12
FN6335.1
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 120Ω bead in series between V
CC
and DV
CC
. This
reduces the coupling between the charge pump and the
analog amplifier supplies.
• Add a 20pF capacitor between the back load 75Ω 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 150Ω 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 150Ω 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 75Ω cables) will
drive 10pF loads in parallel with 1kΩ with less than 5dB of
peaking. If less peaking is required, a small series resistor,
usually between 5Ω to 50Ω, 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
ISL59837
