TDA8708 datasheet(5/24 Pages) PHILIPS

5 / 24 page

June 1994

Philips Semiconductors

Product speciﬁcation

Video analog input interface

TDA8708A

FUNCTIONAL DESCRIPTION

The TDA8708A provides a simple interface for decoding

video signals.

The TDA8708A operates in configuration mode 1 (see

Fig.4) when the video signals are weak (i.e. when the gain

of the AGC amplifier has not yet reached its optimum

value). This enables a fast recovery of the synchronization

pulses in the decoder circuit. When the pulses at the

GATE A and GATE B inputs become distinct (GATE A and

GATE B pulses are synchronization pulses occurring

during the sync period and rear porch respectively) the

TDA8708A automatically switches to configuration mode 2

(see Fig.5).

When the TDA8708A is in configuration mode 1, the gain

of the AGC amplifier will be roughly adjusted (sync level to

a digital output level of 0 and the peak level to a digital

output level of 255).

In configuration mode 2 the digital output of the ADC is

compared to internal digital reference levels. The resultant

outputs control the charge or discharge current of a

capacitor connected to the AGC pin. The voltage across

this capacitor controls the gain of the video amplifier. This

is the gain control loop.

The sync level comparator is active during a positive-going

pulse at the GATE A input. This means that the sync pulse

of the composite video signal is used as an amplitude

reference. The bottom of the sync pulse is adjusted to

obtain a digital output of logic 0 at the converter output. As

the black level is at digital level 64, the sync pulse will have

a digital amplitude of 64 LSBs.

The peak-white control loop is always active. If the video

signal tends to exceed the digital code of 248, the gain will

be limited to avoid any over-range of the converter.

The use of nominal signals will prevent the output from

exceeding a digital code of 213 and the peak-white control

loop will be non-active.

The clamp level control is accomplished by using the same

techniques as used for the gain control. The black-level

digital comparator is active during a positive-going pulse at

the GATE B input. The clamp capacitor will be charged or

discharged to adjust the digital output to code 64.

LIMITING VALUES

In accordance with the Absolute Maximum Rating System (IEC 134).

THERMAL CHARACTERISTICS

SYMBOL

PARAMETER

MIN.

MAX.

UNIT

VCCA

analog supply voltage

−0.3

+7.0

VCCD

digital supply voltage

−0.3

+7.0

VCCO

output supply voltage

−0.3

+7.0

∆VCC

supply voltage difference between VCCA and VCCD

−1.0

+1.0

supply voltage difference between VCCO and VCCD

−1.0

+1.0

supply voltage difference between VCCA and VCCO

−1.0

+1.0

input voltage

−0.3

VCCA

output current

+10

Tstg

storage temperature

−55

+150

°C

Tamb

operating ambient temperature

+70

°C

junction temperature

+125

°C

SYMBOL

PARAMETER

VALUE

UNIT

Rth j-a

thermal resistance from junction to ambient in free air

SOT117-1

K/W

SOT136-1

K/W

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