ISL59530 Datasheet, PDF (20/22 Pages) Intersil Corporation – 16x16 Video Crosspoint

ISL59530
V_REF
V_IN
R_TERM
C_IN
V_S
-
+
Q1
V_OUT
2uA
FIGURE 49. DC RESTORE BLOCK DIAGRAM
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 2µA 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 ISL59530. 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).
.
FIGURE 50. DC RESTORE VIDEO WAVEFORMS
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 =
20
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 ISL59530 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 ISL59530’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 ISL59530’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:
PD_MAX
=
T_{----J---M-----A----X-----–-----}T_{----A---M-----A----X--}
Θ_JA
FN6220.1
June 12, 2006