7

FN6094.3

November 5, 2004

Detailed Description

The ISL83699 is a bidirectional, dual double-pole/

double-throw (DPDT) analog switch that offers precise

switching capability from a single 1.65V to 3.6V supply with

low on-resistance (0.26

Ω) and high speed operation

for portable battery powered equipment due to its low

operating supply voltage (1.65V), low power consumption

(2.7

µW max), low leakage currents (45nA max), and the tiny

QFN and TSSOP packages. The ultra low on-resistance and

Ron flatness provide very low insertion loss and distortion to

applications that require signal reproduction.

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 8). 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 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 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 acceptable for the signal path inputs.

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 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 signal range is

reduced and the resistance may increase, especially at low

supply voltages.

Power-Supply Considerations

The ISL83699 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 ISL83699 4.7V

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 1.65V but the

part will operate with a supply below 1.5V. 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 1.8V CMOS compatible (0.5V and 1.4V)

over a supply range of 2.0V to 3.6V (See Figure 17). At 3.6V

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 50

Ω systems, the signal response is reasonably flat even

past 30MHz with a -3dB bandwidth of 104MHz (See Figure

15). 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, while Crosstalk indicates

the amount of feedthrough from one switch to another.

Figure 16 details the high Off Isolation and Crosstalk

rejection provided by this part. At 100kHz, Off Isolation is

about 68dB in 50

Ω 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.

FIGURE 8. OVERVOLTAGE PROTECTION

GND

OPTIONAL PROTECTION

V+

DIODE

OPTIONAL PROTECTION

DIODE

OPTIONAL

PROTECTION

RESISTOR

ISL83699