7
FN6025.5
June 9, 2014
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Detailed Description
The ISL84514 and ISL84515 analog switches offer precise
switching capability from a single 2.4V 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 (2.4V),
low power consumption (5µW), low leakage currents (2nA
max), and the tiny SOT-23 packaging. High frequency
applications also benefit from the wide bandwidth, and the
very high off-isolation.
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 6
). 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 6
). 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 6
). 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 ISL8451x construction is typical of most CMOS analog
switches, except that there are only two supply pins: V+ and
GND. Unlike switches with a 13V maximum supply voltage,
the ISL8451x 15V maximum supply voltage provides plenty
of room for the 10% tolerance of 12V supplies, as well as
margin for overshoot and noise spikes.
The minimum recommended supply voltage is 2.4V. 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 beginning on page 3
and “Typical Performance Curves” beginning on page 9
for
details.
V+ and GND power the internal CMOS switches and set
their analog voltage limits. These supplies also power the
internal logic and level shifters. The level shifters convert the
input logic levels to switched V+ and GND signals to drive
the analog switch gate terminals.
FIGURE 5. CAPACITANCE TEST CIRCUIT
Test Circuits and Waveforms (Continued)
V+
C
GND
NO OR NC
COM
IN
IMPEDANCE
ANALYZER
0V OR V+
FIGURE 6. OVERVOLTAGE PROTECTION
GND
V
COM
V
NO OR NC
OPTIONAL PROTECTION
V+
IN
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
ISL84514, ISL84515
