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ISL84524

Low-Voltage, Single Supply, 4 to 1 Multiplexer and DPDT Analog Switches

Intersil 

ISL84524, ISL84525

Detailed Description

The ISL84524 and ISL84525 operate from a single 2V to

12V supply with low on-resistance and high speed operation.

The devices are especially well suited to portable battery

powered equipment thanks to the low operating supply

voltage (2.7V), low power consumption (3µW), low leakage

currents (25nA max), and the tiny MSOP packaging. High

frequency applications also benefit from the wide bandwidth,

and the very high off isolation (75 dB) and crosstalk rejection

(-85dB).

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 GND (see

Figure 8). To prevent forward biasing these diodes, V+ must

be applied before any input signals, and input signal voltages

must remain between V+ and GND. 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.

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 8). These

additional diodes limit the analog signal from 1V below V+ to

1V above GND. The low leakage current performance is

unaffected by this approach, but the switch resistance may

increase, especially at low supply voltages.

OPTIONAL

PROTECTION

RESISTOR

FOR LOGIC

INPUTS

1kΩ

1kΩ

ADDX

INH

VNO or NC

OPTIONAL PROTECTION

DIODE

V+

VCOM

GND

OPTIONAL PROTECTION

DIODE

FIGURE 8. OVERVOLTAGE PROTECTION

Power-Supply Considerations

The ISL8452X construction is typical of most CMOS analog

switches, except that they have only two supply pins: V+ and

GND. V+ and GND drive the internal CMOS switches and

set their analog voltage limits. Unlike switches with a 13V

maximum supply voltage, the ISL8452X 15V maximum

supply voltage provides plenty of room for the 10% tolerance

of 12V supplies, as well as room for overshoot and noise

spikes.

The minimum recommended supply voltage is 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 specification tables and Typical Performance

curves 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 is TTL compatible (0.8V and 2.4V) over a

supply range of 2V to 11V. At 12V the VIH level is about 2.5V.

This is still below the TTL guaranteed high output minimum

level of 2.8V, but noise margin is reduced. For best results

with a 12V supply, use a logic family that provides a VOH

greater than 3V.

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, signal response is reasonably flat even past

100MHz (see Figure 13). Figure 13 also illustrates that the

frequency response is very consistent over 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

14 details the high Off Isolation and Crosstalk rejection

provided by this family. At 10MHz, Off Isolation is about 55dB

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.

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