ISL59837

FN6335 Rev 1.00

Page 12 of 14

March 5, 2007

THE CHARGE PUMP

The ISL59837 charge pump provides a bottom rail up to 1.6V

below ground while operating on a 0V to 3.3V power supply.

The charge pump is internally regulated to one-half the

potential of the positive supply. This internal multi-phase

charge pump is driven by a 110MHz differential ring oscillator

driving a series of inverters and charge storage circuitry. Each

series inverter charges and places parallel adjoining charge

circuitry slightly out of phase with the immediately preceding

block. This generates a negative rail of about -1.6V with a low

amplitude ripple voltage from the charge pump action. Some of

this ripple is coupled into the output signals at a very low

amplitude, as seen in Figure 21. The ripple on the outputs is

typically well below the noise floor of the signal.

There are two ways to further reduce the output supply noise:

• Add a 120

reduces the coupling between the charge pump and the

analog amplifier supplies.

• Add a 20pF capacitor between the back load 75

 resistor

and ground (see “ISL59837 + DC-Restore Solution” on

page 10). This will attenuate frequencies above 100MHz.

The system operates at sufficiently high frequencies that any

related charge pump noise is far beyond standard video

bandwidth requirements. Still, appropriate bypassing discipline

must be observed, and all pins related to either the power

supply or the charge pump must be properly bypassed. See

“Power Supply Bypassing and Printed Circuit Board Layout” on

page 13.

THE VREF PIN

Applying a voltage to the VREF pin simply places that voltage

on what would usually be the ground side of the gain resistor of

the amplifier, resulting in a DC-level shift of the output signal.

Applying 100mV to the VREF pin would apply a 100mV DC

level shift to the outgoing signal. The charge pump provides

sufficient bottom room to accommodate the shifted signal.

The ISL59837 buffers the VREF voltage before applying it to

the triple amplifiers, isolating the input from the amplifiers and

allowing it to be driven by moderate-impedance voltage

sources.

THE VEE PIN

The VEE pin is the output pin for the charge pump. A voltmeter

applied to this pin will display the output of the charge pump.

This pin does not affect the functionality of the part. One may

use this pin as an additional voltage source. Keep in mind that

the output of this pin is generated by the internal charge pump

and a fully regulated supply that must be properly bypassed.

We recommend a 0.1µF ceramic capacitor placed as close to

the pin and connected to the ground plane of the board.

INPUT, OUTPUT AND SUPPLY VOLTAGE RANGE

The ISL59837 is designed to operate with a single supply

voltage range of from 0V to 3.3V. The need for a split supply

has been eliminated 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.

VIDEO PERFORMANCE

For good video performance, an amplifier is required to maintain

the same output impedance and the same frequency and phase

response as DC levels are changed at the output. This is

especially difficult when driving a standard video load of 150



because of the change in output current with changing DC

levels. Special circuitry has been incorporated into the ISL59837

for the reduction of output impedance variation with the current

output. This results in outstanding differential gain and

differential phase specifications of 0.06% and 0.1°, while driving

150

 at a gain of +2. Driving higher impedance loads would

result in similar or better differential gain and differential phase

performance.

NTSC

The ISL59837 (generating a negative rail internally) is ideally

suited for NTSC video with its accompanying negative-going

sync signals, which is easily handled by the ISL59837 without

the need for an additional supply as the ISL59837 generates a

negative rail with an internal charge pump referenced at

negative 1/2 the positive supply.

YPbPr

YPbPr signals originating from a DVD player requiring three

channels of very tightly-controlled amplifier gain accuracy present

no difficulty for the ISL59837. Specifically, this standard encodes

sync on the Y-Channel and it is a negative-going signal, which is

easily handled by the ISL59837 without the need for an additional

supply as the ISL59837 generates a negative rail placed at

negative 1/2 the positive supply. Additionally, the Pb and Pr are

bipolar analog signals and the video signals are negative-going,

and again, easily handled by the ISL59837.

DRIVING CAPACITIVE LOADS AND CABLES

The ISL59837 (internally-compensated to drive 75

 cables) will

drive 10pF loads in parallel with 1k

 with less than 5dB of

peaking. If less peaking is required, a small series resistor, usually

between 5

 to 50, can be placed in series with the output. This

will reduce peaking at the expense of a slight closed loop gain

reduction. When used as a cable driver, double termination is

always recommended for reflection-free performance. For those

applications, a back-termination series resistor at the amplifier's

output will isolate the amplifier from the cable and allow extensive

capacitive drive. However, other applications may have high

capacitive loads without a back-termination resistor. Again, a

small series resistor at the output can help to reduce peaking. The

ISL59837 is a triple amplifier designed to drive three channels;

simply deal with each channel separately as described in this

section.