AD723 Analog Devices, AD723 Datasheet - Page 11

• Manufacturer Part Number: AD723
• Description: 2.7 V to 5.5 V RGB-to-NTSC/PAL Encoder with Load Detect and Input Termination Switch
• Manufacturer: Analog Devices

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.
CURRENT
(POWER
CHECK
DOWN)
CE = HIGH (AWAKE/SLEEP)
CHECK
CSYNC
TVDET
PULSE
LOAD
LOAD
TEST
EVAL
CE
H = 63.5 s
4.7 s
TIME LEGEND:NTSC (PAL)
(8.2 s)
8.2 s
(14.3 s)
15.9 s
2.3 s
18 s
(0.9 s)
1.1 s
(7.3 s)
9.1 s
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.
AD723