12
FN6699.1
June 5, 2008
diodes, VDD must be applied before any input signals, and
the signal voltages must remain between VDD and -3V and
the logic voltage must remain between VDD and ground.
If these conditions cannot be guaranteed, then precautions
must be implemented to prohibit the current and voltage at
the logic pin and signal pins from exceeding the maximum
ratings of the switch. The following two methods can be used
to provided additional protection to limit the current in the
event that the voltage at a signal pin goes below ground by
more than -3V or above the VDD rail and the logic pin goes
below ground or above the VDD rail.
Logic inputs can be protected by adding a 1k
Ω resistor in
series with the logic input (see Figure
8). The resistor limits
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
purpose of using a low rON switch. Connecting Schottky
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 and to clamp when the voltage
reaches the overvoltage limit.
.
High-Frequency Performance
In 50
Ω systems, the ISL54405 has a -3dB bandwidth of
230MHz (see Figure
29). The frequency response is very
consistent over varying analog signal levels.
An OFF-switch acts like a capacitor and passes higher
frequencies with less attenuation, resulting in signal
feedthrough from a switch’s input to its output. Off-Isolation
is the resistance to this feedthrough, while crosstalk
indicates the amount of feedthrough from one switch to
another. Figure
30 details the high Off-Isolation and crosstalk
rejection provided by this part. At 1MHz, Off-Isolation is
about 100dB in 50
Ω systems, 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.
FIGURE 8. OVERVOLTAGE PROTECTION
GND
VCOM
VNX
VDD
LOGIC
OPTIONAL
PROTECTION
RESISTOR
OPTIONAL
SCHOTTKY
DIODE
OPTIONAL
SCHOTTKY
DIODE
INPUT
ISL54405