THS7375IPWR datasheet(20/36 Pages) TI

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Level

Shift

Internal

Circuitry

800kW

Input

0.1 mF

Comparator

STCLPF

THS7375

SBOS449 – SEPTEMBER 2008......................................................................................................................................................................................... www.ti.com

up to 3 mA of current to increase the input voltage

It should be noted that this STC system is dynamic

level on the THS7375 input side of the coupling

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

capacitor. As soon as the voltage goes above the 0-V

depends on the voltage that appears at the input pin

level, the loop stops sourcing current and becomes

at any given point in time. The STC filtering helps

very high impedance.

minimize

level

shift

problems

associated

with

switching noises or very short spikes on the signal

One of the concerns about the sync tip clamp level is

line. This feature helps ensure a very robust STC

how the clamp reacts to a sync edge that has

system.

overshoot—common in VCR signals or reflections

found in poor printed circuit board (PCB) layouts.

When the ac STC operation is used, there must also

Ideally, the STC should not react to the overshoot

be some finite amount of discharge bias current. As

voltage of the input signal. Otherwise, this issue could

previously described, if the input signal goes below

result in clipping on the remainder of the video signal

the 0-V clamp level, the internal loop of the THS7375

because it may raise the bias voltage too much.

sources current to increase the voltage appearing at

the input pin. As the difference between the signal

To help minimize this input signal overshoot problem,

level and the 0-V reference level increases, the

the control loop in the THS7375 has an internal

amount

source

current

increases

low-pass filter as shown in Figure 48. This filter

proportionally—supplying up to 3 mA of current.

reduces the response time of the STC circuit. This

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

delay is a function of how far the voltage is below

voltage can be very short. If the difference is very

ground, but in general it is close to an 800-ns delay.

small, then the source current is also very small to

This filter slows down the response of the control loop

account for minor voltage droop.

so as not to clamp on the input overshoot voltage, but

rather the flat portion of the sync signal.

However, if the input signal goes above the 0-V input

level, a problem arises. The problem is that the video

signal is always above this level and must not be

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

Figure 48. Equivalent AC Sync-Tip Clamp Input

amount of line tilt (droop).

Circuit

If the discharge current is very small, then the amount

of tilt is very low, which is a generally a good thing.

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

However, the amount of time for the system to

apparent voltage shift. The amount of shift depends

capture the sync signal could be too long. This effect

on the amount of droop in the signal as dictated by

is also called hum rejection. Hum arises from the ac

the input capacitor and the STC current flow.

line voltage frequency of 50-Hz or 60-Hz. The value

Because the sync is primarily for timing purposes,

of the discharge current and the ac-coupling capacitor

with synchronization occurring on the edge of the

combine to dictate the hum rejection and the amount

sync signal, this shift is transparent in most systems.

of line tilt.

While this feature may not fully eliminate overshoot

To allow for both dc-coupling and ac-coupling in the

issues on the input signal in case of severe overshoot

same part, the THS7375 incorporates an 800-k

Ω

and/or ringing, the STC system should help minimize

resistor to ground. Although a true constant-current

improper clamping levels. As an additional method to

sink is preferred over a resistor, there are significant

help minimize this issue, an external capacitor (such

issues when the voltage is near ground. This

as 10 pF to 47 pF) to ground in parallel with the

condition can cause the current sink transistor to

external termination resistors can help filter overshoot

saturate and cause potential problems with the signal.

problems.

Also, this resistor is large enough to not impact a

dc-coupled DAC termination. For discharging an

ac-coupled source, Ohm’s Law is applied. If the video

signal is 1 V, then there is 1 V/800 k

Ω = 1.25

µA of

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