Quad, Rail-to-Rail, Fault-Protected,

SPST Analog Switches

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9

Overview of MAX4511/MAX4512/MAX4513

The MAX4511/MAX4512/MAX4513 differ considerably

from traditional fault-protection switches, with several

advantages. First, they are constructed with two paral-

lel FETs, allowing very low on-resistance when the

switch is on. Second, they allow signals on the NC_ or

NO_ pins that are within or slightly beyond the supply

rails to be passed through the switch to the COM termi-

nal, allowing rail-to-rail signal operation. Third, when a

signal on NC_ or NO_ exceeds the supply rails by

about 50mV (a fault condition), the voltage on COM_ is

limited to the appropriate polarity supply voltage.

Operation is identical for both fault polarities. The fault-

protection extends to ±36V from GND.

During a fault condition, the NO_ or NC_ input pin

becomes high impedance regardless of the switch

state or load resistance. If the switch is on, the COM_

output current is furnished from the V+ or V- pin by

“booster” FETs connected to each supply pin. These

FETs can typically source or sink up to 10mA.

When power is removed, the fault protection is still in

effect. In this case, the NO_ or NC_ terminals are a vir-

tual open circuit. The fault can be up to ±40V.

The COM_ pins are not fault protected; they act as nor-

mal CMOS switch pins. If a voltage source is connect-

ed to any COM_ pin, it should be limited to the supply

voltages. Exceeding the supply voltage will cause high

currents to flow through the ESD protection diodes,

possibly damaging the device (see

Absolute Maximum

Ratings).

Pin Compatibility

These switches have identical pinouts to common non-

fault-protected CMOS switches. Care should be exer-

cised in considering them for direct replacements in

existing printed circuit boards, however, since only the

NO_ and NC_ pins of each switch are fault protected.

Internal Construction

Internal construction is shown in Figure 1, with the ana-

log signal paths shown in bold. A single normally open

V+

NO_

(NC_)

IN_

GND

V-

HIGH

FAULT

LOW

FAULT

ON

N1

P1

P2

COM_

N2

NC SWITCH

-ESD DIODE

NORMALLY OPEN SWITCH CONSTRUCTION

Figure 1. Block Diagram