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

VDD = 3.3V

ENABLE

MICROCONTROLLER

RP

2kΩ

DATA

Rx

INT

Tx

VCC

MAX9940

GND

1-Wire

DEVICE

Rx

EXT

Tx

Figure 1. Recommended Scheme for Battery-Operated Devices that Need to Shut Down the MAX9940 and Prevent Power Draw

During Short to GND Faults

In all three schemes, the MAX9940 protects the micro-

controller from both DC fault voltages above VCC and

ESD strikes on EXT. The difference in the three

schemes lies in the impact on power consumption in

battery-operated devices during normal and short to

GND fault conditions.

Figure 1 shows a recommended configuration for battery-

operated devices that need to conserve power both on

a continuous basis as well as during short to GND fault

conditions. In this scheme, the ENABLE port of a micro-

controller supplies the quiescent current for the

MAX9940 as well as that required for digital communi-

cation (i.e., RP pullup resistor). By forcing a 0 on the

ENABLE digital output port of the microcontroller, the

MAX9940 is in a zero-power shutdown mode, while also

preventing any power drain to occur in the event of a

short to GND fault on EXT. As stated earlier, EXT main-

tains the ability to withstand DC voltages up to 28V

even when VCC = 0.

In Figure 2, the ENABLE port of a microcontroller pow-

ers the MAX9940. The low 13µA operating current

allows standard digital I/O ports to easily supply the

operating current of the MAX9940 without any substan-

tial voltage drop (VOH ≈ VDD). By forcing a 0 on the

ENABLE port of the microcontroller, the MAX9940 can

be put into a zero-power mode, thus conserving battery

power. It should be noted that there is no internal ESD

diode from INT to VCC. This allows the voltage at INT to

stay at VDD even though VCC = 0, thus drawing no cur-

rent from RP or the battery. However, an internal diode

does exist from INT to EXT, and therefore, in the event

of a short to GND fault on EXT, current is drawn through

RP, causing a power drain from VDD, and can potential-

ly reduce battery life.

In Figure 3, VDD powers the MAX9940 directly, and

consumes quiescent current on a continuous basis. In

this mode, the internal FET between INT and EXT is

kept on as long as the voltage on EXT is below VCC. As

a result, in the event of a short to GND fault on EXT,

current is drawn through RP, causing a power drain

from VDD and potentially reducing battery life.

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