__________Applications Information

Power-Supply Considerations

Overview

The MAX4536/MAX4537/MAX4538 construction is typi-

cal of most CMOS analog switches. These devices have

three supply pins: V+, V-, and GND. V+ and V- drive the

internal CMOS switches and set the limits of the analog

voltage on any switch. Reverse ESD-protection diodes

are internally connected between each analog-signal

pin, and both V+ and V-. One of these diodes conducts

if any analog signal exceeds V+ or V-. These reverse-

biased ESD diodes leak during normal operation, form-

ing 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 either 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 analog switch-

es’ gates. This drive signal is the only connection

between the logic supplies and the analog supplies.

V+, and V- have ESD-protection diodes to GND. The

logic-level inputs have ESD protection to V+ and to V-.

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+ is +5V. The threshold increases slightly as V+

is raised. When V+ reaches +12V, the level threshold is

about 3.1V, above the TTL output high-level minimum

of 2.8V, but still compatible with CMOS outputs.

Bipolar Supplies

The MAX4536/MAX4537/MAX4538 operate with bipolar

supplies between ±2.0V 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 MAX4536/MAX4537/MAX4538’s 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 dam-

age the part and/or external circuits.

CAUTION: The absolute maximum V+ to V- differen-

tial voltage is 13.0V. Typical ±6V or +12V supplies

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

voltage can damage the MAX4536/MAX4537/MAX4538.

Even ±5% tolerance supplies may have overshoot or

noise spikes that exceed 13.0V.

Single Supplies

The MAX4536/MAX4537/MAX4538 operate from single

supplies between +2.0V and +12V when V- is connect-

ed 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 in turning the

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

like a capacitor and passes higher frequencies with

less attenuation. At 10MHz, off-isolation is about -44dB

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 with Enable

8

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