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參數(shù)資料
| 型號： | ISL59532 |
| 廠商： | Intersil Corporation |
| 英文描述： | 32x32 Video Crosspoint(32x32視頻矩陣開關(guān)) |
| 中文描述： | 32x32視頻交叉點（32x32視頻矩陣開關(guān)） |
| 文件頁數(shù)： | 22/24頁 |
| 文件大?。?/td> | 1305K |
| 代理商： | ISL59532 |
22
FN7432.3
July 24, 2006
When a video signal is applied to V
IN
, the most negative
signal will be the sync tip. If the sync tip goes below V
REF
,
Q1 will turn on and quickly source enough current into C
IN
so that the sync tip is forced to be equal to V
REF
. After the
sync tip, the video jumps up by 300mV or more, so V
OUT
becomes >> V
REF
, so Q1 will not turn on for the rest of the
video line. However the 2
μ
A current source continues to
slowly discharge C
IN
, so that by the end of the video line, the
next sync tip will again be slightly below V
REF
, forcing Q1 to
source some current into C1 to make V
OUT
= V
REF
during
the sync tip.
This is how the video is “DC-restored” after being AC
coupled into the ISL59532. The sync tip voltage will be equal
to V
REF
, on the right side of C
IN
, regardless of the DC level
of the video on the left side of C
IN
. Due to various sources of
offset in the actual clamp function, the actual sync tip level is
typically about 75mV higher than V
REF
(for V
REF
= 0.5V).
It is important to choose the correct value for C
IN
. Too small
a value will generate too much droop, and the image will be
visibly darker on the right than on the left. A C
IN
value that is
too large may cause the clamp to fail to converge. The droop
rate (dV/dt) is i
PULLDOWN
/C
IN
volts/second. In general, the
droop voltage should be limited to <1 IRE over a period of
one line of video; so for 1 IRE = 7mV, I
B
= 10μA maximum,
and an NTSC waveform we will set C
IN
> 10μA*60μs/7mV =
0.086μF. Figure 50 shows the result of C
IN
= 0.1μF
delivering acceptable droop and C
IN
= 0.001μF producing
excessive droop
When the clamp function is disabled in the CONTROL
register (Clamp = 0) to allow DC-coupled operation, the
I
CLAMP
current sinks/sources are disabled and the input
passes through the DC Restore block unaffected. In this
application V
REF
may be tied to GND.
Overlay Operation
The ISL59532 features an overlay feature, that allows an
external video signal or DC level to be inserted in place of
that output channel’s video. When the OVER
N
signal is
taken high, the output signal on the OUT
N
pin is replaced
with the signal on the VOVER
N
pin.
There are several ways the overlay feature can be used.
Toggling the OVER
N
signal at the frame rate or slower will
replace the video frame(s) on the OUT
N
pin with the video
supplied on the VOVER
N
pin.
Another option (for OSD displays, for example), is to put a
DC level on the VOVER
N
line and toggle the OVER
N
signal
at the pixel rate to create a monocolor image “overlaid” on
channel N’s output signal.
Finally, by enabling the OVER
N
signal for some portion of
each line over a certain amount of lines, a picture-in-picture
function can be constructed.
It’s important to note that the overlay inputs do not have the
DC Restore function previously described - the overlay
signal is DC coupled into the output. It is the system
designer’s responsibility to ensure that the video levels are
in the ISL59532’s linear region and matching the output
channel’s offset and amplitude. One easy way to do this is to
run the video to be overlaid through one of the ISL59532’s
unused channels and then into the VOVER
N
input.
The OVER
N
pins all have weak pulldowns, so if they are
unused, they can either be left unconnected or tied to GND.
Power Dissipation and Thermal Resistance
With a large number of switches, it is possible to exceed the
150°C absolute maximum junction temperature under
certain load current conditions. Therefore, it is important to
calculate the maximum junction temperature for an
application to determine if load conditions or package types
need to be modified to assure operation of the crosspoint
switch in a safe operating area.
The maximum power dissipation allowed in a package is
determined according to:
-
+
V
IN
V
S
R
TERM
C
IN
Q1
V
REF
2uA
V
OUT
FIGURE 49. DC RESTORE BLOCK DIAGRAM
FIGURE 50. DC RESTORE VIDEO WAVEFORMS
PD
MAX
T
--------------------------------------------
T
JA
–
=
ISL59532
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