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FN6503.0

May 31, 2007

the input current below the threshold that produces

permanent damage, and the sub-microamp input current

produces an insignificant voltage drop during normal

operation.

This method is not acceptable for the signal path inputs.

Adding a series resistor to the switch input defeats the

diodes to the signal pins as shown in Figure 8 will shunt the

fault current to the supply or to ground thereby protecting the

switch. These schottky diodes must be sized to handle the

expected fault current.

Power-Supply Considerations

The ISL54047 construction is typical of most single supply

CMOS analog switches, in that they have two supply pins:

V+ and GND. V+ and GND drive the internal CMOS

switches and set their analog voltage limits. Unlike switches

with a 4V maximum supply voltage, the ISL54047 5.5V

maximum supply voltage provides plenty of room for the

10% tolerance of 4.3V supplies, as well as room for

overshoot and noise spikes.

The minimum recommended supply voltage is 1.65V. It is

important to note that the input signal range, switching times,

and on-resistance degrade at lower supply voltages. Refer

to the Electrical Specification Tables on page 5 and Typical

Performance Curves on page 9 for details.

V+ and GND also power the internal logic and level shifters.

The level shifters convert the input logic levels to switched

V+ and GND signals to drive the analog switch gate

terminals.

This family of switches cannot be operated with bipolar

supplies, because the input switching point becomes

negative in this configuration.

Logic-Level Thresholds

This switch family are 1.8V CMOS compatible (0.5V and

1.4V) over a supply range of 2.7V to 4.5V (see Figure 18). At

CMOS guaranteed low output maximum level of 0.5V, but

noise margin is reduced.

The digital input stages draw supply current whenever the

digital input voltage is not at one of the supply rails. Driving

the digital input signals from GND to V+ with a fast transition

time minimizes power dissipation.

The ISL54047 has been designed to minimize the supply

current whenever the digital input voltage is not driven to the

supply rails (0V to V+). For example driving the device with

2.85V logic (0V to 2.85V) while operating with a 4.2V supply

VIN = 2.85V).

Frequency Performance

In 50

Ω systems, the ISL54047 has a -3dB bandwidth of

27MHz (see Figure 21). The frequency response is very

consistent over a wide V+ range, and for varying analog

signal levels.

An OFF switch acts like a capacitor and passes higher

frequencies with less attenuation, resulting in signal feed

through from a switch’s input to its output. Off Isolation is the

resistance to this feed through, while Crosstalk indicates the

amount of feed through from one switch to another. Figure

22 details the high Off Isolation and Crosstalk rejection

provided by this part. At 100kHz, Off Isolation is about 102dB

in 50

Ω systems, 118dB into 8Ω, and 124dB into 4Ω,

decreasing approximately 20dB per decade as frequency

increases. Higher load impedances decrease Off Isolation

and Crosstalk rejection due to the voltage divider action of

the switch OFF impedance and the load impedance.

Leakage Considerations

Reverse ESD protection diodes are internally connected

between each analog-signal pin and both V+ and GND. One of

these diodes conducts if any analog signal exceeds V+ or

GND.

Virtually all the analog leakage current comes from the ESD

diodes to V+ or GND. Although the ESD diodes on a given

signal pin are identical and therefore fairly well balanced,

they are reverse biased differently. Each is biased by either

V+ or GND and the analog signal. This means their leakages

will vary as the signal varies. The difference in the two diode

leakages to the V+ and GND pins constitutes the analog-

signal-path leakage current. All analog leakage current flows

between each pin and one of the supply terminals, not to the

other switch terminal. This is why both sides of a given

switch can show leakage currents of the same or opposite

polarity. There is no connection between the analog signal

paths and V+ or GND.

FIGURE 8. OVERVOLTAGE PROTECTION

GND

V+

OPTIONAL

PROTECTION

RESISTOR

OPTIONAL

SCHOTTKY

DIODE

OPTIONAL

SCHOTTKY

DIODE

ISL54047