Fault-Protected, High-Voltage

Single 8-to-1/Dual 4-to-1 Multiplexers

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11

Normal Operation

Two comparators continuously compare the voltage on

the NO_ pin with V+ and V- supply voltages. When the

signal on NO_ is between V+ and V-, the multiplexer

behaves normally, with FETs N1 and P1 turning on and

off in response to A_ signals (Figure 1). The parallel

combination of N1 and P1 forms a low-value resistor

between NO_ and COM_ so that signals pass equally

well in either direction.

Positive Fault Condition

When the signal on NO_ exceeds V+ by about 150mV,

the positive fault comparator output goes high, turning

off FETs N1 and P1 (Figure 1). This makes the NO_ pin

high impedance, regardless of the switch state. If the

switch state is “off,” all FETs turn off, and both NO_ and

COM_ are high impedance. If the switch state is “on,”

FET P2 turns on, clamping COM_ to V+.

Negative Fault Condition

When the signal on NO_ goes about 150mV below V-,

the negative fault comparator output goes high, turning

off FETs N1 and P1 (Figure 1). This makes the NO_ pin

high impedance, regardless of the switch state. If the

switch state is “off,” all FETs turn off, and both NO_ and

COM_ are high impedance. If the switch state is “on,”

FET N2 turns on, clamping COM_ to V-.

Transient Fault Condition

When a fast rising or falling transient on NO_ exceeds

V+ or V-, the output (COM_) follows the input (NO_) to

the supply rail with only a few nanoseconds delay. This

delay is due to the switch on-resistance and circuit

capacitance to ground. When the input transient

returns to within the supply rails, however, there is a

longer output recovery time. For positive faults, the

recovery time is typically 2.5µs (see

Typical Operating

is typically 1.3µs. These values depend on the COM_

output resistance and capacitance. The delays do not

depend on the fault amplitude. Higher COM_ output

resistance and capacitance increase the recovery

times.

COM and A_

FETs N2 and P2 can source about ±10mA from V+ or

V- to the COM_ pin in the fault condition (Figure 1).

Ensure that if the COM_ pin is connected to a low-

impedance load, the absolute maximum current rating

of 30mA is never exceeded, either in normal or fault

conditions.

The GND, COM_, and A_ pins do not have fault protec-

tion. Reverse ESD protection diodes are internally con-

nected between GND, COM_, A_, and both V+ and V-.

If a signal on GND, COM_, or A_ exceeds V+ or V- by

more than 300mV, one of these diodes will conduct.

During normal operation, these reverse-biased ESD

diodes leak a few nanoamps of current to V+ and V-

Fault Protection Voltage and Power Off

The maximum fault voltage on the NO_ pins is ±40V

from ground when the power is off. With ±15V supply

voltages, the highest voltage on NO_ can be V- + 40V,

and the lowest voltage on NO can be V+ - 40V.

Exceeding these limits can damage the chip.

Logic Level Thresholds

The logic level thresholds are CMOS and TTL compati-

ble with V+ = 13.5V to V+ = 16.5V.

Applications Information

Ground

There is no connection between the analog signal

paths and GND. The analog signal paths consist of an

N-channel and a P-channel MOSFET with their sources

and drains paralleled and their gates driven out of

phase to V+ and V- by the logic-level translators.

V+ and GND power the internal logic and logic level

translators and set the input logic thresholds. The logic-

level translators convert the logic levels to switched V+

and V- signals to drive the gates of the multiplexers.

This drive signal is the only connection between the

power supplies and the analog signals. GND, A_, and

COM_ have ESD protection diodes to V+ and V-.

Supply Current Reduction

When the logic signals are driven rail-to-rail from 0 to

+15V or -15V to +15V, the current consumption will be

reduced from 370µA (typ) to 200µA.

Power Supplies

The MAX4508/MAX4509 operate with bipolar supplies

between ±4.5V and ±20V. The V+ and V- supplies

need not be symmetrical, but their sum cannot exceed

the 44V absolute maximum rating.

The MAX4508/MAX4509 operate from single supplies

between +9V and +36V when V- is connected to GND

.