參數(shù)資料
型號(hào)： THS7375IPWRG4
廠商： TEXAS INSTRUMENTS INC
元件分類： 音頻／視頻放大
英文描述： VIDEO AMPLIFIER, PDSO14
封裝： GREEN, PLASTIC, TSSOP-14
文件頁數(shù)： 15/39頁
文件大?。?/td> 973K
代理商： THS7375IPWRG4
Level
Shift
Internal
Circuitry
+V
S
800kW
Input
Pin
Input
0.1 mF
Comparator
+V
S
STCLPF
SBOS449A – SEPTEMBER 2008 – REVISED JANUARY 2011
www.ti.com
One of the concerns about the sync tip clamp level is
When the ac STC operation is used, there must also
how the clamp reacts to a sync edge that has
be some finite amount of discharge bias current. As
overshoot—common in VCR signals or reflections
previously described, if the input signal goes below
found in poor printed circuit board (PCB) layouts.
the 0-V clamp level, the internal loop of the THS7375
Ideally, the STC should not react to the overshoot
sources current to increase the voltage appearing at
voltage of the input signal. Otherwise, this issue could
the input pin. As the difference between the signal
result in clipping on the remainder of the video signal
level and the 0-V reference level increases, the
because it may raise the bias voltage too much.
amount
of
source
current
increases
proportionally—supplying up to 3 mA of current.
To help minimize this input signal overshoot problem,
Thus, the time period to re-establish the proper STC
the control loop in the THS7375 has an internal
voltage can be very short. If the difference is very
low-pass filter as shown in Figure 50. This filter
small, then the source current is also very small to
reduces the response time of the STC circuit. This
account for minor voltage droop.
delay is a function of how far the voltage is below
ground, but in general it is close to an 800-ns delay.
However, if the input signal goes above the 0-V input
This filter slows down the response of the control loop
level, a problem arises. The problem is that the video
so as not to clamp on the input overshoot voltage, but
signal is always above this level and must not be
rather the flat portion of the sync signal.
altered in any way. But if the sync level of the input
signal is above this 0-V level, then the internal
discharge (sink) current reduces the ac-coupled bias
signal to the proper 0-V level.
This discharge current must not be large enough to
alter the video signal appreciably or picture quality
issues may arise. This issue is often seen by looking
at the tilt (droop) of a constant luma signal being
applied and observing the resulting output level. The
associated change in luma level from the beginning
of the video line to the end of the video line is the
amount of line tilt (droop).
If the discharge current is very small, then the amount
Figure 50. Equivalent AC Sync-Tip Clamp Input
of tilt is very low, which is a generally a good thing.
Circuit
However, the amount of time for the system to
capture the sync signal could be too long. This effect
is also called hum rejection. Hum arises from the ac
As a result of this delay, the sync may have an
line voltage frequency of 50-Hz or 60-Hz. The value
apparent voltage shift. The amount of shift depends
of the discharge current and the ac-coupling capacitor
on the amount of droop in the signal as dictated by
combine to dictate the hum rejection and the amount
the input capacitor and the STC current flow.
of line tilt.
Because the sync is primarily for timing purposes,
with synchronization occurring on the edge of the
To allow for both dc-coupling and ac-coupling in the
sync signal, this shift is transparent in most systems.
same part, the THS7375 incorporates an 800-k
resistor to ground. Although a true constant-current
While this feature may not fully eliminate overshoot
sink is preferred over a resistor, there are significant
issues on the input signal in case of severe overshoot
issues when the voltage is near ground. This
and/or ringing, the STC system should help minimize
condition can cause the current sink transistor to
improper clamping levels. As an additional method to
saturate and cause potential problems with the signal.
help minimize this issue, an external capacitor (such
Also, this resistor is large enough to not impact a
as 10 pF to 47 pF) to ground in parallel with the
dc-coupled DAC termination. For discharging an
external termination resistors can help filter overshoot
ac-coupled source, Ohm’s Law is applied. If the video
problems.
signal is 1 V, then there is 1 V/800 k
= 1.25 mA of
It should be noted that this STC system is dynamic
discharge current. If more hum rejection is desired or
and does not rely upon timing in any way. It only
there is a loss of sync occurring, simply decrease the
depends on the voltage that appears at the input pin
0.1-mF input coupling capacitor. A decrease from 0.1
at any given point in time. The STC filtering helps
mF to 0.047 mF increases the hum rejection by a
minimize
level
shift
problems
associated
with
factor of 2:1. Alternatively, an external pull-down
switching noises or very short spikes on the signal
resistor to ground may be added that decreases the
line. This feature helps ensure a very robust STC
overall
resistance
and
ultimately
increases
the
system.
discharge current.
22
Copyright 2008–2011, Texas Instruments Incorporated
Product Folder Link(s): THS7375
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