ADG465

–7–

REV. A

APPLICATIONS INFORMATION

Overvoltage and Power Supply Sequencing Protection

The ADG465 is ideal for use in applications where input over-

voltage protection is required and correct power supply sequencing

cannot always be guaranteed. The overvoltage protection en-

sures that the output voltage of the channel protector will not

exceed the threshold voltages set by the supplies (see Circuit

Information section) when there is an overvoltage on the input.

When the input voltage does not exceed these threshold volt-

ages, the channel protector behaves like a series resistor (80

Ω

typ). The resistance of the channel protector does vary slightly

with operating conditions (see Typical Performance Graphs).

The power sequencing protection is afforded by the fact that

when the supplies to the channel protector are not connected,

the channel protector becomes a high resistance device. Under

this condition all transistors in the channel protector are off and

the only currents that flow are leakage currents, which are at the

µA level.

+5V

–5V

EDGE

CONNECTOR

ADG465

ADC

CONTROL

LOGIC

ANALOG IN

–2.5V TO +2.5V

LOGIC

LOGIC

GND

Figure 12. Overvoltage and Power Supply Sequencing

Protection

Figure 12 shows a typical application requiring overvoltage and

power supply sequencing protection. The application shows a

Hot-Insertion rack system. This involves plugging a circuit

board or module into a live rack via an edge connector. In this

type of application it is not possible to guarantee correct power

supply sequencing. Correct power supply sequencing means that

the power supplies should be connected prior to any external

signals. Incorrect power sequencing can cause a CMOS device

to “latch up,” see Trench Isolation section. This is true of most

CMOS devices, regardless of the functionality. RC networks

are used on the supplies of the channel protector (Figure 12)

to ensure that the rest of the circuitry is powered up before the

channel protectors. In this way, the outputs of the channel

capacitors are charged. The diodes ensure that the supplies on

the channel protector never exceed the supply rails of the board

when it is being disconnected. Again, this ensures that signals

on the inputs of the CMOS devices never exceed the supplies.

High Voltage Surge Suppression

The ADG465 are not intended for use in high voltage applica-

tions such as surge suppression. The ADG465 has breakdown

power supplies are connected. When the power supplies are

disconnected, the breakdown voltages on the input of the chan-

nel protector are

±35 V. In applications where inputs are likely

to be subject to overvoltages exceeding the breakdown voltages

quoted for the channel protectors, transient voltage suppressors

(TVSs) should be used. These devices are commonly used to

protect vulnerable circuits from electric overstress such as that

caused by electrostatic discharge, inductive load switching and

induced lightning. However, TVSs can have a substantial

standby (leakage) current (300

µA typ) at the reverse standoff

voltage. The reverse standoff voltage of a TVS is the normal

peak operating voltage of the circuit. In addition, TVSs offer no

protection against latchup of sensitive CMOS devices when the

power supplies are off. To provide the best leakage current

specification and circuit protection, the best solution is to use a

channel protector in conjunction with a TVS.

Figure 13 shows an input protection scheme that uses both a

TVS and channel protector. The TVS is selected with a reverse

standoff voltage much greater than the operating voltage of the

circuit (TVSs with higher breakdown voltages tend to have

better standby leakage current specifications), but inside

the breakdown voltage of the channel protector. This circuit

protects the circuitry whether or not the power supplies are

present.

ADG465

ADC

TVSs

BREAKDOWN

VOLTAGE = 20V

Figure 13. High Voltage Protection