THS7368IPW datasheet(23/45 Pages) TI2

23 / 45 page

Level

Shift

Internal

Circuitry

800kW

Input

0.1 mF

STCLPF

THS7368

www.ti.com

SBOS497 – DECEMBER 2009

INPUT MODE OF OPERATION: AC SYNC TIP

As a result of this delay, sync may have an apparent

CLAMP

voltage shift. The amount of shift depends on the

amount of droop in the signal as dictated by the input

Some video DACs or encoders are not referenced to

capacitor and the STC current flow. Because sync is

ground but rather to the positive power supply. The

used primarily for timing purposes with syncing

resulting video signals are generally at too great a

occurring on the edge of the sync signal, this shift is

voltage for a dc-coupled video buffer to function

transparent in most systems.

properly. To account for this scenario, the THS7368

incorporates a sync-tip clamp circuit. This function

requires a capacitor (nominally 0.1

μF) to be in series

with the input. Although the term sync-tip-clamp is

used throughout this document, it should be noted

that the THS7368 would probably be better termed as

a dc restoration circuit based on how this function is

performed. This circuit is an active clamp circuit and

not a passive diode clamp function.

The input to the THS7368 has an internal control loop

that sets the lowest input applied voltage to clamp at

ground (0 V). By setting the reference at 0 V, the

THS7368 allows a dc-coupled input to also function.

Therefore, the sync-tip-clamp (STC) is considered

Figure 39. Equivalent AC Sync-Tip-Clamp Input

Circuit

transparent because it does not operate unless the

input signal goes below ground. The signal then goes

through the same 150-mV level shifter, resulting in an

While this feature may not fully eliminate overshoot

output voltage low level of 300 mV. If the input signal

issues on the input signal, in cases of extreme

tries to go below 0 V, the THS7368 internal control

overshoot and/or ringing, the STC system should help

loop sources up to 6 mA of current to increase the

minimize improper clamping levels. As an additional

input voltage level on the THS7368 input side of the

method to help minimize this issue, an external

coupling capacitor. As soon as the voltage goes

capacitor (for example, 10 pF to 47 pF) to ground in

above the 0-V level, the loop stops sourcing current

parallel with the external termination resistors can

and becomes very high impedance.

help filter overshoot problems.

One of the concerns about the sync-tip-clamp level is

It should be noted that this STC system is dynamic

how the clamp reacts to a sync edge that has

and does not rely upon timing in any way. It only

overshoot—common in VCR signals, noise, DAC

depends on the voltage that appears at the input pin

overshoot, or reflections found in poor printed circuit

at any given point in time. The STC filtering helps

board (PCB) layouts. Ideally, the STC should not

minimize

level

shift

problems

associated

with

react to the overshoot voltage of the input signal.

switching noises or very short spikes on the signal

Otherwise, this response could result in clipping on

line. This architecture helps ensure a very robust

the rest of the video signal because it may raise the

STC system.

bias voltage too much.

When the ac STC operation is used, there must also

To help minimize this input signal overshoot problem,

be some finite amount of discharge bias current. As

the control loop in the THS7368 has an internal

previously described, if the input signal goes below

low-pass filter, as shown in Figure 39. This filter

the 0-V clamp level, the internal loop of the THS7368

reduces the response time of the STC circuit. This

sources current to increase the voltage appearing at

delay is a function of how far the voltage is below

the input pin. As the difference between the signal

ground, but in general it is approximately a 400-ns

level and the 0-V reference level increases, the

delay for the SD channel filters and approximately a

amount

source

current

increases

150-ns delay for the SF filters. The effect of this filter

proportionally—supplying up to 6 mA of current.

is to slow down the response of the control loop so as

Thus, the time to re-establish the proper STC voltage

not to clamp on the input overshoot voltage but rather

can be very fast. If the difference is very small, then

the flat portion of the sync signal.

the source current is also very small to account for

minor voltage droop.

Submit Documentation Feedback

Product Folder Link(s): THS7368

THS7368IPW Datasheet(PDF) 23 Page - Texas Instruments

THS7368IPW Datasheet(HTML) 23 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Datasheet Download

Link URL