MAX366CSA+ Datasheet MAX367EWN+T, MAX366ESA+, MAX366CSA+ | P7-P9

MAX366/MAX367

Signal-Line Circuit Protectors

_______________________________________________________________________________________ 7

The current through each protector should never exceed

30mA. Always calculate the power dissipated by all the

protectors in worst-case conditions (maximum voltage

and current through each protector) to ensure the pack-

age dissipation limit is not reached.

With single-supply operation, grounded loads will have

zero voltage (and current) whenever the input voltage is

below approximately 2V. In effect, both the IN and OUT

pins are in fault condition.

A special case arises when power is off: The part is in a

perpetual fault condition but no fault current flows

because all the internal FETs are off.

Single-Supply Output Operation

Single-supply operation is a special case. Signals can-

not go to ground, since from 0V to approximately +2V is

a fault condition.

Extremely Low-Current Operation

Figure 3 shows the typical high-impedance transfer

characteristics with a 100MΩ load. Compared to the

transfer characteristic at 1MΩ (see

Typical Operating

Characteristics

), the two knees are closer to the supply

voltages and the slopes of the flat portions of the curve

(fault conditions) are steeper. As the load resistance is

increased even further, the positive and negative knees

increase, and the slopes in fault conditions increase

even more. Eventually, at some extremely high output

resistance (e.g., Tera ohms), the output voltage can

exceed the supply voltage during fault conditions. This

is due to extremely low leakage currents from the input

to output.

When the protector’s output side is connected to very

high-resistance, very low-current loads (such as op-

amp inputs), a small leakage current flows from the

input to the output during fault conditions. This current

is typically below a nano-ampere (<10

-9

A) but, if the

output resistance is high enough, it can cause the out-

put voltage to exceed the supply voltages during fault

conditions.

This condition can be self-correcting, however, if the

high-resistance load has protection diodes to the sup-

ply rails (either external or internal to the op amp).

These diodes conduct the leakage current to the supply

rails and safely limit the output voltage. An alternative is

to add a high-value resistor to ground in parallel with

the load. This resistor may be as low as 1000MΩ; its

value must be determined experimentally at the highest

anticipated operational temperature.

The fault protectors will not normally be used with high-

impedance FET-input amplifiers that lack input protection

diodes. Such amplifiers are fragile and are normally

reserved for use when ultra-low leakage (pA) is needed.

The MAX366/MAX367 have nano-amperes of leakage,

which would negate the low leakage of the unprotected

amplifier.

Low-Voltage Operation

The MAX366/MAX367 “operate” with supply voltages

all the way down to 0V, but what they do to the signal is

not obvious. With a total supply voltage of 3.5V, the

protector is in a fault condition with nearly any input that

is not close to 2.0V. Below 3.5V (including power off),

the protector is perpetually in a fault condition (i.e., high

impedance).

When the supply voltage(s) ramps up (and/or down)

from zero, the signal path is initially in a fault condition

(open), until the supply voltage passes the input volt-

age. The output starts at zero and is delayed from

reaching the input voltage as the part comes out of the

fault condition. If the supply voltage exceeds about

3.5V, but never exceeds the input voltage, the output

will follow the supply, always remaining about 1.3V

below the positive supply voltage or 2V above the neg-

ative supply voltage. If the input voltage subsequently

comes out of the fault condition, the output returns to

the input value. This set of conditions is exactly

reversed when power ramps down to zero.

Since the input and output pins are identical and inter-

changeable, predicting whether or not the part is in a

fault condition is easy: If either IN or OUT exceeds V+

or V-, a fault condition exists and the current that flows

will be just enough to cause the other signal pin (OUT

or IN) to approach the appropriate supply rail.

-1

-2

-3

-4

-30 30

MAX366/7-fig03

INPUT VOLTAGE (V)

OUTPUT VOLTAGE (V)

0-20 -10 2010

V+ = +5V

V- = -5V

OUT

= 100MΩ

Figure 3. High-Impedance Transfer Characteristic

MAX366/MAX367

Signal-Line Circuit Protectors

8 _______________________________________________________________________________________

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

OUT11

10µF

IN1 7

+5V

-5V

OP AMP

OUT22 IN2 6

OUT33 IN3 5

V+

V-

100k

Figure 4. Turn-On Delay

MAX366/MAX367

Signal-Line Circuit Protectors

_______________________________________________________________________________________ 9

Figure 6 shows a circuit that can give reliable results.

This circuit uses a 100mV voltage source and a low-

voltage-drop ammeter as the measuring circuit, and an

adjustable supply to sweep the analog voltage across

its whole range. The ammeter must have a voltage

drop of less than one millivolt (at any current) for accu-

rate results. (A Keithley Model 617 Electrometer has a

suitable ammeter circuit, appropriate ranges, and a

built-in voltage source designed for this type of mea-

surement.) Measurements are made by setting the

analog voltage, measuring the current, and calculating

the path resistance. The procedure is repeated at

each analog voltage and supply voltage.

It is important to use a voltage source of 100mV or less.

As shown in Figure 4, this voltage is added to the V

voltage to form the V

OUT

voltage. Using a higher volt-

age could cause the OUT pin to go into a fault condi-

tion prematurely.

High-Frequency Performance

In 50Ω systems, signal response is reasonably flat up

to several megahertz (see

Typical Operating

Characteristics

). Above 5MHz, the response has sev-

eral minor peaks, which are highly layout dependent.

Because the path resistance is dependent on the sup-

ply voltage and signal amplitude, the impedance is not

controlled. Adjacent channel attenuation up to 5MHz is

about 3dB above that of a bare IC socket, and is due

entirely to capacitive coupling.

Pulse response is reasonable, but because the imped-

ance changes rapidly, fast rise times may induce ringing

as the signal approaches the fault voltage. At very high

amplitudes (such as noise spikes), the capacitive cou-

pling across the signal pins will transfer considerable

energy, despite the fact that the DC path is a virtual open

circuit.

MAX366

OUT_V

IN_ V

OUT

V+

V-

V+

V-

PATH RESISTANCE = 100mV/A

100mV

ADJUSTABLE ANALOG VOLTAGE

Figure 6. Path-Resistance Measuring Circuit

MAX366

OUT11 IN1 7

+5V

-5V

OUT22 IN2 6

OUT33 IN3 5

V+

V-

100k

SWITCHED +5V

OP AMP

Figure 5. Power-Supply Sequencing

P1-P3 P4-P6 P7-P9 P10-P12

MAX366CSA+

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Hot Swap Voltage Controllers Signal Line Circuit Protector

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MAX366CSA+

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