ADG466/ADG467

–8–

REV. A

When a negative overvoltage is applied to the channel protector

circuit, the PMOS transistor enters a saturated mode of opera-

low. As in the case of the positive overvoltage, the other MOS

devices are nonsaturated.

NMOS

PMOS

NMOS

NEGATIVE

OVERVOLTAGE

(–20V)

(–13V)

NEGATIVE

OVERVOLTAGE

(–20V)

NON-

SATURATED

NON-

SATURATED

SATURATED

Figure 20. Negative Overvoltage on the Channel Protector

The channel protector is also functional when the supply rails

are down (e.g., power failure) or momentarily unconnected

(e.g., rack system). This is where the channel protector has an

advantage over more conventional protection methods such as

subnano-ampere levels (see Figure 21).

NMOS

PMOS

NMOS

POSITIVE OR

NEGATIVE

OVERVOLTAGE

(0V)

OFF

OFF

OFF

Figure 21. Channel Protector Supplies Equal to Zero Volts

TRENCH ISOLATION

The MOS devices that make up the channel protector are iso-

lated from each other by an oxide layer (trench) (see Figure 22).

When the NMOS and PMOS devices are not electrically iso-

lated from each other, there exists the possibility of “latch-up”

caused by parasitic junctions between CMOS transistors. Latch-

up is caused when P-N junctions that are normally reverse bi-

ased become forward biased, causing large currents to flow,

which can be destructive.

CMOS devices are normally isolated from each other by Junc-

tion Isolation. In Junction Isolation, the N and P wells of the

CMOS transistors form a diode that is reverse-biased under

normal operation. However, during overvoltage conditions, this

diode becomes forward biased. A Silicon-Controlled Rectifier

(SCR) type circuit is formed by the two transistors causing a

significant amplification of the current that, in turn, leads to

latch-up. With Trench Isolation, this diode is removed; the

result is a latch-up proof circuit.

P-CHANNEL

P+

P+

N–

N-CHANNEL

N+

N+

P–

T

R

E

N

C

H

T

R

E

N

C

H

T

R

E

N

C

H

BURIED OXIDE LAYER

SUBSTRATE (BACKGATE)

Figure 22. Trench Isolation