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Detailed Description

The ISL84514 and ISL84515 analog switches offer precise

switching capability from a single 2.4V 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 (2.4V),

low power consumption (5

µW), low leakage currents (2nA

max), and the tiny SOT-23 packaging. High frequency

applications also benefit from the wide bandwidth, and the

very high Off Isolation.

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 GND (see

Figure 6). 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 1k

Ω

resistor in series with the input (see Figure 6). 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.

Adding a series resistor to the switch input defeats the

diodes can be added in series with the supply pins to provide

overvoltage protection for all pins (see Figure 6). 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.

Power-Supply Considerations

The ISL8451X construction is typical of most CMOS analog

switches, except that there are only two supply pins: V+ and

GND. Unlike switches with a 13V maximum supply voltage,

the ISL8451X 15V maximum supply voltage provides plenty

of room for the 10% tolerance of 12V supplies, as well as

margin for overshoot and noise spikes.

The minimum recommended supply voltage is 2.4V. 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 power the internal CMOS switches and set

their analog voltage limits. These supplies 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. For a

±5V single SPST switch,

see the ISL84516, ISL84517 data sheet.

Logic-Level Thresholds

This switch family is TTL compatible (0.8V and 2.4V) over a

supply range of 3V to 11V, and the full temperature range

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

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 5

0Ω systems, signal response is reasonably flat to

20MHz, with a -3dB bandwidth exceeding 200MHz (see

Figure 13). Figure 13 also illustrates that 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. Figure 14 details the high

Off Isolation provided by this family. At 10MHz, off isolation is

about 50dB in 5

0Ω 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.

FIGURE 6. OVERVOLTAGE PROTECTION

GND

OPTIONAL PROTECTION

V+

IN

DIODE

OPTIONAL PROTECTION

DIODE

OPTIONAL

PROTECTION

RESISTOR

ISL84514, ISL84515