±15kV ESD-Protected, EMC-Compliant, 230kbps
RS-232 Serial Port for Motherboards/Desktop PCs
MAX3185
Human Body Model
Figure 2a shows the Human Body Model, and Figure
2b shows the current waveform it generates when dis-
charged into a low impedance. This model consists of
a 100pF capacitor charged to the ESD voltage of inter-
est, which is then discharged into the device through a
1.5kΩ resistor.
IEC1000-4-2
The IEC1000-4-2 standard covers ESD testing and per-
formance of finished equipment; it does not specifically
refer to integrated circuits. The MAX3185 helps you
design equipment that meets Level 4 (the highest level)
of IEC1000-4-2, without additional ESD-protection com-
ponents.
The main difference between tests done using the
Human Body Model and IEC1000-4-2 is higher peak
current in IEC1000-4-2. Because series resistance is
lower in the IEC1000-4-2 ESD test model (Figure 1a),
the ESD withstand voltage measured to this standard is
generally lower than that measured using the Human
Body Model. Figure 1b shows the current waveform for
the ±8kV IEC1000-4-2 Level 4 ESD Contact-Discharge
test.
The Air-Gap test involves approaching the device with a
charge probe. The Contact-Discharge method connects
the probe to the device before the probe is energized.
Machine Model
The Machine Model for ESD testing uses a 200pF stor-
age capacitor and zero-discharge resistance. It mimics
the stress caused by handling during manufacturing
and assembly. Of course, all pins (not just RS-232
inputs and outputs) require this protection during man-
ufacturing. Therefore, the Machine Model is less rele-
vant to the I/O ports than are the Human Body Model
and IEC1000-4-2.
__________Applications Information
Use proper layout to ensure other devices on your
board are not damaged in an ESD strike. Currents as
high as 60A can instantaneously pass into ground, so
be sure to minimize the ground-lead return path to the
power supply. A separate return path to the power sup-
ply is recommend. Trace widths should be greater than
40 mils. Bypass V
CC
, V
DD
, and V
SS
with 0.1µF capaci-
tors as close to the part as possible to ensure maxi-
mum ESD protection.
Tie any unused transmitter inputs to GND or V
CC
. No
external protection diodes are needed because the
MAX3185 is not sensitive to power-supply sequencing.
R
C
1M
CHARGE-CURRENT
LIMIT RESISTOR
HIGH-
VOLTAGE
DC
SOURCE
R
D
1500Ω
DISCHARGE
RESISTANCE
DEVICE
UNDER
TEST
I
P
100%
90%
AMPERES
Ir
PEAK-TO-PEAK RINGING
(NOT DRAWN TO SCALE)
Cs
100pF
STORAGE
CAPACITOR
36.8%
10%
0
0
t
RL
TIME
t
DL
CURRENT WAVEFORM
Figure 2a. Human Body ESD Test Model
6
Figure 2b. Human Body Model Current Waveform
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