TPF133A
Low Power, 3-channel Video Filter for HD Video
capacitance is 0.1μF, the maximum droop voltage is
about 1mV which is restored by the clamp circuit. The
maximum pull-up current of the clamp circuit is 1.7mA.
For a 4μs sync tip width and 0.1μF capacitor, the
maximum restoration voltage is about 80mV.
Application Information
The TPF133A is targeted for systems that require three
high-definition (HD) video outputs. Although it can be
used for numerous other applications, the needs and
requirements of the video signal are the most important
design parameters of the TPF133A. The TPF133A
incorporates many features not typically found in
integrated video parts while consuming very low power.
The line droop voltage will increase if a smaller
AC-coupling capacitance is used. For the same
reason, if larger capacitance is used the line droop
voltage will decrease. Table 1 is droop voltage and
maximum restoration voltage of the clamp for typical
capacitance.
Internal Sync Clamp
The typical embedded video DAC operates from a
ground referenced single supply. This becomes an
issue because the lower level of the sync pulse output
may be at a 0V reference level to some positive level.
The problem is presenting a 0V input to most single
supply driven amplifiers will saturate the output stage
of the amplifier resulting in a clipped sync tip and
degrading the video image. A larger positive reference
may offset the input above its positive range.
Table 1. Maximum restoration voltage and droop voltage
of Y signals for different capacitance
CAP VALUE
(nF)
DROOP IN 60μs
CHARGE IN 4μs
(mV)
(mV)
100
1.2
68
1,000
0.12
6.8
Low Pass Filter--Sallen Key
The Sallen Key is a classic low pass configuration.
This provides a very stable low pass function, and in
the case of the TPF133A, the six-pole roll-off at
around 36MHz. The six-pole function is accomplished
with an RC low pass network placed in series with and
before the Sallen Key.
The TPF133A features an internal sync clamp and
offset function to level shift the entire video signal to
the best level before it reaches the input of the
amplifier stage. These features are also helpful to
avoid saturation of the output stage of the amplifier by
setting the signal closer to the best voltage range.
Output Couple
TPF133A output could support both “AC Couple” and
“DC Couple”, if use “AC Couple”, this capacitor is
typically between 220-μF and 1000-μF, although
470-μF is common. This value of this capacitor must
be this large to minimize the line tilt (droop) and/or
field tilt associated with ac-coupling as described
previously in this document.
The simplified block diagram of the TPF133A in
Figure-2. The AC coupled video sync signal is pulled
negative by a current source at the input of the
comparator amplifier. When the sync tip goes below
the comparator threshold the output comparator is
driven negative, The PMOS device turns on clamping
sync tip to near ground level. The network triggers on
the sync tip of video signal.
The TPF133A internal sync clamp makes it possible to
DC couple the output to a video load, eliminating the
need for any AC coupling capacitors, thereby saving
board space and additional expense for capacitors.
This makes the TPF133A extremely attractive for
portable video applications. Additionally, this solution
completely eliminates the issue of field tilt in the lower
frequency. The trade off is greater demand of supply
current. Typical load current for AC coupled is around
1mA, compared to typical 6.6mA used when DC
Droop Voltage and DC
Restoration
Selection of the input AC-coupling capacitance is
based on the system requirements. A typical sync tip
width of a 64μs NTSC line is 4μs during which clamp
circuit restores its DC level. In the remaining 60μs
period, the voltage droops because of a small
constant 2.0μA sinking current. If the AC-coupling
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