ISL8484 View Datasheet(PDF) - Renesas Electronics

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Description

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ISL8484

Ultra Low ON-Resistance, +1.65V to +4.5V, Single Supply, Dual SPDT Analog Switch

Renesas Electronics 

ISL8484

.

V+ C

OPTIONAL

PROTECTION

RESISTOR

100

NO

COM

NC

IN

GND

FIGURE 8. V+ SERIES RESISTOR FOR ENHANCED ESD AND

LATCH-UP IMMUNITY

OPTIONAL

SCHOTTKY

DIODE

OPTIONAL

PROTECTION

RESISTOR

V+

INX

VNX

VCOM

OPTIONAL

SCHOTTKY

DIODE

GND

FIGURE 9. OVERVOLTAGE PROTECTION

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 9). To prevent forward biasing these diodes, V+ must be

applied before any input signals, and the input signal voltages

must remain between V+ and GND.

If these conditions cannot be guaranteed, then precautions

must be implemented to prohibit the current and voltage at the

logic pin and signal pins from exceeding the maximum ratings

of the switch. The following two methods can be used to

provided additional protection to limit the current in the event

that the voltage at a signal pin or logic pin goes below ground

or above the V+ rail.

Logic inputs can be protected by adding a 1k resistor in

series with the logic input (see Figure 9). 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.

FN6128 Rev 5.00

May 12, 2008

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 rON 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 ISL8484 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 ISL8484 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

the “Electrical Specifications” tables, beginning on page 3, and

“Typical Performance Curves”, beginning on page 9, for

details.

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 is 1.8V CMOS compatible (0.5V and 1.4V)

over a supply range of 2.7V to 4.5V (see Figure18). At 2.7V the

VIL level is about 0.53V. This is still above the 1.8V CMOS

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 ISL8484 has 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 Figure17 for VIN = 2.85V).

High-Frequency Performance

In 50 systems, the signal response is reasonably flat even

past 30MHz with a -3dB bandwidth of 120MHz (see Figure 22).

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

Page 8 of 13

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