ISL59111 Просмотр технического описания (PDF) - Renesas Electronics

Номер в каталоге

Компоненты Описание

производитель

ISL59111

8MHz Rail-to-Rail Composite Video Driver

Renesas Electronics 

ISL59111

VDD

SYNC CLAMP

VDD

VDD

+

-

CIN

IN

R1

IN

RIN 100nF

SALLEN KEY LOW PASS FILTER

C2

R2

R3

VDC

+

-

C1

C3

EN

GND

EN=GND: SHUTDOWN IDD~0

EN=VDD: ACTIVE IDD~1.5mA

SAG

AC COUPLING

NETWORK CAPACITOR

OUT

R6

R7 SAG

C5

47µF

C4

ROUT

75 RL

75

22µF

R5

R4

FIGURE 22. BLOCK DIAGRAM

Application Information

The ISL59111 is a single supply rail-to-rail output amplifier

achieving a -3dB bandwidth of around 8MHz and slew rate of

about 40V/µs while demanding only 2mA of supply current.

This part is ideally suited for applications with specific

micropower consumption and high bandwidth demands. As the

performance characteristics above and the features described

below, the ISL59111 is designed to be very attractive for

portable composite video applications.

The ISL59111 features a sync clamp, low pass function, and

SAG network at the output facilitating reduction of typically

large AC coupling capacitors. See Figure 22.

Internal Sync Clamp

The typical embedded video DAC operates from a ground

referenced single supply. This becomes an issue because the

lower level of the sync pulse output may be at a 0V reference

level to some positive level. The problem is presenting a 0V

input to most single supply driven amplifiers will saturate the

output stage of the amplifier resulting in a clipped sync tip and

degrading the video image. A larger positive reference may

offset the input above its positive range.

The ISL59111 features an internal sync clamp and offset

function to level shift the entire video signal to the best level

before it reaches the input of the amplifier stage. These

features are also helpful to avoid saturation of the output stage

of the amplifier by setting the signal closer to the best voltage

range.

The simplified block diagram of the ISL59111 in Figure 22 is

divided into four sections. The first, Section A is the Sync

Clamp. The AC coupled video sync signal is pulled negative by

a current source at the input of the comparator amplifier. When

the sync tip goes below the comparator threshold the output

comparator is driven negative, The PMOS device turns on

clamping sync tip to near ground level. The network triggers on

the sync tip of video signal.

The Sallen Key Low Pass Filter

The Sallen Key is a classic low pass configuration illustrated in

Figure 22. This provides a very stable low pass function, and in

the case of the ISL59111, a three-pole roll-off at around 8MHz.

The three-pole function is accomplished with an RC low pass

network placed in series with and before the Sallen Key. One

pole provided by the RC network and poles two and three

provided by the Sallen Key for a nice three-pole roll-off at

around 8MHz. If more aggressive, multiple-pole roll-offs are

needed, multiple ISL59111 can be placed in series. There will,

of course, be a loss of bandwidth as additional devices are

added.

AC Output Coupling and the SAG Network

Composite video signals carry viable information at

frequencies as low as 30Hz up to 5MHz. When a video system

output is AC coupled it is critical that the filter represented by

the output coupling capacitor and the surrounding resistance

network provide a band pass function with a low pass band low

enough to exclude very low frequencies down to DC, and with

FN6172 Rev 4.00

April 19, 2007

Page 7 of 10

Share Link: