When the NO_ voltage goes beyond supply rails (fault

condition), the NO_ input becomes high impedance

regardless of the switch state or load resistance. When

power is removed, and the fault protection is still in

effect, the NO_ terminals are a virtual open circuit. The

fault can be up to ±40V, with V+ = V- = 0. If the switch

is on, the COM_ output current is furnished from the V+

or V- pin by “booster” FETs connected to each supply

pin. These FETs can source or sink up to 10mA.

The COM_ pins are not fault protected. If a voltage

source is connected to any COM_ pin, it should be lim-

ited to the supply voltages. Exceeding the supply volt-

age will cause high currents to flow through the ESD

protection diodes, damaging the device (see

Absolute

Figure 1 shows the internal construction, with the ana-

log signal paths shown in bold. A single normally open

(NO) switch is shown. 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 simultane-

ously, according to the input fault condition and the

logic level state.

NO_ Input Voltage

The maximum allowable input voltage for safe opera-

tion depends on whether supplies are on or off and the

load configuration at the COM output. If COM is re-

ferred to a voltage other than ground, but within the

less than 40V. For example, if the load is referred to

+10V at COM_, then the NO_ voltage range can be

from +50V to -30V. As another example, if the load is

connected to -10V at COM_, the NO_ voltage range is

limited to -50V to +30V.

If the supplies are ±15V and COM is referenced to

ground through a load, the maximum NO_ voltage is

±25V. If the supplies are off and the COM output is ref-

erenced to ground, the maximum NO_ voltage is ±40V.

Fault-Protected, High-Voltage

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

10

______________________________________________________________________________________

NORMALLY OPEN SWITCH CONSTRUCTION

COM_

P2

MAX4508

MAX4509

P1

N1

ON

LOW

FAULT

HIGH

FAULT

V+

NO_

A-

GWD

ESO CODE

V-

N2

Figure 1. Functional Diagram