MAX9940 データシートの表示（PDF） - Maxim Integrated

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MAX9940

Signal-Line Overvoltage Protector for Low-Voltage Devices

Maxim Integrated 

Signal-Line Overvoltage Protector

for Low-Voltage Devices

Pin Description

PIN

NAME

FUNCTION

1

VCC

Power Supply. Can be connected to a microcontroller enable input.

2

GND

Ground

3

N.C.

No Connection. Not internally connected.

4

INT

Microcontroller I/O Port. Connection to a microcontroller data port.

5

EXT

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 I2C protocols where large series resis-

tance and capacitances cannot be used to provide

protection due to their impact on VIL/VIH 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 I2C

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 38Ω (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 VCC 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 VCC 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 VCC being

connected directly to VDD of the microcontroller.

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