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Signal-Line Overvoltage Protector
for Low-Voltage Devices
Pin Description
PIN
1
2
3
4
5
NAME
V CC
GND
N.C.
INT
EXT
FUNCTION
Power Supply. Can be connected to a microcontroller enable input.
Ground
No Connection. Not internally connected.
Microcontroller I/O Port. Connection to a microcontroller data port.
External Connector Port. Connection to a 1-Wire device.
Detailed Description
The MAX9940 is a signal-line overvoltage protector for
low-voltage devices that provides circuit protection
from high-voltage faults and ESD strikes. The device
provides protection in digital communication lines such
as 1-Wire and I 2 C protocols where large series resis-
tance and capacitances cannot be used to provide
protection due to their impact on V IL /V IH levels and
communication timing. The MAX9940 includes a series
switch that connects INT to EXT. When a high-voltage
fault condition occurs on EXT, the MAX9940 quickly
shuts off the series switch and isolates the low-voltage
device from the fault condition. In addition to providing
DC fault isolation, the MAX9940 also provides up to
±4kV IEC 61000-4 contact ESD protection on EXT. The
MAX9940 is ideal for circuits that require low-voltage
devices that communicate to the outside world over
connector ports that can expose them to hazardous
high-voltage DC faults and ESD strikes.
Series Switch
The MAX9940 features a series switch to connect a
low-voltage device such as a microcontroller to an
external communication device such as a 1-Wire or I 2 C
slave. The internal switch is turned off when disabled or
if a fault condition exists, isolating the microcontroller
from any possible damage. The nominal switch resis-
tance is 38 Ω (typ). The series switch is composed of
parallel DMOS and HV-pMOS devices as shown in the
Block Diagram/Typical Application Circuit. The series
switch cell contains circuitry that ensures the pMOS
device turns off properly when the voltage at EXT
exceeds the supply voltage. The switch can withstand
a maximum voltage of 28V at EXT.
Comparator
The MAX9940 features a low-power, high-speed com-
parator that is used to turn off the series switch if a
high-voltage condition is detected on EXT. The nominal
hysteresis of the comparator is 128mV (typical). Fault
voltages on EXT that are slightly above V CC trigger the
comparator to quickly isolate INT and EXT channels
from each other. In this mode, the MAX9940 is able to
withstand 28V on EXT. Negative voltages on EXT are
allowed as long as they are current-limited to less than
20mA.
Typical Application Circuits
The innovative design of the MAX9940 allows it to with-
stand large DC voltages up to 28V at INT and EXT even
when V CC is 0. This allows application-specific power-
saving and fault-protection schemes to be implemented.
Figures 1 and 2 show two methods of powering the
MAX9940 from an ENABLE digital output port of the
microcontroller. Figure 3 shows the conventional method
of operating the MAX9940 with both pullup resistor for
digital communication on DATA (RP) and V CC being
connected directly to V DD of the microcontroller.
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