STV9212 datasheet(15/34 Pages) STMICROELECTRONICS

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STV9212

Functional Description

2.9

Output Infra-black Level, Cut-off and Brightness

The schematic diagram of these functions is shown in Figure 14.

2.9.1

Output Infra-black Level

The infra-black level of the video signal at the video outputs OUT1, OUT2 and OUT3 is positioned

to the Vib reference by the output clamp circuit, thus defining the Output infra-black level. If the

output clamp circuit is furnished with a correctly timed OCP (see corresponding sections), the

output infra-black level equals Vib. Vib is composed of a fixed DC voltage (Vibmin), a variable DC

voltage (Vibof) applied on all three channels and a per-channel variable DC voltage (Vibl (1,2,3)) as

shown in Figure 15. In AC-coupling mode (bit

MOD = 1), the Vibl part is suppressed and the Vib is

therefore equal on all three channels, only varying with bits

IBOF[5:0] acting on Vibof. This can be

used to match the device’s outputs to the input of the video amplifier used (biasing). In DC-coupling

mode (bit

MOD = 0), Vibl (1,2,3) are separately set via bits IBL1[7:0], IBL2[7:0] and IBL3[7:0],

respectively. This serves to adjust the cut-off points of the three CRT cathodes. In this case, Vibof

can serve to pre-position the cut-off ranges in the factory adjustment procedure or/and to provide a

rough brightness control.

Figure 14: Cut-off and Brightness Control Block Diagram

AC:1

DC:0

MOD

Output stage

Vibmin

ViblDC

Cut-off

IBLRG

Brightness

range

IBOF

1 for 3 channels

1 per channel

Channel 3

BRIG

CO1

OUT1

Vib

DC:0

AC:1

Vib

VRef

VCOmin

Note: Identical for CO1, CO2 and CO3 outputs except for “brightness” (VbriDC) that is only output at CO3 while MOD=0.

ViblAC

VbriAC

VbriDC

The switches are drawn in their default positions.

VCO

GND

BRIGRG

I²C-bus field

IBL1

IBL2

IBL3

VCO

VCOmax

VCOmin

MOD=1 MOD=0

Video DC level

Only

~40k

Ω

OUT2

OUT3

CO2

CO3

ViblAC

VbriAC

VbriDC

~0V*

Ch3 Ch1

Ch2

* ~0V when the output is left open

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