ISL54048_0706 Datasheet, PDF (8/12 Pages) Intersil Corporation – Ultra Low ON-Resistance, +1.65V to +4.5V, Single Supply, Dual SPST Analog Switch

ISL54048, ISL54049
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.
Power-Supply Considerations
The ISL54048 and ISL54049 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 ISL54048
and ISL54049 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 “Electrical Specifications” on page 3 and the Typical
Performance Curves on page 9 for details.
OPTIONAL
SCHOTTKY
DIODE
OPTIONAL
PROTECTION
RESISTOR
V+
IN_X
V_NX
V_COM
OPTIONAL
SCHOTTKY
DIODE
GND
FIGURE 8. OVERVOLTAGE PROTECTION
V+ and GND also power the internal logic and level shiftiers.
The level shiftiers 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 logic 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 logic
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 ISL54048 and ISL54049 have 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 V_IN = 2.85V).
Frequency Performance
In 50Ω systems, the ISL54048 and ISL54049 have a -3dB
bandwidth of 120MHz (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
feedthrough from a switch’s input to its output. Off isolation is
the resistance to this feedthrough, while crosstalk indicates
the amount of feedthrough 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 62dB
in 50Ω systems, 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
FN6469.1
June 11, 2007