7
Detailed Description
The ISL84521, ISL84522, ISL84523 quad analog switches
offer precise switching capability from a bipolar
2V to 6V or
a single 2V to 12V supply with low on-resistance (65) and
high speed switching (t
ON
= 45ns, t
OFF
= 15ns). The devices
are especially well suited to portable battery powered
equipment thanks to the low operating supply voltage (2V),
low power consumption (1W) and low leakage currents (1nA
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- (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 (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 (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 ISL8452X 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 ISL8452X 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.
FIGURE 6. CROSSTALK TEST CIRCUIT
FIGURE 7. CAPACITANCE TEST CIRCUIT
Test Circuits and Waveforms (Continued)
0V or 2.4V
ANALYZER
V+
C
NO1 OR NC1
SIGNAL
GENERATOR
R
L
GND
IN2
COM1
IN2
50
0V OR 2.4V
NO
COM2
NO2 OR NC2
C
V-
CONNECTION
V+
GND
NO OR NC
COM
IN
IMPEDANCE
ANALYZER
0V OR 2.4V
V-
FIGURE 8. OVERVOLTAGE PROTECTION
V-
V
COM
V
NO OR NC
OPTIONAL PROTECTION
V+
IN
X
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
ISL84521, ISL84522, ISL84523
