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Embedded design energy efficiency with lower power LCD operation

Don Shannon, Silicon Laboratories

December 16, 2012

Don Shannon, Silicon LaboratoriesDecember 16, 2012

Editor’s Note: In this Product How-To design article, Silicon Labs‘ Don Shannon describes the constraints facing developers of energy-efficient home appliance consumer designs that make use of liquid crystal displays, which can draw significant power if not implemented properly. He outlines how the company’s SIM3L1xx MCUs can be used to improve energy efficiency and lower power.

We are all familiar with small liquid crystal displays (LCDs) that are found everywhere - in household appliances, consumer electronics, cars, toys, personal medical devices, and many other products.. LCDs have been popular for a long time and will continue to be used in countless electronic devices. Embedded LCDs provide effective user feedback while being cost-effective, power-efficient, and easy to design into embedded products.

LCDs are commonly sourced as either modules or raw displays. Modules contain a display, a driver and interconnect between the two components. The driver included with the module typically interfaces with an MCU through a serial bus. Another option is to design with a raw display and an MCU that includes a display driver, eliminating the fixed-function LCD driver from the system and freeing up a serial bus. This approach has numerous advantages over a module, such as greater design freedom, reduced cost, and lower power. Because many low-power and cost-sensitive applications include LCDs, there is a large selection of MCUs that integrate an LCD driver. An integrated LCD driver gives full access to the controller, allowing a high degree of optimization for the display control and performance.

Custom LCDs provide an excellent way to differentiate an embedded product. Off-the-shelf segment and dot-matrix screens have their place, but displays with custom symbols, shapes, and arrangements enhance the user experience and make an embedded product stand out. A custom display has a unique number of LCD segments, but many LCD controllers support a fixed number of LCD pins. The unused LCD pins are useless and increase power consumption due to unnecessary toggling.

One way to solve this problem is through the use of integrated MCU designs such as the Precision32 SIM3L1xx, which incorporates a patented crossbar architecture that allows developers to configure unused LCD pins as GPIOs that can be mapped to various analog and digital peripherals.

Understanding how LCD displays operate will enhance the developer’s ability to optimize the driver settings. An LCD basically consists of two polarizing lenses, two electrodes and a liquid crystal. An electric field applied between the electrodes changes the orientation of the liquid crystal, effectively controlling the transmission or reflection of light and making the segment appear light or dark, as seen in Figure 1.



Click on image to enlarge.

Figure 1: LCD construction and operation

One important property is that segments should not be driven by a dc voltage because it will significantly reduce the lifetime of the display. Therefore, LCD segments are driven by an ac signal, typically a square type wave at 60 Hz. For example, to turn on a segment, the electrodes are driven by a 3 V peak square wave centered at 0 V. The segment is driven between +3 V and -3 V by switching the electrodes between 3 V and 0 V. The dc voltage is 0 V, and the ac magnitude is 6 V peak-to-peak, as illustrated in Figure 2.

Figure 2: LCD segment switching

Other important parts of LCDs are the segment and common pins. The total number of segments in the display can be as high as the product of these two numbers. For example, a 4 x 32 display has 4 common and 32 segment pins and can have up to 128 display segments. Figure 3 shows how the LCD segments are connected between segment and common pins. Because the pins are shared between multiple display segments and must be driven by an ac signal, the waveforms on the pins have an odd appearance. Fortunately, this is all handled by the display driver.


Figure 3: LCD with segment and common pins

One major appeal of using LCDs in embedded applications is their low power consumption. The display does not consume any dc power and does not emit light, making the power consumption orders of magnitude less than other types of displays. All of the power consumed by an LCD is dynamic because the segments are driven by an ac signal. Even if the display is not changing, it still consumes power. Each segment can be viewed as a capacitor, so the average current consumption is CVf where C is the capacitance of the display, V is the ac amplitude and f is the switching frequency.

Power consumption can be optimized by adjusting C, V, and f. Capacitance is a fixed property of the display and interconnects between the driver and LCD. Locating the driver close to the display will help minimize parasitic capacitance of interconnections. The voltage level controls the contrast of the LCD, putting a limit on the minimum voltage that can be used. As the voltage is reduced, so is power consumption, but the contrast goes down, making the display difficult to read. The last adjustable parameter is frequency. As the frequency is decreased, the power will decrease, but the frequency can become too low and cause the display to flicker and become difficult to view. Constraints on the operating voltage and frequency of LCDs limit the minimum power required by an LCD.

For the display to be easily viewed, the system may require an LCD voltage that is higher than the available supply voltage. For this reason, many LCD controllers include a charge pump that can generate a voltage higher than a supply. The efficiency of the charge pump will directly affect power consumption, so it is important to choose an MCU solution with a high-efficiency LCD charge pump integrated on chip.

In many battery-powered systems, the power required by the LCD is a significant driver of cost and size. Reducing power consumption can result in a smaller battery, lower cost, smaller form factor, and a greener solution.

One design that has taken this integrated LCD charge pump approach is our SIM3L1xx MCU. It has been optimized for use in battery-powered systems, first by integrating the LCD driver on chip with the MCU, reducing LCD display current by approximately 40 percent. We use a patent-pending LCD controller design that intelligently resets all LCD segments before each switching cycle to minimize the amount of charge required from the battery during each LCD segment transition.

All of this is handled automatically by the innovative LCD controller. The total power saved by this approach scales with the display, reducing the power of any LCD. Figure 4 shows actual lab data from a real-world example where the total system current was reduced by more than 30 percent when the new LCD segment resetting mode was enabled.


Figure 4: Real-world power savings

Conclusion
LCDs continue to provide an excellent user interface for countless electronic products. They are economical, power efficient, and easy to add to any embedded design. Silicon Labs’ SIM3L1xx MCUs are an optimal choice for a battery-powered system with a display because the MCU's integrated LCD controller reduces the display power by up to 40 percent, thereby reducing the power consumption, size, and cost of embedded designs.

Don Shannon is a Senior Design Engineer in Silicon Laboratories’ microcontroller division where he is involved in the design and implementation in applications of the company’s SIM3L1xx MCUs. Mr. Shannon specializes in analog and mixed signal IC design and has extensive experience with low power analog circuits and mixed signal verification. He joined Silicon Labs in 2009 and holds a master’s and bachelor’s degree in Electrical Engineering from Texas A&M University.

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