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_______________Detailed Description
The MAX4505 protects other ICs from overvoltage by
clamping its output voltage to the supply rails. If the
power supplies to the device are off, the device clamps
the output to 0V. The MAX4505 provides protection for
input signals up to ±36V with the power supplies on
and ±40V with the power supplies off.
The MAX4505 protects other integrated circuits (ICs)
connected to its output from latching up. Latchup is
caused by parasitic SCR(s) within the IC turning on,
and can occur when the supply voltage applied to the
IC exceeds the specified operating range. Latchup can
also occur when signal voltage is applied before the
power-supply voltage. When in a latchup state, the cir-
cuit draws excessive current and may continue to draw
excessive current even after the overvoltage condition
is removed. A continuous latchup condition may dam-
age the device permanently. Such “faults” are com-
monly encountered in modular control systems where
power supplies to interconnected modules may be
interrupted and reestablished at random. Faults can
happen during production testing, maintenance, start-
up, or a power failure.
Figure 1 shows the normal complementary pair (N1
and P1) found in many common analog switches. In
addition to these transistors, the MAX4505 also con-
tains comparators, sensing circuitry, and clamping cir-
cuitry to control the state of N1 and P1. During normal
operation, N1 and P1 remain on with a typical 65
on-
resistance between IN and OUT.
The on-board comparators and sensing circuitry moni-
tor the input voltage for possible overvoltage faults.
Two clamp circuits limit the output voltage to within the
supply voltages. When the power supplies are off, any
input voltage applied at IN turns off both N1 and P1,
and OUT is clamped to 0V.
Normal Operation
When power is applied, the protector acts as a resistor
in series with the signal path. A voltage on the “input”
side of the switch conducts through the protector to the
output (Figure 2).
When the output load is resistive, it draws current
through the protector. The internal resistance is typical-
ly less than 100
. The MAX4505 does not affect high-
impedance loads. The protector’s path resistance is a
function of the supply voltage and the signal voltage
(see
Typical Operating Characteristics).
MAX4505
Fault-Protected, High-Voltage,
Signal-Line Protector
6
_______________________________________________________________________________________
Pin Description
-15V
+15V
P-CHANNEL
DRIVER
N-CHANNEL
DRIVER
+15V
N3
P3
IN
N1
CLAMP
OUT
CLAMP
+V(+15V)
-V(-15V)
N2
P2
P1
COMPARATOR
SENSE
SWITCH
SENSE
SWITCH
Figure 1. Simplified Internal Structure
MAX
SOT
AIN
Analog Fault-Protected Input
3
1
V-
N.C.
AOUT
V+
NAME
Negative Supply Voltage Input
4
5
No Connection. Not internally connected.
2, 5, 6, 7
4
Analog Signal Output
1
3
Positive Supply Voltage Input
8
2
FUNCTION
PIN