Detailed Description

The MAX4711/MAX4712/MAX4713 differ considerably

from traditional fault-protection switches, with several

advantages. First, they are constructed with two paral-

lel FET’s allowing very low on-resistance. 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 sig-

nal operation. Third, when a signal on NC_ or NO_

exceeds the supply rails by about 150mV (a fault con-

dition) the voltage on COM_ is limited to the same

polarity supply voltage. Operation is identical for both

fault polarities.

During a fault condition, the NO_ or NC_ input

becomes high impedance regardless of the switch

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

output current is supplied from V+ or V- by the clamp

FET’s that are connected from COM to each supply.

These FET’s can typically source or sink up to 15mA.

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 ±12V.

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

mal CMOS switch terminals. If a voltage source is con-

nected 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-protected

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 while considering them for direct replacements in

existing printed circuit boards 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 NO switch is

shown; the NC configuration is identical except the

logic-level translator is inverting. The analog switch is

formed by the parallel combination of N-channel FET

(N1) and P-channel FET (P1), which are driven on and

off simultaneously according to the input fault condition

and the logic-level state.

Normal Operation

Two comparators continuously compare the voltage on

the NO_ (or NC_) pin with V+ and V-. When the signal

on NO_ or NC_ is between V+ and V- the switch acts

normally, with FETs N1 and P1 turning on and off in

response to IN_ signals. The parallel combination of

N1 and P1 forms a low-value resistor between NO_ (or

Fault-Protected, Low-Voltage,

Quad SPST Analog Switches

10

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Pin Description

PIN

MAX4711

MAX4712

MAX4713

NAME

FUNCTION

1, 16, 9, 8

1, 16, 9, 8

1, 16, 9, 8

IN1, IN2, IN3,

IN4

Logic Inputs. Fault-protected to (V- + 12V).

2, 15, 10, 7

2, 15, 10, 7

2, 15, 10, 7

COM1,

COM2,

COM3, COM4

Analog Switch Common Terminals

3, 14, 11, 6

——

NC1, NC2,

NC3, NC4

Fault-Protected Analog Switch Normally Closed Terminals

—

3, 14, 11, 6

—

NO1, NO2,

NO3, NO4

Fault-Protected Analog Switch Normally Open Terminals

——

3, 6

NO1, NO4

Fault-Protected Analog Switch Normally Open Terminals

——

14, 11

NC2, NC3

Fault-Protected Analog Switch Normally Closed Terminals

44

4

V-

Negative Supply Voltage Input. Connect to GND for single-supply

operation.

55

5

GND

Ground

12

12

12

N.C.

No Connection. Not internally connected.

13

13

13

V+

Positive Supply Input