Datasheet

MAX9940
In all three schemes, the MAX9940 protects the micro-
controller from both DC fault voltages above V
CC
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., R
P
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 V
CC
= 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 (V
OH
≈ V
DD
). 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 V
CC
. This allows the voltage at INT to
stay at V
DD
even though V
CC
= 0, thus drawing no cur-
rent from R
P
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
R
P
, causing a power drain from V
DD
, and can potential-
ly reduce battery life.
In Figure 3, V
DD
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 V
CC
. As
a result, in the event of a short to GND fault on EXT,
current is drawn through RP, causing a power drain
from V
DD
and potentially reducing battery life.
Signal-Line Overvoltage Protector
for Low-Voltage Devices
6 _______________________________________________________________________________________
MICROCONTROLLER
EXT
GND
INT
ENABLE
V
CC
R
P
2kΩ
V
DD
= 3.3V
MAX9940
DATA
Rx
Tx
1-Wire
DEVICE
Rx
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