adg465 Analog Devices, Inc., adg465 Datasheet - Page 6

• Manufacturer Part Number: adg465
• Description: Single Channel Protector In An Sot-23 Package
• Manufacturer: Analog Devices, Inc.

ADG465
When a negative overvoltage is applied to the channel protector
circuit, the PMOS transistor enters a saturated mode of operation
as the drain voltage exceeds V
the case of the positive overvoltage, the other MOS devices are
nonsaturated.
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
diode clamping (see Applications Information). When V
V
microampere levels (see Figure 10).
Figure 10. Channel Protector Supplies Equal to Zero Volts
Figure 9. Negative Overvoltage on the Channel Protector
OVERVOLTAGE
SS
OVERVOLTAGE
POSITIVE OR
equal 0 V, all transistors are off and the current is limited to
NEGATIVE
NEGATIVE
(–20V)
*V
TP
V
= PMOS THRESHOLD VOLTAGE (+2V)
DD
NMOS
V
NMOS
(+15V)
DD
NON-
SATURATED
(0V)
OFF
OVERVOLTAGE
NEGATIVE
(–20V)
SS
– V
V
SS
PMOS
V
PMOS
TP
(–15V)
SS
SATURATED
. See Figure 9 below. As in
OFF
(0V)
(0V)
V
SS
(–13V)
– V
V
DD
NMOS
TP
NMOS
V
(+15V)
DD
*
NON-
SATURATED
(0V)
OFF
DD
and
–6–
TRENCH ISOLATION
The MOS devices that make up the channel protector are
isolated from each other by an oxide layer (trench) (see Figure
11). When the NMOS and PMOS devices are not electrically
isolated from each other, there exists the possibility of “latchup”
caused by parasitic junctions between CMOS transistors. Latchup
is caused when P-N junctions that are normally reverse biased,
become forward biased, causing large currents to flow. This can
be destructive.
CMOS devices are normally isolated from each other by
Junction 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
latchup. With Trench Isolation, this diode is removed; the result
is a latchup-proof circuit.
T
R
E
N
C
H
V
P
N
S
+
–
P-CHANNEL
Figure 11. Trench Isolation
V
G
SUBSTRATE (BACKGATE)
BURIED OXIDE LAYER
V
P
D
+
T
R
E
N
C
H
V
N
P
S
–
+
N-CHANNEL
V
G
V
N
D
+
REV. A
T
R
E
N
C
H