THS7375IPW.B datasheet(22/42 Pages) TI2

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Level

Shift

Internal

Circuitry

800kW

Input

0.1 mF

Comparator

STCLPF

THS7375

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

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

Ω

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

Ω = 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.

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