adg465 Analog Devices, Inc., adg465 Datasheet - Page 7

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

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
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
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
REV. A
A level.
Figure 12. Overvoltage and Power Supply Sequencing
Protection
–2.5V TO +2.5V
ANALOG IN
LOGIC
LOGIC
GND
+5V
–5V
CONNECTOR
EDGE
ADG465
V
DD
V
SS
CONTROL
LOGIC
ADC
–7–
channel protectors. In this way, the outputs of the channel
protectors are clamped well below V
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
voltages of V
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 A 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.
SS
Figure 13. High Voltage Protection
– 20 V and V
V
DD
DD
= +5V
TVSs
BREAKDOWN
VOLTAGE = 20V
ADG465
+ 20 V on the inputs when the
V
SS
DD
= –5V
and V
SS
ADG465
ADC
until the