ISL84581 データシートの表示（PDF） - Renesas Electronics

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ISL84581

Low-Voltage, Single and Dual Supply, 8-to-1 Multiplexer

Renesas Electronics 

ISL84581

Detailed Description

The ISL84581 multiplexer offers precise switching capability

from bipolar ±2V to ±6V supplies or a single 2V to 12V

supply. When powered with dual ±5V supplies the part has

low ON-resistance (39) and high speed operation

(tON = 38ns, tOFF = 19ns).

It has an inhibit pin to simultaneously open all signal paths.

The device is especially well suited for applications using

±5V supplies. With ±5V supplies the performance (rON,

Leakage, Charge Injection, etc.) is best in class.

High frequency applications also benefit from the wide

bandwidth and 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 V- (see

Figure 7). To prevent forward biasing these diodes, V+ and

V- must be applied before any input signals, and input signal

voltages must remain between V+ and V-. 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

resistor in series with the input (see Figure 7). 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 7). These

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

1V above V-. 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

OPTIONAL PROTECTION

DIODE

V+

1k

LOGIC

VNOx

VCOM

V-

OPTIONAL PROTECTION

DIODE

FIGURE 7. INPUT OVERVOLTAGE PROTECTION

FN6416 Rev 3.00

April 13, 2009

Power-Supply Considerations

The ISL84581 construction is typical of most CMOS analog

switches, in that it has three supply pins: V+, V-, and GND.

V+ and V- drive the internal CMOS switches and set their

analog voltage limits, so there are no connections between

the analog signal path and GND. Unlike switches with a 13V

maximum supply voltage, the ISL84581 15V maximum

supply voltage provides plenty of room for the 10% tolerance

of 12V supplies (±6V or 12V single supply), as well as room

for overshoot and noise spikes.

The part performs equally well when operated with bipolar or

single voltage supplies.The minimum recommended supply

voltage is 2V single supply or ±2V dual supply. 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 4 and “Typical

Performance Curves” on page 11 for details.

V+ and GND power the internal logic setting the digital

switching point of the level shifters. The level shifters convert

the logic levels to switched V+ and V- signals to drive the

analog switch gate terminals.

Logic-Level Thresholds

V+ and GND power the internal logic stages, so V- has no

affect on logic thresholds. This ISL84581 is TTL compatible

(0.8V and 2.4V) over a V+ supply range of 2.7V to 10V. At

12V the VIH level is about 3.3V. This is still below the CMOS

guaranteed high output minimum level of 4V, but noise

margin is reduced. For best results with a 12V supply, use a

logic family that provides a VOH greater than 4V.

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 50 systems, signal response is reasonably flat even past

100MHz (see Figures 16 and 17). Figures 16 and 17 also

illustrate 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. Figure 18 details the high off

isolation of the ISL84581. At 10MHz, off-isolation is about

55dB in 50 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.

Page 10 of 15

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