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TDA9556

7.5 NS TRIPLE-CHANNEL HIGH VOLTAGE VIDEO AMPLIFIER

STMicroelectronics 

TDA9556

6 THEORY OF OPERATION

6.1 General

The TDA9556 is a three-channel video amplifier supplied by a low supply voltage: VCC (typ.12V) and a

high supply voltage: VDD (up to 115V).

The high values of VDD supplying the amplifier output stage allow direct control of the CRT cathodes (DC

coupling mode).

In DC coupling mode, the application schematic is very simple and only a few external components are

needed to drive the cathodes. In particular, there is no need of the DC-restore circuitry which is used in

classical AC coupling applications.

The output voltage range is wide enough (Figure 2) to provide simultaneously :

– Cut-off adjustment (typ. 25V)

– Video contrast (typ. up to 40V),

– Brightness (with the remaining voltage range).

In normal operation, the output video signal must remain inside the linear region whatever the cut-off,

brightness and contrast adjustments are.

Figure 2. Output signal, level adjustments

VDD

15V

(A) Top Non-Lin ear Region

Blanking pulse

Video Signal

(B) Cut-off Adjust. (25V Typ.)

(C) Brightness Adjust. (10V Typ.)

(D) Contrast Adjust. (40V Typ.)

17V

(E) Bottom Non-Linear Region

GND

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