Precision, Single-Supply,

SPST Analog Switches

6

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__________Applications Information

Logic Levels

The MAX323/MAX324/MAX325 are TTL compatible

when powered from a single +5V supply. When pow-

ered from other supply voltages, TTL compatibility is

not guaranteed, and the logic inputs should be driven

from rail to rail. For example, with a 15V supply, IN1

and IN2 should be driven low to 0V and high to 15V.

Similarly, with a 3.3V supply, IN1 and IN2 should be dri-

ven low to 0V and high to 3.3V

Driving IN1 and IN2 rail-to-rail minimizes power con-

sumption.

Analog Signal Levels

Analog signals that range over the entire supply voltage

(V+ to GND) can be switched with very little change in

on-resistance over the entire voltage range (see

Typical

Operating Characteristics). All the switches are bidirec-

tional, so NO_, NC_, and COM_ pins can be used as

either inputs or outputs.

Power-Supply Sequencing

and Overvoltage Protection

Do not exceed the absolute maximum ratings, because

stresses beyond the listed ratings may cause permanent

damage to the devices.

Proper power-supply sequencing is recommended for

all CMOS devices. Always apply V+ before applying

analog signals or logic inputs, especially if the analog

or logic signals are not current-limited. If this sequenc-

ing is not possible, and if the analog or logic inputs are

not current-limited to <30mA, add a small-signal diode

(D1) as shown in Figure 1. If the analog signal can dip

below GND, add D2. Adding protection diodes reduces

the analog signal range to a diode-drop (about 0.7V)

below V+ (for D1), and to a diode-drop above ground

(for D2). Leakage is unaffected by adding the diodes.

On-resistance increases by a small amount at low sup-

ply voltages. Maximum supply voltage (V+) must not

exceed 17V.

Adding protection diodes causes the logic thresholds

to be shifted relative to the power-supply rails. This can

be significant when low supply voltages (5V or less) are

used. With a 5V supply, TTL compatibility is not guaran-

teed when protection diodes are added. Driving IN1

and IN2 all the way to the supply rails (i.e. to a diode-

drop higher than the V+ pin, or to a diode-drop lower

than the GND pin) is always acceptable.

Protection diodes D1 and D2 also protect against

some overvoltage situations. With Figure 1’s circuit, if

the supply voltage is below the absolute maximum rat-

ing, and if a fault voltage up to the absolute maximum

rating is applied to an analog signal pin, no damage

will result. For example, with a +5V supply, analog sig-

nals up to ±8V will not damage the circuit of Figure 1.

If only a single fault signal is present, the fault voltage

can rise to +17V or to -12V without damage occurring.

_____________________Pin Description

POSITIVE SUPPLY

COM

NO

D2

D1

GND

Vg

V+

Figure 1. Overvoltage Protection Using Two External Blocking

Diodes

Normally Closed Analog Switch Terminal

(MAX324/MAX325)

NC2

Normally Open Analog Switch Terminal

(MAX323 only)

NO2

5

Ground

GND

4

Logic Inputs

IN2, IN1

3, 7

PIN

Analog Switch Common Terminal

COM1,

COM2

2, 6

Normally Closed Analog Switch Terminal

(MAX324 only)

NC1

Normally Open Analog Switch Terminal

(MAX323/MAX325)

NO1

1

FUNCTION

NAME

Positive Supply

V+

8