Pin Description

Note

The pin designations CV R G B and Sync are as-

signed for the convenience of description and are not in-

tended to be a limitation For example RGB could be YUV

or they could all be independent signal sources

Pin 1

Composite video input 1 (CV1) biased internally

via 18 kX to

2

a

1V

Pin 2

Composite video input 2 (CV2) biased as for pin

1 (CV1) above

Pin 3

Composite video input 3 (CV3) biased as for pin

1 (CV1) above

Pin 4

RGB input R1 This pin is internally biased via

a clamp circuit to

2

a

1V and should be AC

coupled to a low impedance source

The input coupling capacitor also acts as a

clamp capacitor see application notes

Pin 5

RGB input G1 biased as for pin 4 (R1) above

Pin 6

RGB input B1 biased as for pin 4 (R1) above

Pin 7

Sync input S1 biased internally via 25k to

2

a

1V

Pin 8

RGB input R2 biased as for pin 4 (R1) above

Pin 9

RGB input G2 biased as for pin 4 (R1) above

Pin 10

RGB input B2 biased as for pin 4 (R1) above

Pin 11

Sync input S2 biased as for pin 7 (S1) above

Pin 12

Negative supply (GND)

Pin 13

Connect a capacitor to GND to decouple the

internal bias of the RGB amplifiers

Pin 14

Internal bias for the CV and Sync Amplifiers

decouple with a capacitor to GND

Pin 15

B Output

Pin 16

G Output

Pin 17

R Output

Pin 18

This is the clamp pulse input pin A positive

going pulse activates the RGB input bias

clamps

See application notes

Pin 19

Channel select input control C

Pin 20

Channel select input control B

Pin 21

Channel select input control A

Pin 22

Enable input for control latches Channel

selection is locked while this input is low and is

updated when high The minimum enable pulse

width is 5 ms

Pin 23

CV output or Sync output when an RGB channel

is selected

Pin 24

decoupled at high frequencies a 100 nF

capacitor connected close to the supply pins is

normally adequate

Application Notes

DEVICE DESCRIPTION

The LM1044 video switch circuit has a configuration as illus-

trated in

Figure 1 and consists of a 3 input to 1 output 5

MHz switch with 6 dB gain three 2 input to 1 output 30

MHz 0 dB gain switches coupled together with a 2 input to

1 output switch sharing the 3 way switch output All switch

stages are current switched differential amplifers with feed-

back providing low impedance buffered outputs Latched

logic inputs with control decoding are provided for switch

control and a DC clamp facility is available on the 30 MHz

channels

The principle application of this device is the selection be-

tween various composite video (CV) or Red Green and

Blue (RGB) sources now found in video systems using vari-

ous signal sources eg

VCR’s satellite receivers home

computers and video games Other possible application ex-

amples for example security camera switching are shown

towards the end of these notes

The 5 MHz channels are ideally suited for the switching of

composite video sources and have a gain of 6 dB to allow

amplification from terminated inputs back up to internal sig-

nal levels The 30 MHz channels are suitable for direct RGB

inputs to display high quality graphics and will also handle

high quality linear signals The fourth switch channel shares

the CV output pin and is ideal for routing synchronization

signals from the RGBYUV sources into the path to the

sync separator and timebase circuits

CHANNEL SELECTION

The switch selections are made via the enable and 3 logic

control inputs according to the truth table shown on the

following page This gives a choice of 3 CV video signal

sources or 2 RGB plus Sync signals on the video display

TLH9252 – 3

FIGURE 1

5