Video drivers in low-power applications.
The two main considerations for power consumption in portable devices are operating current and standby current. Both are important; keeping operating current to a minimum is essential to long overall battery life, while standby current is especially important when dealing with auxiliary functions. For example, driving an external TV monitor from a personal multimedia player (PMP) would be considered an auxiliary function, as video from the PMP is typically viewed on the PMP's screen, not on an external monitor.
Video outputs are becoming more prevalent in today's portable electronic devices. Depending on the PMP, video outputs can range from composite to S-Video to component video. Inexpensive PMPs typically feature standard-definition composite video outputs, so this article will highlight a few composite video driver applications.
Power Savings with a Video DAC In a Low-Power Mode
A video driver provides the interface between the PMP's video DAC and the outside world. In applications that are not power sensitive, the video DAC can directly drive a video output line. In portable applications, however, it is much more efficient to run the video DAC in a low-power mode, using an external low-power amplifier (video driver) to drive the video output line. A 40 percent savings in power consumption (versus the video DAC driving the load directly) is not uncommon for this configuration. From a practical standpoint, it also makes sense to use an inexpensive external driver to buffer a valuable video DAC, especially where the IC may have some exposure to the outside world.
The video driver is typically configured as an active filter, also known as a reconstruction or anti-imaging filter. This low-pass filter can be discrete or an integrated solution; both options will be discussed. The purpose of the reconstruction filter is twofold: it blocks the higher frequency components (above the Nyquist frequency) that were introduced into the video signal as part of the digitization process, and it provides gain to drive the external 75[ohm] cable to the monitor. A typical composite video signal has a bandwidth of approximately 4.5 MHz. To ensure faithful signal reproduction, the filter must have a flat response out to at least 6 MHz. The filter should also provide good rejection at 27 MHz, a common sampling frequency used in video DACs.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Designers using discrete reconstruction filters have the benefit of selecting low power amplifiers to craft filters that meet their specific needs. The drawback to this method is the area and number of components required. As in most designs, tradeoffs need to be considered; in this case, power consumption versus area. Figure 1 shows the schematic for a discrete three-pole, 6 MHz Sallen-Key LPF with 6 dB gain using an ADA4853-1 low-power, high-speed video amplifier. Consuming only 1.3 mA in operating mode and 100 nA in standby mode, it is suited for portable applications. As previously mentioned, devices used for auxiliary functions should have as low a standby current as possible to minimize power consumption. In this type of application, the amplifier and other video processing devices are all shut down when not needed.
Consider Integrated Solutions
To help alleviate the dilemma of choosing between power consumption and size, integrated solutions should be considered. These devices include a gain block, filter and, in some cases, a charge pump and load detector. Portable devices typically run from a single supply (battery), so no negative supply is available. This is important when designing DC-coupled applications. The sync tip of a video signal extends below ground by approximately 300 mV. In order to achieve good video integrity, the synctip voltage must be maintained. In DC-coupled systems this isn't a problem, as the DC level can be restored later. But in DC-coupled applications, the DC level must be maintained throughout the signal path.
For example, the ADA4431-1 video filter integrates a third-order low-pass filter, gain of +2 amplifier, charge pump and load detector into a 3 mm X 3 mm LFCSP package. The total supply current with the charge pump on is 4.7 mA; with the charge pump off, the current drops to 1.6 mA --rivaling that of the discrete design. As a "smart part," it powers down without prompting from the system or user. The ADA4431-1 video filter is equipped with a load detector feature. The load detector senses whether there is a load present at the output.
If it detects a short circuit or no load at the output, the device shuts down and provides a low logic level on its load detect output (LDO) pin. The LDO pin signals the microcontroller to shut down all ICs in the signal path for maximum power conservation. When a load is returned to the output, the LDO pin goes high, enabling the rest of the signal path to be powered back up. Figure 2 shows a typical schematic for the ADA4431-1 video driver/reconstruction filter.
When designing portable electronics, keep trade-offs in mind. This article discussed trade offs in size, power consumption (both operating and standby), flexibility, simplicity and intelligence. Familiarity with the available options is critical in making the best decision for any given application.
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by John Ardizzoni, Analog Devices www.analog.com
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|Title Annotation:||Low power Design|
|Publication:||ECN-Electronic Component News|
|Date:||Mar 1, 2008|
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