ISL54047 Datasheet, PDF (8/12 Pages) Intersil Corporation – Ultra Low ON-Resistance, High Off- Isolation, Single Supply, Diff SPST Analog Switch

ISL54047
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 acceptable for the signal path inputs.
Adding a series resistor to the switch input defeats the
purpose of using a low R_ON switch. Connecting schottky
diodes to the signal pins as shown in Figure 8 will shunt the
fault current to the supply or to ground thereby protecting the
switch. These schottky diodes must be sized to handle the
expected fault current.
OPTIONAL
SCHOTTKY
DIODE
OPTIONAL
PROTECTION
RESISTOR
V+
IN_X
V_NX
V_COM
OPTIONAL
SCHOTTKY
DIODE
GND
FIGURE 8. OVERVOLTAGE PROTECTION
Power-Supply Considerations
The ISL54047 construction is typical of most single supply
CMOS analog switches, in that they have two supply pins:
V+ and GND. V+ and GND drive the internal CMOS
switches and set their analog voltage limits. Unlike switches
with a 4V maximum supply voltage, the ISL54047 5.5V
maximum supply voltage provides plenty of room for the
10% tolerance of 4.3V supplies, as well as room for
overshoot and noise spikes.
The minimum recommended supply voltage is 1.65V. 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 on page 5 and Typical
Performance Curves on page 9 for details.
V+ and GND 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.
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 are 1.8V CMOS compatible (0.5V and
1.4V) over a supply range of 2.7V to 4.5V (see Figure 18). At
2.7V the V_IL level is about 0.53V. This is still above the 1.8V
CMOS guaranteed low output maximum level of 0.5V, but
noise margin is reduced.
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.
The ISL54047 has been designed to minimize the supply
current whenever the digital input voltage is not driven to the
supply rails (0V to V+). For example driving the device with
2.85V logic (0V to 2.85V) while operating with a 4.2V supply
the device draws only 12μA of current (see Figure 16 for
VIN = 2.85V).
Frequency Performance
In 50Ω systems, the ISL54047 has a -3dB bandwidth of
27MHz (see Figure 21). 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 feed
through from a switch’s input to its output. Off Isolation is the
resistance to this feed through, while Crosstalk indicates the
amount of feed through from one switch to another. Figure
22 details the high Off Isolation and Crosstalk rejection
provided by this part. At 100kHz, Off Isolation is about 102dB
in 50Ω systems, 118dB into 8Ω, and 124dB into 4Ω,
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.
Leakage Considerations
Reverse ESD protection diodes are internally connected
between each analog-signal pin and both V+ and GND. One of
these diodes conducts if any analog signal exceeds V+ or
GND.
Virtually all the analog leakage current comes from the ESD
diodes to V+ or GND. Although the ESD diodes on a given
signal pin are identical and therefore fairly well balanced,
they are reverse biased differently. Each is biased by either
V+ or GND and the analog signal. This means their leakages
will vary as the signal varies. The difference in the two diode
leakages to the V+ and GND pins constitutes the analog-
signal-path leakage current. All analog leakage current flows
between each pin and one of the supply terminals, not to the
other switch terminal. This is why both sides of a given
switch can show leakage currents of the same or opposite
polarity. There is no connection between the analog signal
paths and V+ or GND.
8
FN6503.0
May 31, 2007