ISL54504 Datasheet, PDF (8/12 Pages) Intersil Corporation – +1.8V to +5.5V, 2.5OHM, Single SPST Analog Switches

ISL54504, ISL54505
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 7) 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 7. OVERVOLTAGE PROTECTION
Power-Supply Considerations
The ISL54504, ISL54505 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 ISL54504, ISL54505 5.5V
maximum supply voltage provides plenty of room for
the 10% tolerance of 3.6V supplies, as well as room for
overshoot and noise spikes.
The minimum recommended supply voltage is 1.8V but
the part will operate with a supply below 1.8V. It is
important to note that the input signal range, switching
times, and ON-resistance degrade at lower supply
voltages. Refer to the “Electrical Specifications” tables
starting on page 3 and the “Typical Performance
Curves” starting on page 9 for details.
V+ and GND also power the internal logic and level
shiftier. The level shiftier converts 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 is 1.8V CMOS compatible (0.5V and
1.4V) over a supply range of 2V to 3.6V (see
Figure 14). At 3.6V the V_IH level is about 0.95V. This is
still below the 1.8V CMOS guaranteed high output
minimum level of 1.4V, 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.
High-Frequency Performance
In 50Ω systems, the ISL54504/ISL54505 has a -3dB
bandwidth of 250MHz (see Figure 15). The frequency
response is very consistent over a wide V+ range and
for varying analog signal levels.
An OFF switch behaves like a capacitor and passes
higher frequencies with less attenuation, resulting in
signal feedthrough from a switch’s input to output. Off
isolation is the resistance of this signal feedthrough.
Figure 16 details the high off isolation provided by the
ISL54504, ISL54505. At 1MHz, off isolation is about
70dB in 50Ω systems, decreasing approximately 20dB
per decade as frequency increases. Higher load
impedances decrease off isolation 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
FN6552.2
October 23, 2009