7
Detailed Description
The ISL8323–ISL8325 dual analog switches offer precise
switching capability from a single 2.7V to 12V supply with
low on-resistance and high speed operation. The devices
are especially well suited to portable battery powered
equipment thanks to the low operating supply voltage (2.7V),
low power consumption (5W), low leakage currents (100pA
max), and the small SOIC packaging. High frequency
applications also benefit from the wide bandwidth, and the
very high off isolation and crosstalk rejection.
Supply Sequencing And Overvoltage Protection
With any CMOS device, proper power supply sequencing is
required to protect the device from excessive input currents
which might permanently damage the IC. All I/O pins contain
ESD protection diodes from the pin to V+ and to GND (see
Figure 8). To prevent forward biasing these diodes, V+ must
be applied before any input signals, and input signal
voltages must remain between V+ and GND. If these
conditions cannot be guaranteed, then one of the following
two protection methods should be employed.
Logic inputs can easily be protected by adding a 1k
resistor in series with the 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.
Adding a series resistor to the switch input defeats the
purpose of using a low R
ON
switch, so two small signal
diodes can be added in series with the supply pins to provide
overvoltage protection for all pins (see Figure 8). These
additional diodes limit the analog signal from 1V below V+ to
1V above GND. The low leakage current performance is
unaffected by this approach, but the switch resistance may
increase, especially at low supply voltages.
Power-Supply Considerations
The ISL832X construction is typical of most CMOS analog
switches, except that they have only two supply pins: V+ and
GND. V+ and GND drive the internal CMOS switches and
set their analog voltage limits. Unlike switches with a 13V
maximum supply voltage, the ISL832X 15V maximum supply
voltage provides plenty of room for the 10% tolerance of 12V
supplies, as well as room for overshoot and noise spikes.
The minimum recommended supply voltage is 2.7V. 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 shifters.
The level shifters convert the 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 is TTL compatible (0.8V and 2.4V) over a
supply range of 3V to 11V (see Figure 12). At 12V the V
IH
level is about 2.5V. This is still below the TTL guaranteed
high output minimum level of 2.8V, but noise margin is
reduced. For best results with a 12V supply, use a logic
family the provides a V
OH
greater than 3V.
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.
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
COM
V
NO or NC
OPTIONAL PROTECTION
V+
IN
X
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
ISL8323, ISL8324, ISL8325
