ADG438 查看數據表（PDF） - Analog Devices
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ADG438 Datasheet PDF : 8 Pages
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ADG438F/ADG439F
THEORY OF OPERATION
The ADG438F/ADG439F multiplexers are capable of with-
standing overvoltages from –40 V to +55 V, irrespective of
whether the power supplies are present or not. Each channel of
the multiplexer consists of an n-channel MOSFET, a p-channel
MOSFET and an n-channel MOSFET, connected in series.
When the analog input exceeds the power supplies, one of the
MOSFETs will switch off, limiting the current to sub-microamp
levels, thereby preventing the overvoltage from damaging any
circuitry following the multiplexer. Figure 12 illustrates the
channel architecture that enables these multiplexers to with-
stand continuous overvoltages.
When an analog input of VSS + 1.2 V to VDD – 0.8 V is applied
to the ADG438F/ADG439F, the multiplexer behaves as a
standard multiplexer, with specifications similar to a standard
multiplexer, for example, the on-resistance is 180 Ω typically.
However, when an overvoltage is applied to the device, one of
the three MOSFETs will turn off.
Figures 10 to 13 show the conditions of the three MOSFETs for
the various overvoltage situations. When the analog input ap-
plied to an ON channel approaches the positive power supply
line, the n-channel MOSFET turns OFF since the voltage on
the analog input exceeds the difference between VDD and the
+55V
OVERVOLTAGE
Q1
Q2
Q3
n-CHANNEL
MOSFET IS
OFF
VDD
VSS
Figure 10. +55 V Overvoltage Input to the ON Channel
n-channel threshold voltage (VTN). When a voltage more nega-
tive than VSS is applied to the multiplexer, the p-channel
MOSFET will turn off since the analog input is more negative
than the difference between VSS and the p-channel threshold
voltage (VTP).
When the power supplies are present but the channel is off,
again either the p-channel MOSFET or one of the n-channel
MOSFETs will remain off when an overvoltage occurs.
Finally, when the power supplies are off, the gate of each
MOSFET will be at ground. A negative overvoltage switches on
the first n-channel MOSFET but the bias produced by the
overvoltage causes the p-channel MOSFET to remain turned
off. With a positive overvoltage, the first MOSFET in the series
will remain off since the gate to source voltage applied to this
MOSFET is negative.
During fault conditions, the leakage current into and out of the
ADG438F/ADG439F is limited to a few microamps. This pro-
tects the multiplexer and succeeding circuitry from over stresses
as well as protecting the signal sources which drive the multi-
plexer. Also, the other channels of the multiplexer will be
undisturbed by the overvoltage and will continue to operate
normally.
+55V
OVERVOLTAGE
Q1
Q2
Q3
n-CHANNEL
MOSFET IS
OFF
Figure 12. +55 V Overvoltage with Power OFF
–40V
OVERVOLTAGE
Q1
Q2
Q3
n-CHANNEL
MOSFET IS
ON
VSS
p-CHANNEL
MOSFET IS
VDD
OFF
Figure 11. –40 V Overvoltage on an OFF Channel with
Multiplexer Power ON
–40V
Q1
OVERVOLTAGE
n-CHANNEL
MOSFET IS
ON
Q2
Q3
p-CHANNEL
MOSFET IS
OFF
Figure 13. –40 V Overvoltage with Power OFF
Test Circuits
IDS
V1
S
D
VS
RON = V1/IDS
Test Circuit 1. On Resistance
VDD
IS (OFF)
A
VS
VD
VDD
S1
S2
S8
VSS
VSS
D
EN
+0.8V
VDD
VSS
VDD
VSS
S1
ID (OFF)
S2
D
A
S8
VS
VD
EN
+0.8V
Test Circuit 2. IS (OFF)
Test Circuit 3. ID (OFF)
–6–
REV. D
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