MAX354 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – Fault-Protected Analog Multiplexers

Fault-Protected Analog Multiplexers
Figure 10 shows the condition of an off channel with V+
and V- present. As with Figures 8 and 9, either an N-
channel or a P-channel device will be off for any input
voltage from -40V to +40V. The leakage current with
negative overvoltages will immediately drop to a few
nanoamps at +25°C. For positive overvoltages, that
fault current will initially be 10µA or 20µA, decaying
over a few seconds to the nanoamp level. The time
constant of this decay is caused by the discharge of
stored charge from internal nodes and does not com-
promise the fault-protection scheme.
Figure 11 shows the condition of the on channel with
V+ and V- present. With input voltages less than ±10V,
all three FETs are on and the input signal appears at
the output. If the input voltage exceeds V+ minus the
N-channel threshold voltage (V_TN), the N-channel FET
will turn off. For voltages more negative than V- minus
the P-channel threshold (V_TP), the P-channel device will
turn off. Since V_TN is typically 1.5V and V_TP is typically
3V, the multiplexer’s output swing is limited to about -12V
to +13.5V with ±15V supplies.
Switching Characteristics
and Charge Injection
Table 1 shows typical charge injection levels versus
power-supply voltages and analog input voltage. The
charge injection that occurs during switching creates a
voltage transient whose magnitude is inversely propor-
tional to the capacitance on the multiplexer output.
Table 1. MAX354 Charge Injection
Supply Voltage
±5V
±10V
±15V
Analog Input Level
+2V
0V
-2V
+5V
0V
-5V
+10V
0V
-10V
Injected Charge
52pC
35pC
16pC
105pC
65pC
25pC
180pC
80pC
15pC
Test Conditions: C_L, = 1000pF on mux output; the tabulated
analog input level is applied to channel 1; channels 2–8 inputs
are open circuited. EN = +5V, V_A1 = V_A2 = 0V, V_O is toggled at
a 2kHz rate between 0V and 3V. +100pC of charge creates a
+100mV step when injected into a 1000pF load capacitance.
-25V
Q1 -25V Q2
Q3
OVERVOLTAGE ^S
DS
DS
D
N-CHANNEL MOSFET
IS TURNED ON
G
G
G
BECAUSE V_GS = +25V
P-CHANNEL
MOSFET IS OFF
Figure 8. -25V Overvoltage with Multiplexer Power Off
-15V
+15V
-15V
+25V
Q1
Q2
Q3
OVERVOLTAGE ^S
DS
DS
D
N-CHANNEL MOSFET
IS TURNED OFF
G
G
G
BECAUSE V_GS = -25V
Figure 9. +25V Overvoltage with Multiplexer Power Off
-15V
+15V
-15V
-25V
Q1
Q2
OVERVOLTAGE
N-CHANNEL MOSFET
IS TURNED ON
BECAUSE V_GS = +10V
-15V FROM +15V FROM
DRIVERS DRIVERS
P-CHANNEL
MOSFET IS OFF
Q3 _{+25V FORCED}
ON COMMON
OUTPUT LINE BY
EXTERNAL CIRCUITRY
N-CHANNEL
MOSFET IS OFF
Figure 10. -25V Overvoltage on an Off Channel with
Multiplexer Power Supply On
+25V
OVERVOLTAGE
Q1 _13.5V Q2
N-CHANNEL MOSFET
IS TURNED OFF
BECAUSE V_GS = -10V
V_TN = 1.5V
+15V FROM -15V FROM
DRIVERS DRIVERS
Q3 _13.5V
OUTPUT
N-CHANNEL
MOSFET IS ON
Figure 11. +25V Overvoltage Input to the On Channel
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