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AD723ARU-REEL датащи(PDF) 11 Page - Analog Devices |
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AD723ARU-REEL датащи(HTML) 11 Page - Analog Devices |
11 / 20 page REV. 0 AD723 –11– Power-Down Load Checking One of the main uses of the TVDET signal is for plug-and-play operation. When this feature is used, a VGA controller or other IC polls the AD723 at regular intervals (such as once per second) to see if a load has been attached to either output. If a load is found, active video and sync signals can be generated for TV encoding if CE is held. To facilitate this use, the AD723 supports sleep-mode load checking while powered down. This feature is activated with the timing sequence shown in Figure 5. CE is temporarily raised high while a single full-width horizontal sync pulse followed by a single half-width horizontal sync pulse are applied. The spacing between these two pulses should nominally be one H. Load check- ing is performed just after the half-width pulse (this simulates the beginning of the vertical blanking interval) and the TVDET signal becomes valid approximately 18 µs after the pulse’s lead- ing edge (for both NTSC and PAL). CE is held high until TVDET is valid and is then pulled low to avoid powering up the rest of the chip. To make this mode possible, the AD723 is designed to activate only the digital and sleep mode load check sections of the IC when CE is initially pulled high. The rest of the chip is only activated when CE remains high for four consecutive rising edges of CSYNC. CE (POWER DOWN) CSYNC TVDET H = 63.5 s TIME LEGEND:NTSC (PAL) 2.3 s 18 s 4.7 s LOAD CHECK TEST CURRENT LOAD CHECK EVAL PULSE (8.2 s) 8.2 s (14.3 s) 15.9 s (7.3 s) 9.1 s (0.9 s) 1.1 s CE = HIGH (AWAKE/SLEEP) Figure 5. Timing Diagram for Load Check The advantage of this two-tiered power-up sequence is that the total time required to poll for TV presence is kept short, and standby power is kept low. When the entire chip is powered up, a settling time as long as 100 ms may be required before the load check signal becomes valid, due to settling of the input clamp. If this settling time was part of the plug-and-play update loop, then an on-time duty cycle of 10% would result for a load check interval of once per second. This would result in substantial current con- sumption. With power-down load checking, and reasonable duty cycle, a standby current less than 1 µA can be maintained. Some important points to keep in mind when using the TVDET signal are as follows. When power-down load check is used, the TVDET pin reflects the status at the time of checking. The addi- tion or removal of loads afterwards is not be reflected without checking again. When CE is high, however, the TVDET output will be updated about once per second, provided a valid CSYNC signal is applied (or HSYNC and VSYNC). The TVDET output is the logical OR of the TVDET flags for the Y and CV outputs. Another important consideration when using the TVDET signal is that it is temporarily invalid at full power-up while the input dc restore circuit settles. The settling time can be up to 100 ms for large input coupling capacitors. This means that it is not advisable to use the TVDET signal to directly gate CE. This arrangement may lead to a limit cycle. Suitable delay should be included after turning the AD723 on before deciding to turn it off again because no load is detected. DC-Coupled Outputs The video outputs of the AD723 (Y, C and CV) are all dc- coupled. The advantages of this are two-fold. First, the need for large ac-coupling capacitors (220 µF typically) at the output is eliminated. Second, it becomes possible to perform load checking. The disadvantage with dc-coupled outputs is that there is more dc current to dissipate. Reducing the supply voltage to 3 V can minimize this. Here, the typical power consumption will be similar to ac-coupled voltage drivers. As a result of dissipating dc current, there are two different power consumption numbers: one for a typical picture, and one for a worst-case all-white screen. The all-white screen requires a significant amount of power to be dissipated, but it is very uncommon for both RGB computer graphics and video to be in this condition. |
Аналогичный номер детали - AD723ARU-REEL |
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Аналогичное описание - AD723ARU-REEL |
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