ISL43120_15 Datasheet, PDF (8/13 Pages) Intersil Corporation – Low-Voltage, Single Supply, Dual SPST Analog Switches

ISL43120, ISL43121, ISL43122
Detailed Description
The ISL43120, ISL43121, ISL43122 bidirectional dual SPST
analog switches offer precise switching capability from a
single 2.7V to 12V supply with low ON-resistance (19Ω) and
high speed operation (t_ON = 28ns, t_OFF = 20ns). The devices
are especially well suited to portable battery-powered
equipment thanks to the low operating supply voltage (2.7V),
low power consumption (5µW), low leakage currents
(100pA max), and the tiny SOT-23 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 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 1kΩ 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.
OPTIONAL
PROTECTION
RESISTOR
IN_X
V_{NO OR NC}
OPTIONAL PROTECTION
DIODE
V+
V_COM
GND
OPTIONAL PROTECTION
DIODE
Power-Supply Considerations
The ISL43120, ISL43121, ISL43122 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 ISL43120,
ISL43121, ISL43122 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 Specifications” tables starting on page 3 and
“Typical Performance Curves” (starting on page 9) for details.
V+ and GND also power the internal logic and level shifter. The
level shifter 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 is TTL compatible (0.8V and 2.4V) over a
supply range of 3V to 11V (see Figure 15). 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.
High-frequency Performance
In 50Ω systems, signal response is reasonably flat even past
300MHz (see Figure 16). Figure 16 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 17 details the high off-isolation and crosstalk rejection
provided by this family. At 10MHz, off isolation is about 50dB
in 50Ω 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
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8
FN6033.6
August 17, 2015