11

FN7489.6

May 4, 2006

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 ISL59830

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 11).

The Charge Pump

The ISL59830 charge pump provides a bottom rail up to

1.65V 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 160MHz 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. The overall effect is sequential

discharge and generation of a very low ripple of about 10mV

that is applied to the amplifiers providing a negative rail of up

to -1.65V.

There are two options to reduce the output supply noise.

• Add a 120

further reduce ripple.

Add a 20pF capacitor between the back load 75

Ω resistor

and ground (see the ISL59830A + DC-Restore Solution

schematic on page 10).

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" in this section.

To maximize resistance to latch-up, a diode should be added

between the VEEOUT pin (anode) and GND (cathode), as

shown in the Demo Board Schematic. This prevents VEE

from rising more than 0.7V above ground during startup.

(VEE > 1V above GND can cause latchup under some

conditions.)

BIAS

OUT

FIGURE 27.

TIME (20ns/DIV)

FIGURE 28. CHARGE PUMP OSCILLATION (AMP OUTPUT)

ISL59830