ISL43L110, ISL43L111
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 R
ON
switch. Connecting schottky
diodes to the signal pins as shown in Figure 6 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.
Logic-Level Thresholds
This switch family is 1.8V CMOS compatible (0.5V and 1.4V)
over a supply range of 2V to 3.6V (See Figure 15). At 3.6V
the V
IH
level is about 1.1V. This is still below the 1.8V CMOS
guaranteed high output minimum level of 1.4V, 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.
OPTIONAL
SCHOTTKY
DIODE
V+
OPTIONAL
PROTECTION
RESISTOR
High-Frequency Performance
In 50Ω systems, signal response is reasonably flat even past
100MHz (See Figure 16). The frequency response is very
consistent over a wide V+ range, and for varying analog
signal levels.
V
COM
IN
X
V
NX
OPTIONAL
SCHOTTKY
DIODE
GND
FIGURE 6. OVERVOLTAGE PROTECTION
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. Figure 17 details the high
Off Isolation rejection provided by this family. At 100kHz, Off
Isolation is about 65dB in 50Ω systems, decreasing
approximately 20dB per decade as frequency increases.
Higher load impedances decrease Off Isolation due to the
voltage divider action of the switch OFF impedance and the
load impedance.
Power-Supply Considerations
The ISL43L11X 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 ISL43L11X 4.7V maximum
supply voltage provides plenty of room for the 10% tolerance
of 4.2V supplies, as well as room for overshoot and noise
spikes.
The minimum recommended supply voltage is 1.1V. 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 and
Typical Performance
curves for details.
V+ and GND also power the internal logic and level shiftier.
The level shiftier converts 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.
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.
8
FN6130.0
March 15, 2005
INTERSIL [ INTERSIL CORPORATION ]
