ADG4612/ADG4613
Rev. 0 | Page 19 of 24
APPLICATIONS INFORMATION
There are many application scenarios that benefit from the
functionality offered on the ADG4612/ADG4613 switches.
The ADG4612/ADG4613 offer power-off protection, ensuring
the switch is guaranteed off and inputs are high impedance with no
power supplies present. This isolation mode is a useful feature
that isolates the inputs from the outputs where input signals
may be present before supplies. The isolation mode also protects
the system against positive fault conditions that can occur in
applications, ensuring that the switch turns off and protects
downstream circuitry. For example, a module can be connected
to a live backplane, supplying signals to the board before supplies
are present. This is common in hot swap applications where a
card could be hot plugged in a shelf where there are others cards
already working and powered on.
The ADG4612/ADG4613 allow negative signals, down to −5.5 V
to be passed without a negative supply. This can be very useful
in applications that need to pass negative signals but do not
have a negative supply available. This cannot be done with
conventional CMOS switches because ESD protection diodes
turn on and clamp the signals.
Theses features ensure the system is very robust to power
supply sequencing issues that can be present in conventional
CMOS devices.
09005-030
LIVE BACKPLANE
HOT SWAP MODULES
CONTROLLER
HOT SWAP MODULES
HOT SWAP MODULES
POWER SUPPLY
SW
Figure 32. Typical Application
Signals on Inputs with No Power Present
In conventional CMOS switches, ESD protection diodes can be
found on the analog and digital inputs to V
DD
and GND or V
SS
(see Figure 33, for example). If an input voltage is present on the
switch inputs with no power supplies applied, current can flow
through the ESD protection diodes. If this current is not limited
to a safe level, it is possible to damage the ESD protection diodes
and, hence, the switch. Input signals may pass through the switch
to the output affecting downstream circuitry. The user may also
be exceeding the absolute maximum ratings of the devices, and,
therefore, affecting the long-term reliability of the device.
V
S
V
SS
DD
R
S
R
L
Sx
GND
FORWARD
CURRENT
V
S
> V
D
FORWARD
CURRENT
FLOWS
LOAD
CURRENT
Dx
09005-031
Figure 33. ESD Protection Diodes on Conventional CMOS Switch
Some users add external diodes or add current-limiting resistors to
protect the device against the conditions shown in Figure 33.
However, these solutions all have disadvantages in that they add
extra board area, extra component count, and cost. The system
level performance can also be affected by the higher on resistance
from the current-limiting resistors or the higher leakage from
external Schottky diodes. Using external diodes for protection
still creates the problem where a floating V
DD
line can be pulled
up to a diode drop from the input signal.
V
S
V
SS
DD
R
S
R
L
Sx
GND
Dx
09005-032
FORWARD
CURRENT
LOAD
CURRENT
V
S
> V
D
FORWARD
CURRENT
FLOWS
Figure 34. External Protection Added to Protect Switch Against Damage If
Signals Present on Inputs Without Power Supplies
The ADG4612/ADG4613 eliminate the concerns shown in
Figure 34. There are no internal ESD diodes from the analog or
digital inputs to V
DD
or V
SS
. If signals are present on the ADG4612/
ADG4613 inputs before power is present, the switch is in isolation
mode, which means that the inputs have high impedance to
V
DD
, GND, and the output. This prevents current flow and
protects the device from damage.
