Fault-Protected, High-Voltage,

Dual Analog Switches

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9

_______________Detailed Description

The MAX4631/MAX4632/MAX4633 are fault-protected

analog switches with special operation and construc-

tion. Traditional fault-protected switches are construct-

ed using three series CMOS devices. This combination

produces good fault-protection but fairly high on-resis-

tance when the signals are within 3V of each supply

rail. These series devices are not capable of handling

signals up to the power-supply rails.

These devices differ considerably from traditional fault-

protection switches, with three advantages. First, they

are constructed with two parallel 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_ terminal, allowing rail-to-rail signal opera-

tion. Third, when a signal on NC_ or NO_ exceeds the

supply rails by about 50mV (a fault condition), the volt-

age on COM_ is limited to the appropriate polarity sup-

ply voltage. Operation is identical for both fault

polarities. The fault-protection extends to ±25V

(MAX4632) or ±36V (MAX4631/MAX4633) with power

on and ±40V with power off.

The MAX4631/MAX4632/MAX4633 have a parallel N-

channel and P-channel MOSFET switch configuration with

input voltage sensors. The simplified structure is shown

in Figure 1. The parallel N1 and P1 MOSFETs form the

switch element. N3 and P3 are sensor elements to sam-

ple the input voltage and compare it against the power-

supply rails.

During normal operation of a conducting channel, N1

and P1 remain on with a typical 62

Ω on-resistance

between NO_ (or NC_) and COM_. If the input voltage

exceeds either supply rail by about 50mV, the parallel

combination switches (N1, P1) are forced off through

the driver and sensing circuitry. At the same time, the

output (COM_) is clamped to the appropriate supply

rail by the clamp circuitry (N2, P2). Two clamp circuits

limit the output voltage to the supply voltages.

Pin Compatibility

These switches have identical pinouts to common non-

fault-protected CMOS switches (DG401, DG403,

DG405). Exercise care in considering them as direct

replacements in existing printed circuit boards, since

only the NO_ and NC_ pins of each switch are fault pro-

tected.

Normal Operation

Two comparators continuously compare the voltage on

the NO_ (or NC_) pin with V+ and V- supply voltages

(Figure 1). When the signal on NO_ (or NC_) is between

V+ and V-, the switch behaves normally, with FETs N1

and P1 turning on and off in response to NO_ (or NC_)

signals.

For any voltage between the supply rails, the switch is

bidirectional; therefore, COM_ and NO_ (or NC_) are

interchangeable. Only NO_ and NC_ can be exposed

to overvoltages beyond the supply range and within the

specified breakdown limits of the device.

Fault Condition

The MAX4631/MAX4632/MAX4633 protect devices

connected to their outputs (COM_) through their unique

fault-protection circuitry. When the input voltage is

raised 50mV above either supply rail, the internal sense

and comparator circuitry (N3 and N-channel driver or

P3 and P-channel driver) disconnect the output (COM_)

from the input (Figure 1).

If the switch driven above the supply rail has an on

state, the clamp circuitry (N2 or P2) connects the out-

put to the appropriate supply rail. Table 1 summarizes

the switches’ operation under normal and fault conditions.

-15V

-15V

+15V

P-CHANNEL

DRIVER

N-CHANNEL

DRIVER

+15V

N3

P3

INPUT

NC_

or

NO_

N1

CLAMP

OUTPUT COM_

CLAMP

+V (+15V)

-V (-15V)

N2

P2

P1

COMPARATOR

COMPARATOR

SENSE

SWITCH

SENSE

SWITCH

Figure 1. Simplified Internal Structure