As long as the signal is within these limits, all three tran-
sistors conduct and a low-resistance path is maintained
from the IN to OUT pin. (Note that, since the device is
symmetrical, IN and OUT pins can be interchanged.)
When the signal is beyond the gate threshold of either
Q2 or Q1/Q3, the path resistance rises dramatically.
When power is off, none of the transistors have gate
bias, so the circuit from IN to OUT is open.
Normal Operation
In normal operation, the protector is placed in series
with the signal line and the power supplies are con-
nected to V+ and V- (see Figure 2). V- is ground when
operating with a single supply. When power is applied,
each protector acts as a resistor in the signal path.
Any voltage source on the “input” side of the switch will
be conducted through the protector to the output. (Note
that, since the protector is symmetrical, IN and OUT
pins can be interchanged.)
If the output load is resistive, it will draw current, and a
voltage divider will be formed with the internal resistance
so the output voltage will be lower than the input voltage.
Since the internal resistance is typically less than 100Ω,
high-impedance loads will be relatively unaffected by the
presence of the protector. The protector’s path resis-
tance is a function of the supply voltage and the signal
voltage (see
Typical Operating Characteristics
).
Power Off
When power is off (i.e., V+ = V- = 0V), the protector is a
virtual open circuit, and all voltages on each side are
isolated from each other up to ±40V. With ±40V applied
to the input pin, the output pin will be 0V, regardless of
its resistance to ground.
Fault Conditions
A fault condition exists when the voltage on either sig-
nal pin is within about 1.5V of either supply rail or
exceeds either supply rail. This definition is valid when
power is applied and when it is off, as well as during all
the states as power ramps up or down.
During a fault, the protector acts as a variable resistor,
conducting only enough to sustain the other side of the
switch within about 1.5V of the supply rail. This voltage
is known as the “fault knee voltage,” and is not symmet-
rical. It is approximately 1.3V down from the positive
supply (V+ pin) or approximately 2.0V up from the neg-
ative supply (V- pin). Each fault knee voltage varies
slightly with supply voltage, with output current, and
from device to device.
During a fault condition, all the fault current flows
from one signal pin through the protector and out
the other signal pin. No fault current flows through
either supply pin. (There will be a few pico-amps of
leakage current from each signal pin to each supply
pin, but this is independent of fault current.)
During the fault condition, enough current will flow to
maintain the output voltage at the fault knee voltage, so
the fault current is a function of the output resistance
and the supply voltage. The output voltage and cur-
rent have the same polarity as the fault.
The maximum input fault voltage is 40V from the “oppo-
site-polarity supply rail.” This means the input can go
to ±35V with ±5V supplies or to ±25V with ±15V sup-
plies. The fault voltage is highest (±40V) when the sup-
plies are off (V+ = V- = 0V).
Using the circuit of Figure 2, the approximate fault cur-
rents are as follows:
1) For positive faults:
I
(F)
≈ (V+ - 1.3V - V
LOW
) ÷ R
OUT
2) For negative faults:
I
(F)
≈ (V- + 2V + V
LOW
) ÷ R
OUT
where V
LOW
is the terminating voltage at the far end of
R
OUT
. V
LOW
= 0V when R
OUT
is grounded.
MAX366
OUT11
4
IN1 7
V+V- V+V-
8
V
OUT
V
IN
R
OUT
V
LOW
Figure 2. Application Circuit
MAX366/MAX367
Signal-Line Circuit Protectors
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