voltage drop of about 400mV. If a string of LEDs contains 8 LEDs (VF=8×3.2=25.6V), ISINK will produce a power loss of about 1.5%. If the AMS backlight LED driver does not support voltage offset suppression, the required VSET will be higher, so the power loss in the current sink will be greater.

Power Optimization

As mentioned above, the feedback loop from the LED driver to the SMPS sets the drain voltage to a minimum. There are two ways to implement the output current sink: one is to use a simple deterministic current output driver and an external capacitor (Figure 7a); the other is to use a digital control circuit to set the attack/release time and use a digital-to-analog converter (IDAC) to control the current output (Figure 7b).

Both schemes offer high efficiency and are suitable for all types of SMPS with voltage feedback. It is also possible to connect feedback lines from multiple drivers to the same SMPS, as required by mixed-architecture systems. However, the second digital implementation has some special advantages. In addition to eliminating the need for output capacitors, digital circuits free the designer from defining the start and decay times of the feedback system. Combining a fast start time with decay delay with a relatively slow decay can improve the performance of the display.

The advantages of the above digital circuits are more obvious in scenarios where rapid changes in brightness are required. At this time, when the screen changes from dark to full brightness, the fast start-up time eliminates the perceptible false brightness effect. The analog solution gradually adjusts the output of the LED during a short black frame, so if the next bright frame is full brightness, there will be a perceptible delay.