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ISL59830AIAZ

True Single Supply Video Driver With Power Down

Renesas Electronics 

ISL59830A

Description of Operation and Application

Information

Theory Of Operation

The ISL59830A is a highly practical and robust marriage of

three high bandwidth, high speed, low power, rail-to-rail

voltage feedback amplifiers with a charge pump, to provide a

negative rail without an additional power supply. Designed to

operate with a single supply voltage range of from 0V to 3.3V,

the ISL59830A eliminates the need for a split supply with the

incorporation of a charge pump capable of generating a bottom

rail as much as 1.6V below ground; for a 4.9V range on a

single 3.3V supply. This performance is ideal for NTSC video

with its negative-going sync pulses.

The Amplifier

The ISL59830A fabricated on a dielectrically isolated high

speed 5V Bi-CMOS process with 4GHz PNPs and NPN

transistor exceeding 20GHz - perfect for low distortion, low

power demand and high frequency circuits. While the

ISL59830A utilizes somewhat standard voltage mode

feedback topologies, there are many non-standard analog

features providing its outstanding bandwidth, rail-to-rail

operation, and output drive capabilities. The input signal

initially passes through a folded cascode, a topology providing

enhanced frequency response essentially by fixing the base

collector voltage at the junction of the input and gain stage.

The collector of each input device looks directly into an emitter

that is tied closely to ground through a resistor and biased with

a very stable DC source. Since the voltage of this collector is

"locked stable" the effective bandwidth limiting of the Miller

capacitance is greatly reduced. The signal is then passed

through a second fully-realized differential gain stage and

finally through a proprietary common emitter output stage for

improved rail-to-rail output performance. The result is a highly-

stable, low distortion, low power, and high frequency amplifier

capable of driving moderately capacitive loads with near rail-to-

rail performance.

Input Output Range

The three amplifier channels have an input common mode

voltage range from 0.15V below the bottom rail to within

100mV of the positive supply, VS+ pin (Note: bottom rail is

established by the charge pump at negative one half the

positive supply). As the input signal moves outside the

specified range, the output signal will exhibit increasingly

higher levels of harmonic distortion. And of course, as load

resistance becomes lower, the current drive capability of the

device will be challenged and its ability to drive close to each

rail is reduced. For instance, with a load resistance of 1k the

output swing is within 100mV of the rails, while a load

resistance of 150 limits the output swing to within around

300mV of the rails.

Amplifier Output Impedance

To achieve near rail-to-rail performance, the output stage of the

ISL59830A uses transistors in the common emitter

configuration, typically producing higher output impedance

than the standard emitter follower output stage. The

exceptionally high open loop gain of the ISL59830A and local

feedback reduces output impedance to less than 2 at low

frequency. However, since output impedance of the device is

exponentially modulated by the magnitude of the open loop

gain, output impedance increases with frequency as the open

loop gain decreases with frequency. This inductive-like effect of

the output impedance is countered in the ISL59830A with

proprietary output stage topology, keeping the output

impedance low over a wide frequency range and making it

possible to easily and effectively drive relatively heavy

capacitive loads (See Figure 10).

IN+

IN-

BIAS

FN6233 Rev.3.00

Sep 26, 2006

FIGURE 27.

OUT

Page 11 of 15

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