8
Detailed Description
The ISL8391–ISL8393 quad analog switches offer precise
switching capability from a bipolar ±2V to ±6V or a single 2V
to 12V supply with low on-resistance (20Ω) and high speed
switching (t
ON
= 60ns, t
OFF
= 30ns). The devices are
especially well suited to portable battery powered equipment
thanks to the low operating supply voltage (2V), low power
consumption (1µW), low leakage currents (2.5nA 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 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 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 ISL839X 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 ISL839X 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
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
IN1
COM1
IN2
50Ω
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
ISL8391, ISL8392, ISL8393
