The Peripheral Systems
Maximizing the mobile device's viewing time remains a prime design objective. Beyond the system's basic battery energy capacity, the designer's task is to maximize the efficiency of converting that energy. Not many years ago, a stray 10 milliwatts would have been of little concern, because there would be other power consumers in the system using hundreds of milliwatts. But today, a 10-milliwatt consumer is a significant power sink. A large contribution to the success of Apple's iPhone, for instance, is its outstanding efficiency: 8 hours talk time, 6 hours Internet viewing, and 250 hours in standby mode.
To achieve numbers like these, an optimized mobile multimedia device will exploit every available mechanism for significant power savings. This includes adaptive voltage scaling, which throttles the power supply voltage of power-hungry active subsystems to meet the ever-changing demands for clock cycles. This technique is being used to throttle the power supply voltage of CPU and DSP cores in response to activity-driven changes in clock speed, but it also could be applied to high-power peripheral cores such as a graphics or video processor.
High-performance multimedia devices will also have high-bandwidth interfaces to multimegapixel camera chips and LCD displays, which will require high-bandwidth serial interfaces optimized for low electromagnetic interference (EMI). Backlights will be driven by RGB LED modules, which provide both high efficiency and the ability to adjust the color balance of the backlight.
Speaker drivers will implement bridge configurations and charge pumps to avoid DC-blocking capacitors which are expensive, consume board area, and reduce bass-region audio performance. Sensors at the audio jack are recommended to detect short-circuits and near-shorts.
Low-power display architecture
The display represents an important consumer of energy, not only in the display panel itself, but also in the video interface, video controller, and backlight.
• Mobile pixel link (MPL) interface—provides a high-bandwidth video interface with reduced wire count and emitted electromagnetic interference (EMI).
• Self-refreshing mode—allows shutdown of the video interface and video controller when high-quality video is not needed, for example when the user is listening to music or handling a cellphone call. An on-chip frame buffer memory supports a lower resolution image useful for clocks, music track selection, phone books, and instant messaging.
• RGB LED backlight driver—provides improved white balance at lower power across the full range of brightness levels. Automatically adjusts brightness and white balance to ambient temperature and light conditions.
Mobile pixel-link (MPL) interface
The MPL provides a low pin-count, low EMI, energy-efficient interface for bit-mapped displays. Several features of MPL contribute toward these goals:
• Fewer signal lines—replaces parallel video data buses with a serial interface, typically reducing as many as 28 signal lines to only three or four. This both simplifies the interconnect wiring (typically a flat cable or flex-circuit between the main circuit board and a flat-panel display module) and reduces the number of EMI-emitting antennae.
• Reduced switching current—current-mode signalling reduces the switching current by over an order of magnitude, as compared to TTL and LVCMOS levels.
• Reduced voltage swing—signals have a voltage swing of only 20 mV, as compared to 1.8 volts for TTL and LVCMOS.
Figure 5 shows the architecture of an MPL interface to a flat-panel display driver. The MPL interface is suited to high-bandwidth video, while the SPI interface provides access to the registers of the display driver. When video data is not being transferred, the MPL interface can be shut down to further reduce power consumption.
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Fig. 5: Mobile pixel-link interface
An MPL serializer such as the LM2512A provides an interface between a parallel video bus from a video controller and MPL. The LM2512A can serialize 24-bit RGB video, which is dithered to 18 bits for transmission over MPL, and up to three control signals. Three on-chip 256-by-8 look-up tables provide the option of independent color correction for each color. An SPI interface is used to program the LM2512A's look-up tables and control registers.
The FPD95120 flat-panel display driver shown here has an integrated MPL receiver. For display drivers that lack an MPL interface, an MPL deserializer can be used to regenerate 18-bit parallel video.
Self-refreshing mode
The FPD95120 includes an on-chip 230-kbit partial display memory for refreshing the display when there is no video input, which allows the MPL interface to be shut down. There still may be a need for showing text for instant messaging or a playlist of MP3 tracks; the FPD95120's display memory can be accessed through an SPI interface while the MPL interface is shut down. It supports refreshing a 240-by-320 pixel lower resolution image display with 3 bits per pixel or a 320-by-720 region with 1 bit per pixel.
RGB LED backlight driver
A video-quality display requires a pure white light source that remains white across a wide range of brightness settings, temperature, and multi-sourced flat-panel display vendors. The conventional white LED solution only offers a fixed color balance determined by the white LED vendor. An RGB LED light source synthesizes white light by combining the output of red, green, and blue LEDs, which provides the opportunity to adjust color balance by pulse-width modulation of the driver for each primary color.
(Click on Image to Enlarge)
Fig. 6: RGB LED backlight driver
The LP5520 RGB backlight driver used here (Fig. 6), for example, has a user-programmable calibration memory that holds the brightness vs. temperature curve for each LED color in 16°C increments from -40 to +120°C. If we combine an LM20 temperature sensor mounted close to the LEDs, the LP5520 can automatically maintain the white balance across a wide temperature range. A second input to the on-chip 12-bit A/D converter can be used for an external photodiode to monitor the ambient light level. A host microcontroller sets the LED intensities by accessing the LP5520's control registers through its I2C/SPI interface.
The LP5520's boost converter accepts a 2.9- to 5.5-volt input voltage range and generates an output voltage programmable in increments of one volt from 5 to 20 volts. An adaptive mode saves energy by monitoring the LED driver outputs and reducing the boost voltage to the minimum effective value.
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