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ML6428CS-2

S-Video Filter and 75W Line Drivers with Summed Composite Output

Micro Linear Corporation 

ML6428

FUNCTIONAL DESCRIPTION

The ML6428 is a dual monolithic continuous time video

filter designed for reconstructing the luminance and

chrominance signals from an S-Video D/A source.

Composite video output is generated by summing the Y

and C outputs. The ML6428 is intended for use in AC

coupled input and output applications. The ML6428CS-2,

-4 is intended for AC coupled input and DC coupled

output applications (see Figures 5 and 6).

The filters have a 4th-order Butterworth characteristic with

an optimization toward low overshoot and flat group

delay. All outputs are capable of driving 2VP-P into AC

coupled 150W video loads, with up to 35pF of load

capacitance at the output pin. The CV output can drive

two video loads plus a high-impedance modulator. Thus

the CV output is intended to simultaneously drive a VCR,

a TV, and a high-impedance modulator. Y and C are

capable of driving a 75W load at 1VP-P. The ML6428 is

capable of driving two composite loads and a TV

modulator simultaneously.

All channels are clamped during sync to establish the

appropriate output voltage swing range. Thus the input

coupling capacitors do not behave according to the

conventional RC time constant. Clamping for all

channels settles within 2ms of a change in video input

sources.

In most applications, the ML6428's input coupling

capacitors are 0.1µF. The Y input sinks 1.6µA during

active video, which nominally tilts a horizontal line by

2mV (max) at the Y output (Figure 4). During sync, the

clamp typically sources 20µA to restore the DC level. The

net result is that the average input current is zero.

Any change in the input coupling capacitor's value will

inversely alter the amount of tilt per line. Such a change

will also linearly affect the clamp response times.

The C channel has no pulldown current sources and is

essentially tilt-free. Its input is clamped by a feedback

amp which responds to the CV output. Since CV = Y+C,

the CV output will droop by the same amount as Y during

active video, and will rise by the same amount as Y

during sync.

The ML6428 is robust and stable under all stated load and

input conditions. Capacitavely bypassing both VCC pins

directly to ground ensures this performance. (See Figures

5 and 6)

LUMINANCE (Y) I/O

The luma input is driven by either a low impedance

source of 1VP-P or the output of a 75W terminated line.

The input is required to be AC coupled via a 0.1uF

coupling capacitor which allows for a nominal settling

time of 2ms. The luma output is capable of driving an AC

coupled 150W load at 2VP-P or 1VP-P into a 75W load.

Up to 35pF of load capacitance (at the output pin) can be

driven without stability or slew issues. A 220µF AC

coupling capacitor is recommended at the output

(ML6428-1, -3 only).

CHROMINANCE (C) I/O

The chroma input is driven by a low impedance source of

0.7VP-P or the output of a 75W terminated line. The input

is required to be AC coupled via a 0.1uF coupling

capacitor which allows for a nominal clamping time of

1ms. The chroma output is capable of driving an AC

coupled 150W load at 2VP-P or 1VP-P into a 75W load.

Up to 35pF of load capacitance can be driven without

stability or slew issues. A 220µF AC coupling capacitor is

recommended at the output (ML6428-1, -3 only).

COMPOSITE VIDEO (CV) OUTPUT

The composite video output is capable of driving 2 CV

loads to 2VP-P and a high input impedance CV modulator.

It is intended to drive three devices: TV, VCR, and a

modulator. The TV or VCR input can be shorted to ground

and the other outputs will still meet specifications. Up to

35pF of load capacitance (at the output pin) can be

driven without stability or slew issues.

USING THE ML6428 FOR PAL APPLICATIONS

The ML6428 can be optimized for PAL video by adding

frequency peaking to the composite and S-video outputs.

Figures 7 and 8 illustrate the use of a additional external

capacitor, 330pF, added in parallel to the output source

termination resistor. This raises the frequency response

from 1.6 dB down at 4.8Mhz to 0.35dB down at 4.8MHz

allowing for accurate reproduction of the upper sideband

of the PAL subcarrier. Figure 9 shows the frequency

response of PAL video with various values of peaking

capacitors (0pF, 220pF, 270pF, 330pF) between 0 and

10MHz.

For NTSC applications without the peaking capacitor the

rejection at 27MHz is 42dB (typical) while for PAL

applications with the peaking capacitor the rejection at

27MHz is 38dB (typical). This is shown in Figure 10. The

differential group delay is shown in Figure 11 with and

without a peaking capacitor (0pF, 220pF, 270pF, and

330pF) varies slightly with capacitance, going from 8ns to

13ns.

6

DATASHEET

June, 2000

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