__________Applications Information

Power-Supply Considerations

Overview

The MAX4521/MAX4522/MAX4523 construction is typi-

cal of most CMOS analog switches. They have three

supply pins: V+, V-, and GND. V+ and V- are used to

drive the internal CMOS switches, and they set the lim-

its of the analog voltage on any switch. Reverse ESD-

protection diodes are internally connected between

each analog-signal pin and both V+ and V-. If any ana-

log signal exceeds V+ or V-, one of these diodes con-

ducts. During normal operation these reverse-biased

ESD diodes leak, forming the only current drawn from

V+ or V-.

Virtually all the analog leakage current is through the

ESD diodes. Although the ESD diodes on a given sig-

nal pin are identical and therefore fairly well balanced,

they are reverse biased differently. Each is biased by

either V+ or V- and the analog signal. This means their

leakages vary as the signal varies. The

difference in the

two diode leakages from the signal path to the V+ and

V- pins constitutes the analog-signal-path leakage cur-

rent. All analog leakage current flows to the supply ter-

minals, not to the other switch terminal. This explains

how both sides of a given switch can show leakage

currents of the same or opposite polarity.

There is no connection between the analog-signal

paths and GND. The analog-signal paths consist of an

N-channel and P-channel MOSFET with their sources

and drains paralleled, and their gates driven out of

phase to V+ and V- by the logic-level translators.

V+ and GND power the internal logic and logic-level

translators, and set the input logic thresholds. The

logic-level translators convert the logic levels to

switched V+ and V- signals, to drive the gates of the

analog switches. This drive signal is the only connec-

tion between the logic supplies and the analog sup-

plies. V+ and V- have ESD-protection diodes to GND.

The logic-level inputs and output have ESD protection

to V+ and to GND.

Increasing V- has no effect on the logic-level thresh-

olds, but it does increase the drive to the P-channel

switches, reducing their on-resistance. V- also sets the

negative limit of the analog-signal voltage.

The logic-level thresholds are CMOS/TTL compatible

when V+ = +5V. The threshold increases slightly as V+

is raised, and when V+ reaches +12V, the level thresh-

old is about 3.1V. This is above the TTL output high-

level minimum of 2.8V, but still compatible with CMOS

outputs.

Bipolar Supplies

The MAX4521/MAX4522/MAX4523 operate with bipolar

supplies between ±2V and ±6V. The V+ and V- sup-

plies need not be symmetrical, but their sum cannot

exceed the absolute maximum rating of 13.0V. Do not

connect the MAX4521/MAX4522/MAX4523 V+ to +3V,

and then connect the logic-level-input pins to TTL

logic-level signals. TTL logic-level outputs in excess

of the absolute maximum ratings can damage the

part and/or external circuits.

Caution: The absolute maximum V+ to V- differential

voltage is 13.0V. Typical ±6V or 12V supplies with

±10% tolerances can be as high as 13.2V. This voltage

can damage the MAX4521/MAX4522/MAX4523. Even

±5% tolerance supplies may have overshoot or noise

spikes that exceed 13.0V.

Single Supply

The MAX4521/MAX4522/MAX4523 operate from a

single supply between +2V and +12V when V- is con-

nected to GND. All of the bipolar precautions must be

observed.

High-Frequency Performance

In 50

Ω systems, signal response is reasonably flat up

to 50MHz (see

Typical Operating Characteristics).

Above 20MHz, the on-response has several minor

peaks that are highly layout dependent. The problem

with high-frequency operation is not turning the switch

on, but turning it off. The off-state switch acts like a

capacitor and passes higher frequencies with less

attenuation. At 10MHz, off-isolation is about -52dB in

50

Ω systems, becoming worse (approximately 20dB

per decade) as frequency increases. Higher circuit

impedances also make off-isolation worse. Adjacent

channel attenuation is about 3dB above that of a bare

IC socket, and is due entirely to capacitive coupling.

Quad, Low-Voltage, SPST Analog Switches

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