Low-Voltage, SPST, CMOS Analog Switches

6

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Applications Information

Power-Supply Considerations

The MAX4501/MAX4502 are constructed like most

CMOS analog switches, except they have only two sup-

ply pins: V+ and GND. V+ and GND drive the internal

CMOS switches and set the analog voltage limits of the

switch. Reverse ESD-protection diodes are internally

connected between each analog signal pin and both

V+ and GND. One of these diodes conducts if any ana-

log signal exceeds V+ or GND. During normal opera-

tion, these and other reverse-biased ESD diodes leak,

forming the only current drawn from V+ or GND.

Virtually all the analog leakage current comes from 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 GND and the analog signal. This means

their leakages will vary as the signal varies. The differ-

ence in the two diode leakages to the V+ and GND

pins constitutes the analog signal-path leakage current.

All analog leakage current flows between each pin and

one of the supply terminals, not to the other switch ter-

minal. This is why 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 V+ or GND.

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

level translators, and set the input logic limits. The

logic-level translators convert the logic levels to

switched V+ and GND signals to drive the analog sig-

nal gates. This drive signal is the only connection

between the logic supplies (and signals) and the ana-

log supplies. COM, NO, and NC pins have ESD-protec-

tion diodes to V+ and GND.

The logic-level thresholds are CMOS/TTL compatible

when V+ is +5V. As V+ rises, the threshold increases

slightly. When V+ reaches +12V, the logic-level thresh-

old is about 3V—above the TTL guaranteed high-level

minimum of 2.8V, but still compatible with CMOS out-

puts.

Do not connect the MAX4501/MAX4502’s V+ pin to

+3V and then connect the logic-level pins to TTL

logic-level signals. TTL levels can exceed +3V and

violate the absolute maximum ratings, damaging

the part and/or external circuits.

High-Frequency Performance

In 50

Ω systems, signal response is reasonably flat up

to 250MHz (see Typical Operating Characteristics).

Above 20MHz, the on-response has several minor

peaks that are highly layout dependent. The problem is

not in turning the switch on; it’s in turning it off. The off-

state switch acts like a capacitor and passes higher

frequencies with less attenuation. At 10MHz, off-isola-

tion is about -60dB in 50

Ω systems, decreasing

approximately 20dB per decade as frequency increas-

es. Higher circuit impedances also cause off-isolation

to decrease. Adjacent channel attenuation is about 3dB

above that of a bare IC socket, and is due entirely to

capacitive coupling.

______________________________________________________________Pin Description

Note: NO, NC, and COM pins are identical and interchangeable. Any may be considered as an input or an output; signals pass

equally well in both directions.

FUNCTION

NAME

MAX4501

MAX4502

SO/DIP

SC70-5/SOT23-5

SO/DIP

SC70-5/SOT23-5

Positive Supply-Voltage Input (analog and digital)

PIN

1

1

1

1

COM

Analog Switch Common Terminal

2, 3, 5

—

2, 3, 5

—

N.C.

No Connection. Not internally connected.

4

5

V+

6

4

6

4

IN

Digital Control Input

7

3

7

3

GND

Ground

8

2

—

—

NO

Analog Switch (normally open)

—

—

8

2

NC

Analog Switch (normally closed)

5

4