Signal-Line Circuit Protectors

8

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Bipolar Faults

The MAX366/MAX367 V+ and V- pins are normally con-

nected to a circuit’s most positive and most negative

power supplies. When a circuit has multiple power

supplies (such as ±5V and ±12V) and the MAX366/

MAX367 V+ and V- pins are connected to the lower

supply, it is possible to have fault conditions on both

sides of the signal path at once, if both sides of the

switch have paths to higher voltages. If the polarity of

these faults is the same, the signal path will be open

and there is no conflict.

If the IN and OUT pins are driven in opposite polarities

from low-impedance sources, the lower of the two

impedances will overcome the higher impedance, just

as if the protector were not present. (Make sure the

current does not exceed the 30mA absolute maximum

rating.) As the lower impedance source approaches

and exceeds the fault knee voltage, the protector will

conduct enough current to maintain the other signal pin

near the fault knee voltage.

This means when the fault

knee voltage is reached, the current through the pro-

tector shifts from the higher current capability of the

lower impedance source to the lower current capability

of the higher impedance source.

_______________Typical Applications

Driven Switches

The MAX366/MAX367 have low supply currents

(<1µA), which allows the supply pins to be driven

directly by other active circuitry, instead of connected

directly to the power sources. In this configuration,

the parts can be used as driven fault-protected

switches with V+ or V- pins used as the control pins.

For example, if the V- pin is grounded, you can turn

the V+ pin on and off by driving it with the output of a

CMOS gate. This effectively connects and discon-

nects three or eight separate signal lines at once. (If

bipolar signals or signals that go to ground are being

switched, the V- pin must be driven simultaneously to

a negative potential.) Always ensure that the driving

source(s) does not drive the V+ pin more negative

than the V- pin.

Figure 4 shows a simple turn-on delay that takes

advantage of the MAX366’s low power consumption.

The two RC networks cause gradual application of

power to the MAX366, which in turn applies the input

signals smoothly after the amplifier has stabilized.

The two diodes discharge the two capacitors rapidly

when power is turned off.

This circuit can be tailored to nearly any rate of turn-

on by selecting the RC time constants in the V+ and

V- pins, without affecting the time constant of the

measuring circuit.

Protectors as Circuit Elements

Any of the individual protectors in a MAX366 or MAX367

may be used as a switched resistor, independent of the

functions of other elements in the same package. For

example, Figure 5 shows a MAX366 with two of the pro-

tectors used to protect the input of an op amp, and the

third element used to sequence a power supply.

Combining the circuits of Figures 4 and 5 produces a

delayed action on the switched +5V, as well as smooth

application of signals to the amplifier input.

_________Testing Circuit Protectors

Measuring Path Resistance

Measuring path resistance requires special techniques,

since path resistance varies dramatically with the IN

and OUT voltages relative to the supply voltages.

Conventional ohmmeters should not be used, for two

reasons: 1) the applied voltage and currents are usual-

ly not predictable, and 2) the true resistance is a func-

tion of the applied voltage, which is dramatically altered

by the ohmmeter itself. Autoranging ohmmeters are

particularly unreliable.

MAX366

OUT1

1

10

µF

10

µF

IN1

7

+5V

-5V

OP AMP

OUT2

2

IN2

6

OUT3

3

IN3

5

V+

4

V-

8

100k

100k

Figure 4. Turn-On Delay