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11
FN6054.3
October 28, 2010
Detailed Description
The ISL43231 analog switch offers a precise switching
capability from a bipolar ±2V to ±6V or a single 2V to 12V
supply with low ON-resistance (39
Ω) and high speed
operation (tON =38ns, tOFF = 19ns) with dual 5V supplies.
It has an inhibit and inhibit bar pin to simultaneously open all
signal paths. It also has a latch bar pin to lock in the last
switch address.
The device is especially well suited for applications using
±5V supplies. With ±5V supplies, the performance (RON,
Leakage, Charge Injection, etc.) is best in class.
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 to V- (see
Figure
9). 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
Ω
resistor in series with the input (see Figure
9). 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.
This method is not applicable for the signal path inputs.
Adding a series resistor to the switch input defeats the
purpose of using a low RON switch, so two small signal
diodes can be added in series with the supply pins to provide
overvoltage protection for all pins (see Figure
9). 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 ISL43231 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 ISL43231 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 switch device performs equally well when operated with
bipolar or single voltage supplies. The minimum
recommended supply voltage is 2V or ±2V. It is important to
FIGURE 7. CROSSTALK TEST CIRCUIT
FIGURE 8. CAPACITANCE TEST CIRCUIT
Test Circuits and Waveforms (Continued)
0V or V+
ANALYZER
NOA or NCA
SIGNAL
GENERATOR
RL
GND
ADDX
50
Ω
N.C.
COMB
NOB or NCB
EN
V-
C
V+
C
COMA
EN, LE
GND
NO or NC
COM
ADDX
IMPEDANCE
ANALYZER
0V or V+
EN
V-
C
V+
C
EN, LE
FIGURE 9. INPUT OVERVOLTAGE PROTECTION
V-
VCOM
VNO or NC
OPTIONAL PROTECTION
V+
LOGIC
DIODE
OPTIONAL PROTECTION
DIODE
OPTIONAL
PROTECTION
RESISTOR
FOR LOGIC
INPUTS
1k
Ω
ISL43231