Detailed Description

The MAX9503 completely eliminates the need for

capacitors in the video output by using Maxim’s

DirectDrive technology that includes an inverting charge

pump and linear regulator. The charge pump and linear

regulator create a clean negative supply allowing the

amplifier output to swing below ground. The amplifier

output can swing both positive and negative so that the

video signal black level can be placed at ground. The

MAX9503 features a six-pole, Butterworth filter to per-

form reconstruction filtering on the video input signal

from the DAC.

DirectDrive

Background

Integrated video filter/amplifier circuits operating from a

single, positive supply usually create video output sig-

nals that are level-shifted above ground to keep the

signal within the linear range of the output amplifier. For

applications in which the positive DC level shift of the

video signal is not acceptable, a series capacitor can

be inserted in the output connection in an attempt to

eliminate the positive DC level shift. The series capaci-

tor cannot truly level shift a video signal because the

average level of the video varies with picture content.

The series capacitor biases the video output signal

around ground, but the actual level of the video signal

can vary significantly depending upon the RC time con-

stant and the picture content.

The series capacitor creates a highpass filter. Since

the lowest frequency in video is the frame rate, which

can be between 24Hz and 30Hz, the pole of the high-

pass filter should ideally be an order of magnitude

lower in frequency than the frame rate. Therefore, the

series capacitor must be very large, typically from

220µF to 3000µF. For space-constrained equipment,

the series capacitor is unacceptable. Changing from a

single series capacitor to a SAG network that requires

two smaller capacitors can only reduce space and

cost slightly.

The series capacitor in the usual output connection

also prevents damage to the output amplifier if the con-

nector is shorted to a supply or to ground. While the

output connection of the MAX9503 does not have a

series capacitor, the MAX9503 will not be damaged if

the connector is shorted to a supply or to ground (see

the Short-Circuit Protection section).

Video Amplifier

Typically, the black level of the video signal created by

the video DAC is around 300mV. The MAX9503 shifts

the black level to ground at the output so that the active

video is above ground, and sync is below ground. The

amplifier needs a negative supply for its output stage to

remain in its linear region when driving sync below

ground.

DirectDrive Video Amplifier with

Reconstruction Filter

8

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Pin Description

PIN

QSOP

TQFN

NAME

FUNCTION

115

216

Charge-Pump Negative Power Supply. Bypass with a 1µF capacitor to CPGND.

31

C1N

Charge-Pump Flying Capacitor Negative Terminal. Connect a 1µF capacitor from C1P to C1N.

42

CPGND

Charge-Pump Power Ground

53

C1P

Charge-Pump Flying Capacitor Positive Terminal. Connect a 1µF capacitor from C1P to C1N.

64

Charge-Pump Positive Power Supply. Bypass with a 1µF capacitor to CPGND.

75

BIAS

Common-Mode Voltage. Connect a 100k

Ω resistor from BIAS to SGND.

86

SGND

Signal Ground. Connect to GND.

97

VIDIN

Video Input

10, 14, 15

8, 12, 14

N.C.

No Connection. Not internally connected. Connect to SGND.

11

9

SHDN

12

10

GND

Ground. Connect to SGND.

13

11

Positive Power Supply. Bypass with a 1µF capacitor to SGND.

16

13

VIDOUT

Video Output

—EPEP

Exposed Paddle. Connect to GND.