10
FN6037.4
August 31, 2015
Detailed Description
The ISL43143–ISL43145 quad analog switches offer precise
switching capability from a bipolar
2V to 6V or a single 2V
to 12V supply with low on-resistance (18) and high speed
switching (t
ON
= 52ns, t
OFF
= 40ns). The devices are
especially well suited for portable battery powered
equipment thanks to the low operating supply voltage (2V),
low power consumption (1W), low leakage currents (5nA
max). High frequency applications also benefit from the wide
bandwidth, and the very high OFF isolation and crosstalk
rejection.
Supply Sequencing And Overvoltage Protection
As 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
V- (see Figure 8). To prevent forward biasing these diodes,
V+ and V- must be applied before any input signals, and
input signal voltages must remain between V+ and V-. 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 V-. 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 ISL4314X construction is typical of most CMOS analog
switches, in that they have three supply pins: V+, V-, and
GND. V+ and V- drive the internal CMOS switches and set
their analog voltage limits, so there are no connections
between the analog signal path and GND. Unlike switches
with a 13V maximum supply voltage, the ISL4314X 15V
maximum supply voltage provides plenty of room for the
10% tolerance of 12V supplies (
6V or 12V single supply),
as well as room for overshoot and noise spikes.
This family of switches performs equally well when operated
with bipolar or single voltage supplies, and bipolar supplies
need not be symmetrical. The minimum recommended
supply voltage is 2V or
2V. 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 power the internal logic (thus setting the digital
switching point) and level shifters. The level shifters convert
the logic levels to switched V+ and V- signals to drive the
analog switch gate terminals, so switch parameters -
especially R
ON
- are strong functions of both supplies.
Logic-Level Thresholds
V+ and GND power the internal logic stages, so V- has no
affect on logic thresholds. This switch family is TTL
compatible (0.8V and 2.4V) over a V+ supply range of 2.5V
to 10V (see Figure 17). At 12V the V
IH
level is about 2.8V, so
for best results 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 (see
Figure 18). Driving the digital input signals from GND to V+
with a fast transition time minimizes power dissipation. The
ISL43143-ISL43145 switches have 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 3V logic while operating with
dual or single 5V supplies the device draws only 10A of
current (see Figure 18 for
V
IN
= 3V). Similiar devices of
competitors can draw 8 times this amount of current.
High-Frequency Performance
In 5systems, signal response is reasonably flat even past
200MHz (see Figure 19). Figure 19 also illustrates that 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
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 20 details the high OFF Isolation and
FIGURE 8. OVERVOLTAGE PROTECTION
V-
V
COM
V
NO or NC
OPTIONAL PROTECTION
V+
IN
X
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
ISL43143, ISL43144, ISL43145
